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CHAPTER4

InterplayofstructuralandElectronicproperties
S.Kagoshima,R.Kato,H.Fukuyama,H.seo
andH.Kino
Uπiversiかofrobo,Tokyo,ノαPαπ

丁MTSF

BEDT、πF(ET)<:エ:X江>く工:XX>く工:X江:〕BMDT、TTF

BETS

B0

CX:エ:>(ン:Xン器く工:Xン器
DMET

MDT・TrF

DMET.TseF

繊工:X江:X江ごCX江:>ヘ̲・
1汀、M、TrpDTEDT

Thischapterprovidestheoverviewofthestructure‑electronic‑propertyreauon、
Ship。fsyntheticmetals.1tisorganizedinthreeparts.The6rstpad(byR・Kato)
ismainlydevotedtothe血auenceofthemolecularanangementontheeectronic

Xン:X江>.〔ンXエン'(XX::エ>S.S・DMBEDT、TrF"single̲band"̲versus、"multi、band"systems.Thesecondpart(bys・Kagoshima)
dealswiththestNcturalchangespred0血nantlycausedbythelow、dimension、alit。felectr。nsystem.1tisa"re、visitation"ofthepeierlsandspin、peierls
transiti。n,andanionordedng.TCNQ・andTMTSF・basedsystemsareusedas

Acceptorくミトづ,ノ':1Ξン＼ノノ"textb。。k,examplesbecausetheyarethemoststudiedsystems.Thesub‑section

00

2.5。廿ersam。recunentdiscussionontheinterplayofvariousinstabilitiesin

DCNQI̲cucompounds.Thethirdpart(byH.Fukuyama,H.seoandH・Kino)
givesapropositionforunifyingdassi丘Cationonvariousgroundstatesoserve

N

C

C

^

C

、"1川1N'1、・

TMET、STF

M(dddt)2

StNcture.Twomaintopicsarediscussed:theroleofdimensionalityanthe

inorganicmetals.
TCNQ

1.Molecule,MolecularArran容ement, andElectronicstructure
M。1ecularconductorsexhibitavarietyofphysicalpropenles thatcanbe

SstematicaⅡyunderstoodonthebasisofsimpleandclearelectronlcstructures'

RI,R2・DCNQI:工XエニーンXエンM(mnt)2

M(dmit)2

Recentadvancesinangle‑dependentmagnetoresistancestudiesonmoecu

C。nduct。rshaverevealedthatthesimpletight・bindingbandcalculationisvew

Useful(butnota11mghw)todescribetheFer加Surfacetop010gy[1,2]・The
262

1︑

Variet。bservedinelectronicstructuresheavilydependsonthepac'ng

FigurelExamplesofcomponentmoleculesformolecularconductors.

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Figure3CrystalstruC加reofa・Et2Me2N[Ni(d111iり2]2.

ratherpoorduet0血eb2Syn血etryoftheLUMO,whichisunfavorableforthe【。verlap"),however,providesatwo・dimensionalconductionpathparaⅡeltothe
bcplane.1ndeedtheFer血Surfaceofthiscompoundwas、foundtobetwo、bdimensional[8].Moleculardesignt0工Vardsuchawpeofmolecularanangement>一(D正]2LL

1劃U1︻﹂[1rノ↓1ーーー

Kenhancementoftheinterstackinteraction.Themodelovedap("spanninghasbeenproposedforaxiaⅡysubstitutedphthalocyanineneutralradicals[9]・TheorganicdonormoleculeBEDT‑TTF(ET)removedanimpressionthatthe
町moleculepreferstheface‑to‑facestacMngstNcture,whichwasonegiant
Ieap"forthemolecularconductor.Typicaltwo・dimensionalmolecularan釦ge、
ments(designatedβ,θ,andK)derivedfromtheETmoleculeareshowninHgr
M

4WithcalculatedFen11isudaces.Theβ・typeanangementiscomposedofloosely
StackedETdimers[10].Theshorts・‑scontactsareobservedbetweenstacks,

M

Y

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Figure4
SchematicmoleculararrangementsofETmolecules(end、onprojection)inβ̲
(ET)J3, θ、(ET)21,(averagedS如Cωre),andK・(ET)"3WithcalculatedFe血isurfaces.

Whilethereisnoshons.‑scontactwithinthestacks.suchastruC加ralfeature,
WhichwasnotobseNedintheregularcolU1血Structures,providestwo、
dimensionalinterdimerinteractions.Therearemanyvariationsoftheβ、type

血tennolecularoverlaP血tegralchangessensitivelytotherelativearrangement。fNOETmolecules.

脚a11gement(β',β",ー.etc.),eachofwhichshowsdi丘erentdimensionalityinits

Intheθ'typeanangement,eachETmoleculeissunoundedbsixET

electronicstNcture.Thismeansthatthetwo‑dimensionalelectronlcstNcture
dedvedfromt11e
modincation.The

Y
F

moleculesalmostisotropica11y[12].Thereisnotanyface、to、faceoverlap。fET

blttheacMng

molecules,whichindicatesthattheθ・typeanangemenuargelydeviatesfr。mthe
face'to、facecolunm.Thedihedralangle(θ)betweendonormoleculescanbe

intermolecularoverlapintegralbetweenHOMoswas

CalculatedforapairofETmoleculeswheretwomoleculestakevarlous'加nedbythechoiceofthecounteranion.simplecalculationsindicatethatan
mcreaseofthedihedrala11gleθleadstoasigni五Cantdecreaseofthetransfer

ana11gementswithkeepingtheirlongaxesandtheirmolecularplanesparale゜eachother(asisthecaseoftheβ・sedeS町angements)[1U.Thismapof仕le。verlapintegralasafunctionofcylindricalcoordinatesindicatesthatte、mtegralintheuansversedirection.1nthegeneralphasediagra111fortheθ̲ET
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AdναπCesiHsyπthιticuetals

1πtelP1αyqf'SかιιCtιιrα1απd五leCか0πicproPιπies

269

Salts,thelow‑temperaturegroundstatevariesfrominsulating,tosuper‑
Conducting,tometa11icwithdecreasingtheelectronicconelationparameteru/1形;
Whereuisthee丘ectiveon・sitecoulombenergyandwisthebandwidth[13].
Incontrasttotheβ・typeanangement,theshapeoftheFermisurfacededved

fromtheθ、typearrangementisnotsosensitivetothedihedralangleθ.Theθ・
Wpeisoftencomparedwith山edonoranangementsobservedina・(ET)213n4]
anda、(ET).MHg(SCN)'(M=K,NH小Rb,TD[15].AⅡofthemarebasedonthe"heningbone"arrangementandshorrs‑・scontactsinthetransversedirection.
ThepedodicpatternsoftheETanangementalongthemolecularlongitudinal
axiS血α、(ET)2MHg(SCN)4andina・(ET)213aredi丘erentfromthatintheθ・salts
Ineachcaseofa・(ET)213anda・(ET)2MHg(SCN)4,thecrystalcontainst11ree
Crysta110graphicaⅡyindependentETmolecules,twoofwhicharelocatedonnon・
equivalentinversioncenters.X‑raycrystalstructureanalysissuggeststhatthe
fonnalchargesontheseElmoleculesaredi丘erenttoeachother血α・(ET)213,

BEDT・TrF(ET)/ー＼/ー＼/ーー＼/ー＼
e

e

e

e

Whilethereisnomeaningfuldi丘erenceoftheformalchargeina、(ET)2MHg(SCN)4.TheessentialfeatureoftheFermisurfacefor
α、(ET)2MHg(SCN)4issi加laltothatfortheθ・type.Aremovalofthedegeneracy＼、ーノ＼、̲ノ＼、̲ノ＼}ノ

duet0血esubdest川Cturaldistortion,however,resolvesacylinddcalFemli

Surfaceintotheopenandclosedpieces.α・(ET)213Showsase111i・meta11icband
Structurewithslightbandoverlap.Theintenelationbetweentheelectronic
StNctureofa、(ET)2MHg(SCN)4andthatofa・(ET)213Canbeexplainedinterms
Ofthechangeofthecharacteristicintennolecularinteraction[16].
TheK、typearrangementcontainsnodistinguishablestack[1刀.Thecrystal
Containsconductinglayers,eachofwhichiscomposedoforthogona11yananged
ETdimers.1nterdimerinteractionsaretwo・dimensionalandprovideacylindrical

Fermisurface.ManyET・basedorganicsuperconductorswithratherhightransitiontemperaturesareklowntopossesstheK‑typearrangement.TheK・ET
Saltscanbemappedinthegeneralphasediagramthatiscomposedoffour
Phases;paramagneticmetal,paramagneticinsulator,superconductor,andanti・
fenomagnetiC血Sulator[18].Thisphasediagramisalsounderstoodintermsof

e

e

e

e

＼̲̲ノ

BDT・Trp

BETS

Figure5Side・by・side血termolecular血temctionsinETBETS,a11dTTP.

featureisquitesuitablefortheenhancementofthetr会nsverseoverlap.Anumber
OfBETssaltswit11two・dimensionalelectronicstructureswereprearedand
foundtoexhibitmetaⅡicorsuperconductingbehaviors.The入、typeananement

WaS6rstobservedintheBETSS田tswithtetrahedralanionsP2].Thiswpe。f脚angementisafour・foldquasi・stacMng(Fig.6).Theshonchalcogen...chalc。̲

gencontactsarefoundbetweenstacks,ratherthanwithinthestacks.TheFenm

Surfacededvedfromthistypeofmolecularanangementisalsotwo̲dimensi。nal.
Ingeneral,theelectricalpropeniesofmolecularconductorsarestr。nltheelectronicconelationparameterwhichiSωnedbythedegreeofthe
dimedzation.1tshouldbenotedthat0壮lerdonormolecules(forexample,BETS,
MDT‑TfF,BMDT‑TTETTP'S,S,S・DMBEDT‑TTEDMET,andsoon)also
exhibittheK・1ikeanangements[19].

inauencedbythedisorderinthecTystal.1nthea110ysystem入̲(BETS)GaxY̲,however,thedisordere丘ectontheresistivitybehaviorisnotserious,which

Animporta川featureoftheETmoleculeistheoutersix‑memberedheterodngsthatexpandthe打・conjugatingsystemoftheTTFskeletonand

WherethesizeofaⅡtheheterodngsisuni負ed[241.Thisalsoprovidesaat
Pedpheral"surface"(Fig.5).TheTTP・typedonorsgavealargenumberofcati。nradicalsaltsthatretainmeta11icconductivitydownt010wtemperaωres.The

enhanceinterlnoleculars‑・scontactsinthetransversedirection.Thisunder・

St飢d血gledtoanideathatadjustmentsofpedpheryofthemoleculeconsidedng
Sizesoftheheteroringsa11dthechalcogenatomsaree丘ectiveinthecontr010fthe
dimension址ity[20].TwowpicalexamplesareBETsandTTPS.1ntheBETS
molecule,thefulvenesatomsintheBEDT‑TTFmoleculearereplacedbylarger

Seatomsf2U.1nthetransversedirection,theexpanseofseorbitalsinthe6Ve'

exNbitvadoustypesoftwo・dimensionalmoleculararrangementsincludingβ̲,θ、,andK、types[251.Althoughanysuperconductorshavenotbeendedvedfr。m
血eusualTTP、typedonors,thevinylogousanalogDTEDTwasfoundtof。rma
Superconductingsalt(DTEDT)3AU(CN)2[26].Thissaltcontains Unifonnstacks

ρfdonorsthatleadtoa"uniformβ・type"arrangement.owing totheuniform甜ackingandthelowdegreeofoxidation,thecalculatedFer血 Sudaceistwo‑
d血ensional.

Hνmemberedringisalmostthesamewiththatofsorbitalsinthesix・membereddng,thatis,theperipher址"surface"ofthismoleculeisalmostaat(Fig.5).This

SuggestsstrongtendencyofBETstoretainthemeta11icstateP3]
ThefundamentalframeworkoftheTTP・typedonorsisabis、fusedTTFunit

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thanthosebetweenpe印endicularones.1herefore,thetight・bindingband

Calculationgivesalmostzero(Y・「・X)a11dlarge(X(Y)・M・「)dispersionsalong
thedi丘erentdirectionS加thereciprocalspace.Thischaracteristicdispersion
Providesastar・1ikeshapedFennisurface(Fig.フ).
Theexpansionofthelr・conjugatingsystemwithouterheterorings,theidea
PresentedbytheETmolecule,canbeappliedt01,2・dithiolenemetalcomplexes
Withthedonorcharacter.Forexample,、theM(dddt)2(dddt=5,6・dihydro、

入卜入L ＼滄 ＼卜

゜＼

←＼

←＼

゜＼

1,4・dithiin・2,3・dithi01;M=Ni,pd,pt,AU)molecule,wheret11ecentralc=C
＼

イ〆イ
J‑゜

广
ZZ

ずノ

radicalsalt(DCNQI)2CuisshowninFig.8.planarDCNQlmoleculesstackto
formone‑dimensionalcolulms.TheseDCNQlcolumnsareinterconnectedto

イ〆

J
y‑゜

d/'

＼

y』゜

Z

δ
Z

入卜X＼L

eachothert11roughtetrahedraⅡycoordinatedcuionstoformthethree、、＼、ゞ＼ー＼dimensionalDCNQI‑cunetwork.1fthereisnointeractionbetweencuand

1﹁﹂毛]﹃冒DCNQI,thisstNcturegivesonlyone・dimensiona1井(LUMO)band,asisthe
Caseofordinarymolecularconductors.But加thiscasethecuiS加themixed
ValencestateBuandprovidest11ree・dimensionalbandstNcture.
Figure9ShowsFer111isudaceof(DMe‑DCNQI)2Cucalculatedwiththetight、
bindingapproximation.ThisFer血SurfacehasbeenconhrmedbythedHVA
measurement[5].The6rstprinciplesbandcalculationforthissystemwasalso
Performedandgaveasuf6CientresultB2].1nthissystem,thereisacoexistence
Ofthepurelyone・dimensionalFer血Surface,FSI,whichcomesfromorga11icT
bands,andt11ree・dimensionalFen11isurfaceFS3Whichoriginatesmainlyfrom
the3dband.Thisuniqueelectronicstructureodginatesfromthetetrahedral
Coordinationgeometryofthecopperandtheinterplaybetweendorbitalsa11dP訂

0、0
0、0

叫

j

0'N)

0イ

0、0

Orbitals.

δ

1.2.Fro"1"singlecomponent"to"multicomponent"

U

0^

0"1コ

Rイ

Molecularconductorsatthe丘rststageweresinglecomponentsystems.For

exampletheconductionbandinKcpisaone・dimensionaldZ2band.TTF‑

0、0
0、0

TCNQhasaHOMobandofTTFandaLUMobandofTCNQ,butbothofthem

areone・dimensionalpure7「bands.Recently,however,increasingnumberof^

interestingsystemswhichhave"two"bandswithdi丘erentcharactersnear血e一Fermilevelhavebeenrepoded:forexample,theDCNQI‑cusaltwith打a11d

f、

0

。方一

0'、)

(itineranりdbands,pd(d加り2Saltswithatwo・dimensionalHOMobandandaf'ι

5

One・dimensionalLUMoband,theorganicsuperconductor(TMET‑STF)2BF4.=UN(一0之Un,UΣn)︑0巴=‑UΞの一巴切む.U如ご二M一国

doublebond血ETisreplacedbythetransitionmetalexhibitsvadousmoleculararrangements,someofwhicharesi血lartothosefound血血eETsalts[29].1n
山efronuermolecularorbitalofthel,2・dithiolenecomplexes,the3Porbitalsof
Satomsintheligandshowasigni丘Cantcontribution.1n血issense,themolecular
designforthe12・dithiolenecomplexescanbediscussedinc011血onwiththatfor壮)eorganiC7「molecules.
TheDCNQI‑cusaltindicatesanotherwaytothehigherdimensionalsystem
WiththeuseofthecoordinationbondB田.Thecrystalstructureoftheanion

r

274

Adνα11CesiHsy11theticUιtals

1πtelP1αyqf'st地Ct記rα1απdΞ1ιCか0πicpropertiιSf""

Witht11ree、foldlatticedistonionB4]andastaticchargeordeHngatthecusites('‑CUCU゛CU2‑"')・AroundthecHticalpressure,thesystemexhibitsauniue

FSI

metal‑insulator‑metal,reentranttr飢Sitionwithloweringtemperature(type11D

B5]・Thesizeofsubstituents(RlandR2)hasthesamee丘ectwith山eressure

FS2

J

鴬'.膨'ノ::::::::.咽:冒:.

4勢"::旦:::旦:::.

"群抄'ヴ

Metal

1Π

4,典

Figure9CalculatedFermisudaceof(DMe‑DCNQI)2CU.(Reproduced丘omFig.2in
U01.CかSt.ιiq.C烈St.,284a996)183.)

Brl.

N

1Hψ

NN

ノノノH1

DI"

Cuisdoset04/3+.Therefore,theone・dimensionalorganicplrbandinteracts
WiththecU3dorbital.Anapplicationofpressuretransformsthewpelstate血tothewpe11State.Thetype11StateexhibitsasharP6rst・ordermetal・insulator(M・1)
transition.TheM・1transitionofthissystemisaccompaniedbyacDwformation

CDW(3・f01のCU+:CU2+=2:1
Curie・weiss

DMe、d8

ψN
.ノ

N

00
乙C
N

乙毛.C4C
ノ^4・

π=128
π

α=1223゜α=123.1゜

DCI.

000

乙C

π=122

MeBr.

乙NψN/N
ノノC/C

'ノ

N

π=1.32
ーーーーーー^α=124.フ゜

α=1253゜

ιN‑CU‑N(α)
C仂罵0︒三"り臼めInthemeta11icstate,thecuionisinthe加Xed・valencestatea11dthevalenceof

Insulator

↑'・↑L

DMe"

DCNQD2Cusystem.

1[π十Antiferro.

Ontheotherhand,intheBETssaltwithFeC1410calizeddspinsontheFe3゛ionsinteractwithitinerantlrelectrons.

Asmentionedinl.1,theconductionba11doftheDCNQI‑cusystemis
Composedofthed(d,)orbitalofcuandtheLUMoofDCNQI.Thissystem
exhibitsavariewofphysicalpropertiesdependingontheche血Calmodihcation
Orpressure.FigureloisaschematicphasediagramfortheDCNQI‑cusystem.
Theelectronicstatesofthissystem雛eclassinedintot11reewpesaccordingto
廿ansportproper[ies.Thetypelstateismeta11icdowntothelowesttemperature.

CU

Pressure

Withatwo・dimensionalHOMobandandaone・dimensionalHOMoband[33].

Xvehavementionedthatmolecularconductorsexhibitsimpleandclear
electronicstNctureswherethesimpletight・bindingmethodisagood
approximation.1nmostmolecularmetals,theconductionbandoriginatesfrom
Onlyonefrontiermolecularorbital(HOMofordonor,LUMoforacceptor).This
isbecausetheinter‑moleculartransferenergyissmaⅡerthanenergydi什erencesamongmolecularorbitals.However,itispossiblet010catetwobandswith
di丘erentcharactersneartheFer血level.1nsomecasesinterplayofthesetwo
bandsprovidesuniquephysicalprope札ies.Thetypicalexampleisthe(RbR广

Π::::::.冒

1Nミミ食︑ミトaFS3........

卑旦:::::::咽::::圃::

Iypel

、iil

275

N

U

α

FigureloGeneralphasediagramfortheDCNQI‑cusalts.

α=127.1゜AdναπCιSiπSyHthιticuetals

276
104

10'1

EU(冴︑︑倉>=仂一mu'

ぜ01、

d8

イ,ン

10'ユ

h:X=H

10・2

倉三m一仂UC

10ユ

EU(冴

ノ

10・3

Therefore,thedeviationof山ecuvalencefrom4B+inthemeta11icstateshould

゜"ξ01,

requiretheredistributionofthechargefort11emetal、insulatortransitionand

10werthetransitiontemperature.Thevalenceofcucanbeωnedbythedist0丘ion
OfthecoordinationtetrahedronrepNsentedbytheN・CU・Nangle(α).Thelarger

ノNψC

d2fl,1;のavaluemakesthevalenceofcularger.Thismeansthatthedist0πionofthe

0叫

Coordinationteuahedroninducedbytheche111icalmodi丘Cationorpressuremakes

d8:X=D

themet祉licstateunstable.Arelationbetweenthedist0札ionofthecoordination
tetra11edronandthevalenceofcuisdescdbedinthenextsection.The

10・5

104
h
1
6
0
1

10・6
100

10

T/K

10

100

T/K

Figure11Temperaturedependenceoftheres、istivityforpdstine(typeD, fU11ydeute始ted(typelD,andselectivelydeuterated(typeⅡD(DMe‑DCNQD.CU(che"1icalpressuree丘eCり.onecannotice血atthesma11ersubstituentsmakethemeta11icstateunstable.

Electronicstatesof(DMe‑DCNQI)2CU(RI=R2=CH3),whichissituatedne砥

""

、

27フ

Ordedngatthecusiteshavethesameperioda11dtheenergygainisthelargest.

Nノ

ウN

10゜

1πte1ア1αyqf'SかUctιιrα1απdΞleCか0πicproperties(DCNQI)2CusystemisanamazingsystemwhereverysmaⅡChange血壮leliga11d
丘eldcaninduceadrasticchangeoftheelectronicstate.

ItshouldbeaddedthatsomeDCNQI‑cusalts,forexample(DI‑DCNQD.CU(RI=R2=D,exhibitacomplicatedpressure・temperaωrephasediagramathigher
Pressureregion,WMch血dicateS血atthereshouldbeadditionalfactorsthata丘ect
血eelectronicstateoftheDCNQI‑cusystem[40].1nthesesalts,weakinterstack

LUMO"'LUMointeractionscannotbeneglected.1he6rst、pdnciplesband
Calculationfor(DI‑DCNQD2CusuggeststhattheHOMobandsbecomeS仕ongly
山SpersiveandtheLUMO,HOMoanddstatesareSⅡongly血XedneartheFen11i

theboundary,isquitesensitivetothepressureortheche血Calmodi丘Cauonand丘netuningispossiblebyareplacementofhydrogenatomswithdeuteriumatoms

Ievelundermuchhigherpressures[4U.1fthisisthecase,wemustconsiderthe

B6].Thebondleng壮lbetweencarbona11dhydrogenisreducedbythe
deuteration.1hisgivesane丘ectsi加lartotheapplicationofverylowpressure

A11ionradicalsaltsbasedonasedesofmetaldithiolenecomplexesM(d加t)2(M=Ni,pd,Pり血dicateanotherinterestingmulti・componentsystem[42,43].
TheM(d血t)?moleculeisalracceptorwhichhaSヤrovidedvadousmolecular

Figure11Showsawpicalisotopee什ect.Non・deuteratedsaltismetaⅡicdownto

HOMostateinadditiontotheLUMoanddstates.

thelowesttemperature,whilethecompletelydeuteratedsaltshowsverysh町

Conductors,indudingseveralsuperconductors[19,44].Theconductionba11din

metal、insulatortransition.1nthemeta11icphase,temperature・independentpauli、
Iikebehaviorisobserved.Atthemetal・insulatortransitiontemperature,theNis

血eordinarymolecularconductorsbasedonacceptormoleculesisfonnedbythe
LUMO.1n血eM(d加り2・basedconductors,however,theHOMoba11dalsoplays

anabNptchangeofthemagneticsusceptibility.ThiS血dicatesanappearanceof
thelocalspinsatthecusites.Theselocalspinsobeycunie・圦leisslawdownto

肌importantroleandtherefrequendyoccursa"HOMO‑LUMO"ba11dinversion

about30K.At8K,theanti・fenomagnetictransitionoccursaccompaniedbythe

Weakfenomagnetismduetothecanung.Anintroductionofonlytwodeuterium^コ,ーーi1^^rΞ︑.ーーL︑︼↓

atomsintothemethylgroupS血ducesaremarkablereentranttransition(Fig[45].Thissiωationca11beωnedbythedimerizationofM(d血り2molecules.
ForanidealM(d血り2moleculewithD2hsy11血etry,theLUMohasb2
Sy11血en'ywhiletheHOMohasa?,sy如netry.Theb2SylmnetryoftheLUMO
isnotsuitableforthee血飢Cementoftheintermolecularinteracuonalong血e

ID.

Side・by・sidedirection.ontheotherhal)d,thesiωationfortheHOMowiththe

Beh血dsuchadrasticchangeoftheelectronicstatebyverysmaⅡ
modi丘Cations,the打、dinteractionassociatedwiththeligand負elde丘ectplaysan

a2,sy11血etryisverysinlilart0血atfortheHOM0血TTF・typemolecules(forexalnple,ET)andleadstotheenhancedside・by・side血teraction.Therefore,theSystemcontainstwobandswithdi貸erentdimensionalitythetwo、dilnensional
OMobandandtheone・dimensionalLUMoband.Anotherimponantfactoris
血attheenergyseparation△EbetweentheHOMoandLUMolevelsissmaⅡ.

importantroleB7].Thestabilityof壮lemeta11icstateisconelatedtothevalence
Ofcuinthemeta11icphase.Thevalenceofcuestimatedbyxpsmeasurements

atroomtemperatureisslightlysma11erthan4/3+foreachsaltandapproaches
4/3+withadecrease血thestabilityofthemeta11icstateB81.1tshouldbenoted
thattheaveragedvalenceofcuintheinsulatingstateisexactly4乃+(CU゛:CU2゛=2:Dineverycase.Theoreticalstudieshaveindicatedthat即interplayofthe3、foldpedodicpotentialassociatedwiththenest血gattheone、
dimensionalLUMobandandthestrongconelationatthenanowdbandleads
tothe丘rst、orderM、1transitionf0ⅡOwedbythelargehysteresisandthelocalized
SpinsB9].1nthiscase,boththecDwontheDCNQlcolunmandthech紅ge

Thisispartlybecausethedmetalorbitalscannot加XintotheHOModuetothe

di丘erenceof廿lesylmnetry.Thissma11△EvaluebecomesimportantwhentheM(d血り2moleculesarestronglydimerized.ForeachoftheHOMoandLUMO
States,thedimerizationgeneratestheanti・bondinga11dbondingbandsseparated

bya11energygap.1fthisdimerizationgapislargeenoughcomparedwiththe△E
Value,theanti・bondingHOMobandislocatedhigherthanthebondingLUMO
band("HOMO‑LUMO"bandinversion;Fig.12).whenthef0血alchargeoftherAdναπCeS加SyπthιticMetals

1πtι1ア1αyof'st地Cturα1απdΞleCか0πicpropertiιS0、:1辻・・、〔く口ーー+・・・・゛゛0.11に一肺・","andNMRmeasurementsindicatethatthesesaltsexhibitparamagneticbehavior
f0ⅡOwedbysomeanti・fenomagneticordedngadowtemperatures.Thissuggests
杜lattheyareMott‑insulatorsatambientpressure.
Thepressuree丘ectonsuchasystemwherethetwo・dimensional(andhalf・
五Ⅱe山HOMob飢dislocatedimmediatelyabovetheone・dimensionalLUMO
bandisofspecialinterest.Apossiblescenarioisasf0110WS.Theapplicationof
仕lepressureisexpectedtoenhancetheinter・dimeruansferintegrals.Thisleads

278

ト'・""
鳥1ヘ仟.
"""'H・・・・、0.11111111.

[pd(dmit)2]2、Dimer稔ed
Column(Mottinsulator)
Figure12

Schematicelectronicstructureofthedimedcpd(dlmt)2Systemwiththe
formalchargeofν2・279

toan血CreaseinbandwidthsforbothHOMoandLUMobands.Thereforethe

electronicconelationparameteru/wshouldbereduced.Thereshouldbea

PressureabovethattheHOMobandoverlapswiththeLUMoba11d.1nthiscase,
山eHOMobandisn010ngerhalf・611ed,whichtransformstheMott・insulatorto
血emetal(orthesuperconductor).But,wemustnoticethattheLUMocharacter
increasesattheFer血levelwithanincreaseofpressure.Therefore,atthehigher
Pressurestheone・dimensionalnatureoftheFermisurfacedevelopsandleadsto
SomeinstabilityofthemetaⅡicstate.1ndeed,β・Me4N,β'・Me4Sb,β'・Et2Me2P
Saltsarehlowntobehigh・pressuresuperconductors,andthemeta11icstatethat
ωmsunstableunderhigherpressuresisobservedinsomesalts.Thepressure
e丘ectonthepd(d血t)2Saltsissensitivetothechoiceofthecation.underth'th'b't'tfl・ttdt,乢卜J"一︑︑"干6nsulating,meta11ic,andsuperconducting)accordingtothecation.Thiscation
dependencesuggestS壮)atthemechanismofthepressuree丘ectisnotsosimpleM(d血り2moleculeiSν2・,frequendyobseNedvalue,theconductionbandiS血ehalf、6Ⅱed(anti、bonding)HOMoba11d.Thistwo・dimensionalHOMobandhas[42].

narrowwid血asaresultofthestrongdimerization(thee丘ectiveba11dwidthis
Conelatedtotheinterdimertransferintegrals).Therefore,thesystemtendsto

System.1ncontrasttotheM(d血り2System,however,the"HOMO‑LUMO"bandinversionintheM(dddt)2SystemresultS血theone・dimensionalLUMobandas

becomeaMottinsulator.

血econductionband.

IntheNi(dnliり2、basedconductorswit11thestacMngarrangement,theAnothertypeofmulticomponentsystemistheonewhereitinerant7「electrons

dimedzationisweakandthereforetheconductionbandisthe(quasDone・
dimensionalLUMoba11d(α・Et2Me2N[Ni(d111it)2]2isanexceptionalcaseas'h11'd.1h[Lmentionedinl.D.ontheotherha11d,mostofthepd(dnliり2・basedconductors
Consistofstronglydimedzedpd(d血り2moleculeswith釦eclipsedcon丘guration
andveryshortpd‑.pddista11Ce.Thedimerizationisaresultofthecompetition
betweenthemet址一metalbondandtherepulsivechalcogen・・chalcogeninter・
actions.Thesetwocontributionsdependon壮lespatialextensionof山emetaland

Chalcogenorbitals.TheS廿ongdimedzationleadstothe"HOMO‑LUMO"bandinversioninthepd(d加り2・basedconductors.Forexample,re且ectancespectrafor
卜t

The"HOMO‑LUMO"ba11dinversioncanals0もediscussed加dleM(dddt)2

Me4AS[pd(d血t)2]2andcs[pd(d111it)2]2revealed壮lattheenergyseparation△Eis
0.8‑09evandtheanti、bondingHOMoleve11ieshigherthanthebond血g
LUMolevel[45].Estimatedbandparametersbasedontheopticaldatagivea

PiCωtewhereS廿onglydimedzeddimersinteracttoeachotherwithsma11
anisotropyintheconductionlayer.1nasedesofisostructura1β'(β)・Me4Z(Z=N,
RAS,sb)andβ'、E、Me.Z(Z=RAS,sb)saltsofpd(dmit).with血ef0血al
Chargeofν2、,theconductionbandisconsideredtheHOMoband[42].A11these
Saltsshownodearmeta11icbehavioratambientpressure.susceptibility,ESR,

Possiblefor7「electrons(orholes)tomediatetheexchange加teractionbetween
血elocalizedmagneticmoments.Evenifthelrelectronsarelocalizedthe打electronscanworktocouplethemagneticmomentS壮lroughthesuper・exchangemechanism.Thereareincreasingnumberofexamples;CUPCI[46],(ET)3CUBr4[4刀,(ET)'AFe(C2003C6H5CN(A=H.0,K,NHO[48],入・(BETS)2Fea'[22]andsoon.Amongthem,入・(BETS)2FeC14isofspecialinterest.TheBETS
moleculeprovideS入・typesaltswithGacliandFeCじ.The入・GaC14Saltisan
ambient・pressuresuperconductorwithT=8K,whilethe入・FeC14Saltexhibits
Verysharpmetal・insulatoruansitionatthesametemperature.Thecrystal
Structuresareverysimilartoeachother.Animponantdi貸erenceliesatthe

Counteranion'theFe3゛ionismagneticunlikethediamagneticGa3゛ion.The
ESRmeasurementindicatesthattheFe3゛ionisinthehigh・spinstatea11dthe
metal・insulatortransitioninthe入・FeC14Saltisaccompaniedbyananti・

fenomagneticorderingoftheFe3‑moments.TheapplicationofmagnetiC6elds》10Tsuppressesthesharpmetal・insulatortransition[49].1tisproposedthat

齢Sunusualphenomenon(aneld・restoredhighlyconductingstate;FR11CS)is

r

^・'い

1πterP1αyQf'strιιdurα1απdΞleCか011icproPιrties

C。nnectedtoferromagneticordedngoftheFemomentsunderthehigh

Pedodh'=3.4b[55,56].ThesuperstruCωreisfoundtobefomledbythemagnetiC丘eld.

2.structⅡralchangesandElectronicproperties

・1

Org釦icconductorsfrequentlyundergoachangeinelectronicpropenies
acc。mpaniedbyastNcturalchangewi壮Ivaryingtemperatureorpressureorboth.
Thisispred0血nandyduetothesystemslow・dimensionalnaturewhich

Sinusoidalmoleculardisplacementhavingthewavevectorwhoseb‑component

Ω。=0295h*=a/3.4)b*.Figure15Showsthetemperaturedependenceofthe
intensityofthesuperlatticereaection,whichisapproximatelyproportionaltothe
Squareofthedisplacementamplitudef55‑5刀.
Thistypeofmetal、insulatortransitionisconsideredtobenothingbutthe
PeierlstransitionthathadbeenproposedbyR.E.peierlstoexplaintheelectronic

Prope丘iesofbismuth[5釘.Hepointedoutthataone・dimensionalmetalis
Unstableatlowtemperaturestoundergoametal・insulatortransitionaccompanied

bycharge、densiwwaves(CDW).Aprecursorphenomenonofthepeierls

enhancestheelectron‑1atticeorspin‑1atticeinteractiontocausedramatic

ChangeS血electronicandstNcturalpropenies.Amongthemthemosttypicalhasechangesarethepeierlsandthespin・peierlstransitionsthatwere6rstfoundTTF

andstudied血detailinmolecularconductors.

2.1.ぞeierlSかαπSiガ0π

TCNQ

Inthe血ddle1970smet址licnatureandthetransitionintotheinsulatingstate
Weref。undintwomolecularsystems,KCP(K2Pt(CN)'Bro.3032H20)andTTF‑

TCNQ(tetrathiafulvalene、tetracyanoq山nodimethane),bothofwhichhavehighly。ne̲dimensionalelectronicsystems[50,51].Thesebehaviorsweredi丘erentfrom

th。sehavingbeenfoundin0壮lermolecularconductorsintwopoints;alotof

11

281

AdναπCesiπSyπtheticuetals

280

゛J.子

Unquestionableexpenmentalevidencesfor壮lemeta11icprope武ies,andacle紅Structuralchangeaccompanyingthemetal・insulatortransition・Theodginof.thetransiti。nwasclad丘edtobethepeierlstransition,whichwi11bediscussedbelow・︻L﹂ー

Thedisc。veryofthepeierlstransitioninKcpandTTF‑1CNQweref0ⅡOwedbyth。seinorganicconductorssuchasTseF‑TCNQ,HMTrF‑TCNQ,HMTSF‑
TCNQ,NMP̲TCNQ,TMTSF‑DMICNQ[5U,a11d血inorg釦icconductorslike
Tas,Nbse.,K。..MO0.,(TaseoJand(NbseoJ[52].1nthef0ⅡOwingweshaⅡaCf。CUS。nTTF̲TCNQasagoodexamplefor壮lediscussionofstructure‑properry
relationship

FigureS13and14ShowthecrystalS廿Uctureand杜letemperaωredependence。felectdcalconductivitymeasuredalongtheone・dimensionalaxis,b、axis,0
TTF̲1CNQ[53].Theconductivityincreaseswithdecreasingtemperaturedownt。ab。ut60KbelowwhichtheconductivityischaractedzedbythermaⅡyb
Cactivatednaωre.ThemetaⅡicprope丘iesareasce武ainedbymuchexpedmenta

evidencesuchasoptiC址reaectivity,spin・magneticsusceptibility,a11dthenno、P。wer[54].1ntheinsulatingstatesinlilarmeasurementsalsosuggestthe
resenceofabandgapattheFermilevel.Thesemeasurementssuggesttheme仏、"

insulatortransitiontooccurat53K.

川、1

StNcturalchangeaccompanyingthemetal・insulatortransition:1ntheinsuating

Di丘Usex̲rayandelasticneutronscatteringstudiesrevealedthepresenceoa

Statebel。W53Kasetofsate11itereaectionswasdiscoveredwiththesuperlattice

FigⅡre13CrystalstNctureofTTF‑TCNQ

r

282

AdναπCesiπSyπtheticuetals

1πlelP1αyqfslrιιCtura1απdΞlectr0πicproPιrliιS

283

フ(K)

1」
't

100

104

5040

1.0

20

30

i‑、

Cノ

d)︒(︾琴"コ旦口ω裁武

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...

"︑.[゛一←,

..
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.

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103

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.

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.
.

.
.

.
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.

.

ノ
ノ

ノ

8

ノ

,X̲

7
6

4kF

.5

.3

.

ニ.̲.ニ'ト

2kF

.

.
.

0.2
10

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10

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倉旨ω昔一司S都苓Uの︑(一'EUT◎)︒し

.

/
ノ

0.9

>0.4

.

102

ノ

TTF・TCNQ

2k,(250.)

5

4kF

(500り

10‑2
0

1

2

3

4

5

6
0

11"
,1

制

100/T(K、')

Fi客Ure14TemperaturedependenceofthedcelectricalconductivityofTrF‑TCNQ(ReproducedfromT.1Shiguroetal.,J.phys.SOC.Jpn.,41a976)351,Fig.1(a))
廿ansitionisthephononsoften血gduetotheelectron、phononinteractionwiththe
Wavenumber2kF.1tiscaⅡedtheKohnanomaly[59].

ThemechanismsofthepeierlstransitionisexplainedintuitivelyaS仕lef0ⅡOW血g:Figure16(a)depictsaone・dimensionalmetaⅡicconductionband
OccupieduptotheFenmpotentia1五FandtheFer血Wavenumberk.whenone

introducesaperiodicpotentialduetothemoleculardisplacementhavingthe
Pedodof21「/2kRthesizeofaunitceⅡisenlargedt02打/2k血Whichtwo

electronsresideoccupyingthelowestenergylevel.1nthemomentumspacethe
reciproca11atticehasthesizeof2kF.Therefore,accordingtoelementarysolid

20

30

40

50

Temperature11くJ

血e2k,andthe4k,X、raysateⅡitereaectionsinTTF̲TCNQ.(Rer。ducdf,,.、raysateⅡitereaectionsinTTF‑TCNQ.(Repr。ducedfr。ms.
electronsystemcausesthedensiwresponseρ(r)=ρ Cosqr.1nthelowestorderapproximationoneexpectsthelinearrelation

ρ。=‑V。X(の.

The6rstorderperturbationcalculationgives̲1/(五斜。)ー/(五')
Ⅳ五一Ξ
kkk+q

Statephysics,abandgapopensatk=kFa11d‑kFwheretheelectronenergy
Ξ=五FasshowninFig.16(b).Thissiωationisnothingbutthatofabandinsulator

Wheref(五k)denotestheFen11idiS廿ibutionfunctionfortheelectr。nhavith

Oranintdnsicsemiconductor.1none‑dimensionthismechanismoccurs

Sh゜WS壮leq、dependenceofx(のatr=0.1none̲dimensi。n()hautomatica11ybecauseoftheenergygainbytheamountofthegapenergy.
Thepeierlstransitionisdescribedintermsofthe"poladzationfunction"(orthe"densiw・responsefunction")X(q):Forthepotentialv(カ=vcosqrameta11ic

戸"
1πtιIplayqf'strudιιralaπdΞ1ιCh0πicproPιrtiιS

AdναπCesiHsyhthιticuetals284

χ(Q)/χ(ωーーーーーーーーーーーーー

r1,

EA

U1ーー

a)

285

r=0

EF

.

.

.

..

b)

k

d
1d
2d
3

kF

π^α

k

π^α

.

0

Electron
m

Atom

^ーーー^

α

X1ー

り

E*

0

Figure17k̲electron‑phononinteracdonwiththewavenumber2kF.1tcausestherenormaliza‑
donofthephononfrequencyΩ,Lーーーー・→

Ω.=ω2‑&ω〆VX(ω
q‑q充V

11
0

..

π^α

π^α

11

..

d)

Xvavenumberdependenceofthepoladzationfunctionineachdimension

Ofphononsofteningisca11edthe"giantKohnanomaly"duetothestrong

11

一k

Q

2kF

kF

k

..
m

鄭1
,ノ

J/JJノノJJJ/

It‑h'tt'fthvmber=2k.

In山emetaⅡicstatebelowabout150Kasetofdi丘Usestreakswasfoundin

J二J︑JJJ゛C

decreasingtemperatureforanarbitrarilysmaⅡPotentialampliωdev.This

J山'J/゛ノ宅ノ)ー︑ー﹂ユ,rコーUゞ﹁゛L[ノFigure16AbandpiC加reandadassicalparticleoneforthepeierlstransition.(ωmeta11icband,(b)particlepiCωreofmetal,(C)bandwithdlepeierlsgap,(山Particle
Pictureof杜leinsulatingstatefortheelectrondensiWπ=να

X、raystudiesasshowninFig.18Whichsuggeststheonsetofthepedodiclattice
distonionwithΩ6=0295h*havingnoconelationamongthempe印endicularto
theone・dimensionalb・axis[55,561.conespondingtothesex・raystreaks
inelasticneutronscatteringstudiesrevealedthedecreaseinthephononfrequency'

forthewavevectorΩh=0295h*withdecreasingtemperatureasshowninFig
19[60].Thissoftphononisconsideredtobefrozenoutatthemetal・insulator"曾Ⅱrelsx・raymonochromaticLauepattemofTfF‑TCNQat110K.Thehodzontaltransitiontemperaωre53Kcausingthesuperstructuredescdbedabove.Thiswpe

Phys.Rev、Letters,37a976)437,Fig.1(a))

価rectionisparaⅡeltotheone・dimensionalb・axis.(ReproducedfromJ.P.pougetetal.,
.1

」1,

安

L1

rリ.

AdναπCesiπSyπthehcuaals

286

1πtιJplayqfslrιιdιιrα1α11dΞlectr011icproperties

TTF・TCNQ

kZ

1、D
、、1

ン.ω日]島言ω口od0工:ρ

才ノ
」0

k

1

ーーーーー︑

^

︑

X

︑

^

Q=2k

︑

4

^

︑

k.

2、D

↓

{ンω日一島言ωdodo"ニル

84K

.力

ノ

Kノ

︑

一一

ノ

,、、
一

6

287

'＼.ーー

0

TAphononk^

Polarization 火d0川0

2

'一

'̲̲/̲

k、、Q=2kf̲,^^^^ーー︻FL11E1'コモ︑.'亀

0

Z

3・D

0

0.1

0.2

0.3

0.4

＼>̲

、、
、

/Q=2k

Figure19Kohnanomalyobservedbyinelasticneutronscattering

^

^

^

k

'六11̲̲̲̲

[rl

Whereωandgdenotethebarephononfrequency即dtheelectron‑phonon

叫1︑U1

interactionparameter,respectively,andvisthesystemvolume.Thedivergence
Ofx(ωforq=2kFandT→OcausesthegiantKohnanomaly.Thepeierls
transitionisconsideredtooccuratthetemperatureTpwhereΩq=0.BelowTP廿leIatticedist0武ionwith2kFisfrozenoutleadingtotheonsetofa血Xedwaveof
thepenodiclatticedistortionandtheelectronicdensitywave.1tisca11eda
Charge・densitywave(CDX刃).usua11yx・rayandtheneuuonscattedngsprobethe

FigⅡre20

T11ree・dimensionalrepresentationoftheFer血Sudacesand血eirnestingsin
eachdimension.

2k

IatticecomponentofcDW.
ThepeierlstransitionandthegiantKohnanomalyareddvenbythedivergence

makesonlythek・statesonthetwolinesshowninthe丘guresatisfythe
divetgence Condition.AcontactbetweentheoriginalFer血Surfaceandthe
Shifted One,血e"nesting",d0血natesthedivergenceofx(ω.Thenestingis

ーカ.1畢輩部"エ'一罫邸肘Ofthepoladzationfunctionx(のforq=2kF.conditionsforthedivergenceare
Understood加ωitivelyintermsofthe"nesting"oftheFermisurface:F喰Ure20
Showsthet11reedimensionalrepresentationoftheFenmsurfacesofone・and
two、dimensionalelectronsystems.Thedivergenceofx(ωatT=oisconsidered

Perfectinone・dimension.

ApplyingtheconceptofthepeierlstransitiontoTTF‑1CNQ2kFisevaluatedas0295b*.1tleadstothebandnⅡingupt029.5屍)ofthe丘rstBdⅡOuinzone.The
averagevalencesofTTFandTCNQareexpectedas+0.59and‑0.59,
tespectively.Thisisved丘edbyx・rayphotoelectronspectroscopy(XPS)[6U.

tobeascribedtothecondition,Ξk.q‑Ek=oandf(五k)‑f(Ek+ω=0.1tls

SatiS6edforkontheFer加Surface(solidcurve)whichliesalsoontheFen皿

Surface(brokencuNe)shiftedbyq.1none・dimensionaⅡk・statesontheFemi
Surfacecontdbutetothedivergenceforq=2kF.1ntwo・dimensionanyqlesS血卸

︑1

ノ
ノ
ノ

ぐ0

凡VavenumberQ6(b*)

TheupperpanelofFig.15Showstemperaturedependenceofthetransverse
eomponentofthewavevectorΩofcDW'.Genera11ythetransversecomponents

し

゛

r

289

AdναπCesiπSyπtheticMetals

1πtιIplayofstrudura1απdΞlectr0πicproPιrtiιS。fcDXV,swavevectoraredominatedbythegoodnestingconditionfortheFer血V(r) vrmakestwoeleC廿onsapart丘omeach0壮ler.Thiscoulombenergy
Competeswith血eMneticenergyofeleC廿ons.TheNfore,theratiov/wisakey

interchaininteractionbetweencD圦10neachmolecularstack.1nTTF‑TCNQthe
interchaincoulombinteractionenergyisminimizedbythephasedi丘erenceofT

ParalnetertodiscusstheeleC廿onicpropenies,wherevdenotestheorderofΩintherange53K49KbecausecDwisconsideredtodeveloponlyonthe
TCNQstacks.ThesuccessivetransitioninΩ。血therange49K‑38Kis

The electronswiⅡforma"crystal"inwhichtheyarearrangedwithequal
distance. Thisisca11edtheX和ignercrystal.1none・dimension壮ledistance288

Sudace.1nmanycD圦Isystems,however,theyareexplainedintemlsofthe

magnitudeofthecoulombinteractionevaluatedfor,forexample,r the

betweentwoneighboringcD圦1,P。COSΩχandρOCOS(Ωχ+訂),1eadingtothe
tw。̲f。1dsuperstNcturepara11eltothec・axis,Ω'=0.5C*.Thishasalsoexplained

molecularsize.
Forv/Wシ1electronsarelocalizedand壮lemeta11icconductionisforbidden.

exlainedbytaMngaccountoftheonsetofanothercD圦10ntheTTFstacks

betweenneighboringelectronsisgivenas ν4kFbecause山edistanceiSνπand
kF=打π/2.
Inthedi丘Usex・raymeasurementsofTTF‑TCNQthesuperlatticereaection
Wasfoundwiththewavenumber4kF=0.59h*[56].1tisobservedevenatroom
temperatureandsuggeststheabsenceoftheinterchainconelationabove49K.A
SetofsuperlatticereaectionwasfoundbeloW49Ksuggesting血eformationof
anorderedstructureofthree‑dimension.Thissuperstructureisascnbedtothe
moleculardisplacementcausedbythexvignercrystalofelectronst11roughthe
electron・1atticeinteraction[67].The4kFstNctureisconsideredtobeformed

beloW49K[62].

CD圦Icanbeat釦electriccunent研,hilethesystemisinsulatingbelowTpinthe

Sense。fthesingleparticletransport.ThecurrentiscaniedbyacDxvslidingin
thelatticewithnorestodngforceatl=oif2kFisinc0如nensuntewiththe

Underlyingreciproca11attice.1nrealmaterialsimpuritiesorlatticeeects
1

h
f

h

t

1
b

interactwiththecDwleadingtovariousphenomenasuchasthenonline町
transport,atypeofmode・10CMngetc.163]However, WewiⅡleavethese
1

t

.1

n

.1

.1
.1

t

TCNQstacks.Thisissuggested[681bydetailedanalysesoftheresultsofx・ray,tequal‑ー

neutron,EPRandNMRmeasurements.

Xuhenthelong‑rangecoulombinteractionissmaⅡ,onecantakeaccountof
Onlytheon・sitecoulombinteractionenergyubetweentwoelectronsona
molecule.ThiselectronicsystemisdescribedintermsoftheHubbardmodel.
Theoreticalstudieshaveshownthatforu/Wシ1theelectronsundergotheMott杠ansitionwhichdoesnotnecessadlyinvolvea11ystructuralchanges.The
electronsarelocalizedwithequaldistance.Theyareapparentlythesameasthe圦lignercrystaldescdbedabove.1tisshownthattheMotttransitioniseasyto
Occurwhenthechargedensiwisvmoleculeorvsite.

AdngX入11一a.a.N.ーー

.1

det

0︒esnsMe址

tyowsevdf

nstsp・10n

d0

ec.1ntu

ten.ー
g
soR1

‑b
es
pg
nu
s
e

則Wル血1iー)001

f︒t1ー.ト0

船Cdnu
1.
roe
he
イe
ー

SU︺ann

1TS
.]
.

t

1

r.

e

^ー.r1t^earcsae

n

S

.1

t
rnh
nt.1aaS.1.1n.1Cgt

・1e
yaneh

Suobao.]eof.1Creaoan1.^ystagC1on1.れ1ba1tentetes1ntyohf

1

.1

0

X
D
S
f

、!「

.1

﹃.﹂

1「

1

e

.1
W
D

acc。maniedbtheonsetofspin・densitywaves(SDW)withoutanyst「亀.

Pred0血nantlyontheTI'Fstacks.The2kFsuperstructureisra壮lerascribedto

μBbecauseofthebandod部n.

2.3.spiπPeierlSかαπSiti0π

ιh1

Recentlthecoexistenceof壮le2kFCD圦IwithsDW'hasbeenfoundbya

L1ーミー^︑'引tr︑ー'.

di丘Usex̲rayscattedngSωdyof(TMTSF)2PF6[65].Thishasbeenascdbedtoa
UrelyelectroniccD圦linvolvingnolatticedistonion.1ntheconventionalmoe。fsD圦lthechargedensityshouldbeun辻orm.veryrecentlyatheoryas

SucceededtoexplainthecoexistenceofthepurelyelectroniC2kFCDwwi

SDwintermsofthenext、nearest・neighborcoulombinteractionbetween

electr。ns[661.1tiS血terestingt06ndthecoexistencealsoinothermaterails・﹂ーーーーーーーーー,

2.2.4kFstructure

Thebandmodelemployedinthelastsectiondidnottakeaccountofthecou゜minteractionenergybetweenconductionelectronsv(r)、刃hererdenotestedistancebetweentwoelectrons.Thelong・rangecoulombinteractioni

U圦小en壮lestrongcoulombrepulsionworksbetweenelectrons,theelectronsare
tegardedaslocalizedparticles.1none・dimensionthelocalizedsP加Scanundergo
amagnetictransitioneithertoananti‑fenomagneticstateorasP血Peierlsstate
血athasnospin[69].
Figure21Showstemperaturedependenceofelectdcalconductivityand
magneticsusceptibiliwofMEM(N・methyl・N・ethyl・morpholinium)・(TCNQ)2[7田.Atabout335Kitundergoesametal・insulatortransitionaccompaniedbythe
Onsetofatwo・foldsuperstNctureandatemperaturedependentmagnetic
Susceptibilitycharacteristicoflocalizedmoments.1tisconsideredasdepictedin
Fig.22(a)thatadimedzedTCNQacceptsanelectronlocalizedby,forexample,
theMotttransitionorthewignercrysta11ization.ThesolidcuNeshowninF地.
?1(b)denotesthetheoreticalpredictionforthemagneticsusceptibilityofaone・

11AdναπCesmsy11theticuetals

1πte1つlayofstr互dura1απdΞlectr0πicproperties

弌CMQI/21
づS、・・・ー

L

.

.

MEM
.

上IS、ーー・N2
Fig11re22SchematicviewofthemetaⅡic(τ>340K),4kFcharge・10calized(19くTく340K)andthe2k゛pin・peierls(Tく19K)statesofMEM‑TCNQ.

町{LEU.Sヒ一Φ0"

イCNO 1/21
̲工S、ー・"

0

σ

0

+

dimensionalchainofHeisenbergspinswhosespatialpositionsarehxed.Thisis。+

Ca11edtheBonner‑Fisherwpesusceptibility[7U.BeloW19K,however,the0+magneticsusceptibilitydecreasesrapidlywithdecreasingtemperaturewhilethe
electricalresistanceshowsnospecincchange.stNcturalmeasurementsved丘edtheonsetofafour・foldsuperstNCωrebelowthistemperaωre[721.Thissuggests
tw010calizedelectronsformapairwhosetotalspiniszeroasdepicted血F地.22.
Thismagnetictransitioninvolvingthelatticedist6nionisca11ed血e SP加Peierls

0
0

11

0
0
0

1

0

0

00

0

1
1

0

0

0

0

0

3.8

3.ム

3.0

26

/T(K.)
日ONNER・F15HER
̲̲̲S月N・Fモ1ERLS̲̲CURIE・WEISS
2、01・・'、 0住ansition.Thethree・dimensionalnatureofarealsystemcan IeadtoanotherState,ananti・fenomagneticstate,depend血gonthemagnitude Oftheinterchaininteraction[73].problemsofthespinpeierlstransition havebeenstudiedin
detailalsoinothermatedalssuchasTTF‑MX4C4(CF3)4 [69].
Xん7henmagnitudeoftheelectron・electroncoulombinteractionincreases,theSystemisexpectedtoshowachangefromthesD圦ldominatedstatetothespin・Peierlsone.Thiswasvedhed血anothersystem,(TMTTF(tetramethyltetra・

FONERBA1メNCE.ESRJ̲
OESRι

thiafulvalene))2Xand(TMTSF)2×[74].Theformermatedalshavethenarrower
bandwidththanthelatter.Thismeansthattheroleofcoulomb血teractionis

ーーーー}ー

一山ざΣ3Σ山[,0こX

1 ,1 、

4/21

工

̲工S‑・・、'

ODC、CONDUCTIVITYUI}+MVJ・CONDUCTIVITY^^^^^^^^^^^^^

.

.

.

﹁1ーーーーーー﹂

MEM

丁く19K

19くTく340K
一、允N9‑゛

291

工T>340K

+2

工 工

1千1主ネ

290

moreimponantintheformersystemthaninthelatter.Theformersystemis
Expectedtohavethespin・peierlsstateandthelatteronethesDxvstate.
SystematicstudieshaveproposedthephasediagramshowninFig.23[74].
、^^、、、』、̲、』^^

2.4.Aπi0πorderiπgFigure21Temperaturedependenceofthedcelectdcalconductivityandthestatlc

Iheprecedingthreesectionsdescnbedthesuucturalchangeswhoseddving
forceisthelowdimensionalnatureoftheelectronsystem.sometimesst川Ctural
Changesoccurratherindependentofelectronicproperties.oneofsuchexamples
IS壮leanionordedngtransitionobseNedin(TMTSF)2CI04.
1、1Theanioncl04islocatedatthecenterofinversionsy11血etryinthecrystalof

0πe、Di1πeπSi0παIC011dιιdorS11ed.S.Barisicetal.,(springer,Ber1血,1979)P45.Figs・1'(TMTSF)2CI04althoughtheanionitselfhasthetetragonalshapewithno

1t

0

100

丁{K.}

200

300

ι00

magneticsusceptibilityofMEM・(TCNQ)2.(Reproducedfroms.Huizingaetal.,@如Si
7and8)

292

Adνα11CesiπSyπthιticuetals

1πtelP1αyoj'strιιdιιra1απdΞlectr0πicproperlies︑欝1、quenchinge丘ectin thestructuralchangegivesadirectexplanadonforthese
Phenomena:the impedectordedngofcl04SuppressedtheonsetoftheSuperconductivity.

、、(語化己く缶生W

100

,、
、、
、
、、

Cし

、、

CONDUCTOR

、

、、

293

Therelationbetweentheorientationalorderingandtheonsetofthe、.、Superconductivity Orthespin・densiwwaveisconjecturedasthef0ⅡOwing:The
Odentational Ordedngisexpectedtocause6rstthereductionofthelattice10

Parameter Presumablyduetotheonentationalordenngofthenon、sphericalSPanion,andsecondthere・constructionoftheBriⅡOuinzonebecauseoftheunit̲

SDW

SC
1一』一ーーー→

PRESSURE

5kbor
︑0‑U,(︑切卜Ξ)

占,(︑仁Ξ)゛︑0み(ヒm0↑0亘゛﹂Q,(︑仁Ξ)

0一
.一

U1

Figure23Schematicphasediag皿mofthe(TMTIETMTSF)2Xsystem.(Reproduced
丘omD.Jerome,organicconductors,Ed.J・P.Farge(MarcelDekker,1994)P.420,H.

Inverslonsymmetry.undert11iscircumstancetheanionscanundergoan
Odentationalorderingt11roughtheanion‑anioninteractionpresumablyvia

Ce11doublingalongtheb・axis.The6rste丘ectisrea11yved丘edbythelatticeParametermeasurements[78].Thereductionintheintennoleculardistancealong
小eh・axisisexpectedtoincreasetheconductionbandwidthalongthisdirection.
ItwiⅡleadtothesuppressionoftheFen11isurfacenestingbecauseoftheincrease
inthedegreeofwarpingofthequasione・dimensionalFermisurface.Thiswi11

Suppressthespin・densitywaveandleadtot11esuperconductivity.Thesecond
e丘ectgivesthetwo・foldFer血Surfaceinthere・constructed丘rstBd110uinzone.

Alsoforthiscasethenestingisexpectedtobesuppressedbecauseofthe
Presenceofmanynestingvectors.whenasampleisquenched,thespin、densiw
Waveappearsbelowabout6K.Thedisorderduetothequenchingseemstogive
Ⅱttlee丘ectt0血eonsetofthespin・densitywaves.

TheonentationalordednghasalsobeenfoundinotherTMTSFcompounds

havingasymmetricanions,suchasRe04,N03,FS03[76].1nthesematerialsthe

One・dimensionalaxis,α・axisisdoubledbytheodentationalordedng.Therefore
由eanionordenngcausesametal・insulatortransitionbecausethebandgapatthe

Bd110uinzoneboundaryopensjustattheFer111ilevel.

TMTSFmolecules.

Di貸Usex・raystudiesrevealedtheonsetofasuperstNcturein(TMTSF)CI0

duetotheorderingofcl04atabout24K[75].Figure24Showsschematica11y'壷一一t

theordedngpattemofcl04intheunitceⅡ[761.Theunitce11isdoubledalong
theb・axis.1twasfoundthattheordedngtransitiontookaratherlongtimetoComplete.Theordedngbecomesimperfectifthesampleisquenchedintherange「'

30K‑20KwiththespeedoftheorderoflK/m血orfaster[7刀.1thadbe伽
expehencedthatthesamplehadtobeslowlycooledtorealizethe Super・Conductivity.otherwisethespin・densiwwaveappeaNd.The丘ndingofthe
.

1

d

^

d2.
ーー︑門{牛1r.即︑.1井1王二Figure24Schematicviewoftheorientationalorderingofcl04in(TMTSF)2CI04.

2.5.1πte,P1αyα机0πgναガ0US抗echαπiS抗SThequasione・dimensionalmolecularconductor(RIR2DCNQI)2Cushowsnovel
electronicpropeniesdiscussed血thelastsection.xvith血Creasingpressurethe

meta11icstatebecomesunstableundergoingametal、insulatortransition.1n
additionthesystemundergoesametal・insulator・metalre、enuanttransitionwith

10wedngtemperatureinanarrowpressurerangeaboveacdticalpressureof
abouto.1‑0.5kbarao‑50Mpa)79].

Extensivestudieshaverevealedthemechanismforthesenovelproperties.1tis
血einterplayamongmanymechanisms;thepeierlS血Stabilitycharacteristicof
Pne・dimensionalsystem,theMotttransition血Coulombinteractingsystem,theInlxedvalenceofcu,theJahn・Te11ere丘ectofcU2゛ions,andthecude̲xveiss

Paramagnetismoftheν2・spinofcU2゛.Thekeyroleinshowingthevarietyof
Prope丘ieswasplayedbythestNcturaldefonnationwhichcausesashiftinthe
energylevelofd・electronsofcu.

、.Thed・1ikebandispa丘iaⅡynⅡedupt05/6.Thelr、1ikebandisoccupiedupto

1乃.Thepeierlsinstabilityinthequasione・dimensiona11「、1ikebandcausesa
Superstructurewiththepedodof3C.Thisreducesthe,6rstBdⅡOuinzonet0ν3

1「「,1

AdvaHcesiπSyπtheticuetals

1πtelP1αyqi'strudurα1απdΞlectr011icpropeπies

Oftheodginal.Theuppermostd・1ikebandbecomesjusthalfnⅡedand,according

Capacitywhilethatoftheinsulatingstate‑Tduet0血eentropyofparamagnetic
Spins.ThemetaⅡicstatecanhavethelower丘eeenergyifthepre・factorofeach

294

1

亙

toageneralNleoftheMotttransition,isunstablefortheMotttransitionleading
toachargeordedngwiththepedodof3C.Thusthetwomechanism,theMotttransitionandthepeierlsonecooperatetomakethesystem血SulatingwiththeSuperstNctureof3C.

Theabovescenarioforthemetal・insulatortransitionisvedhedbyx・ray
diffractionstudies:1nthehightemperatureregimeofmeta11icstatetheN‑CU‑N
bondangleawasfoundtoincreasewithdecreasingtemperature[8田.This
Suggestsanup・shiftoftheuppermostd・1evelofcuasexperiencedintheJahn‑
1一1

TeⅡere丘ect.ThisisexpectedtoincreasethequantityofcU2゛1eadingtotheincreaseoftheaveragevalencetoward+4/3,whichispreferabletothemetal、
血Sulatortransitiondiscussedabove.

Intheinsulatingstateathree・foldsuperstNctureisfoundbyx・ray
measurementsasshowninFig.25[8U.Alsoastep・wiseincreaseintheanglea
isfoundatthetransitiontemperature[80,82].Thissuggeststhatthemetal、
insulatortransitionwithincreasingpressureordecreasingtemperatureis

tdggeredbytheincreaseoftheone・dimensionalitythatstimulatesb0壮ltheMott
andthepeierlsuansitions[83].The血Creaseinthetransitiontemperaturewith

1f凹1ー

t"

increasingpressuresuggeststhemorecompressionofthec・axisthanthea.1tis
Consistentwiththelargerthermalcontractioninthec・axis.
Inthere、entranttransitionh'omtheinsulatingtothemeta11icstatewith
decreasingtemperaturejusttherestorationtothemeta11icstatehasbeenfoundinelectdcal,magneticandstructuralpropeπies.1tisnotsU印nsingtohavethere、
entrant廿ansitionbecauseofthefreeenergyof壮lesystem.Themeta11icstate'S
freeenergyisexpectedtobeproportionalto‑Tbecauseoftheelectronicheat295

energytermsatiS負esace此aincondition[84].
3.1heoreticaloverview

3.1.1πかoduCガ0π

Ashasbeenintroducedinsectionsland20fthischapterandinotherchapters,
mevarietyofeleC廿onicpropeniesofmolecularsolidcanhaveandsofarhave
beenrealizedisverybroad.Thisisanothermanifestationoftheunpredictable

Possibilitymaterialsarounduscanhave.W'hileitisimpodantanddecisivefor
由efuturedevelopmenttopursuesuchadiversitybasedontheexpedmental
Studiesoneachsystem,thesearchforsystematicsofmaterialpropertieson壮le血eoreticalgroundisnecessaryfort11eestablishmentofnewconcepts,whichwi11
inωmbetransfenedbackt0壮leexpenmentalactivities.Thissectionisintended
toreviewrecenttheoreticalstudiesai加ng atpossibleclassincationS肌d
加nstNctionsofunifyingviewonthesematerials.
Thetheoreticalconsiderationsarebasedonthefactthatinthesemolecular

Crystalsthemolecularorbitalsaregoodbasisforthedescriptionsoflowenergy
excitations.ACωaⅡytheextendedHnckelwpeof、thetight・b血din容approxima、
廿onforthebandstNcturecalculationsbasedonthesemolecularorbitalswork

叉eryweⅡashasbeencon6rmedby壮leangulardependentmagnetoresistance(AMR)(seeseCⅡonD.Moreoveritistobenotedthattheresultsoftherecent

6rstprinciplebandstructurecalculationson(DCNQD2CU[85]agreewiththedeHaas‑vanAlphenexperiment[86].suchextendedHuckelapproximations,however,failincaseswheresomeMndsofphasetransitionoccura11dtheground;tatearen010ngernormalbuthassomeMndoflongrangeorder.suchstates
IncludethepeierlsorsP血Peierls(SP)state,antifenomagnetismincludingspin

11
tι

1

4ensiwwave(SDXN')andsuperconductivity(SC).Thecausesofsuchlongrange
Ordersaretheelectron‑phononinteractionandthemUωalcoulombinteractions

1

ダー.'L1"ーーt

1

betweeneleC廿ons,theformerofwhichisnowweⅡknowntoleadtothepeierls

Statewithchargedensitywave(CDXV).
Tlnthef0ⅡOwingthereviewiS部Venonthe血eoreticalSωdiesonthee丘ectsof
mutualcoulombinteractionsundertheassumptionthattheelectronicstateat
eachsiteisproperlyNpresentedbyonemolecularorbital.ThentheHa血ltonian
MⅡbeexpressedasf0110WS,"*Σ'.・、0・゛0Σリけ・Σ"・加・.
1,ノ

eretiisthetransfer血tegralsbetweenmolecules,iandj,a11duand
Slteandintersitecoulombinteractions,respectively,and
=

$

V'α,

1

ι,ノ

J

Figure25×・raymonochromatic Lauepattemof(Mecl‑DCNQD2Cuatabout20K・
Thehodzontaldirectionispara11el totheone、dimensionalc・axis.(ReproducedfromR・Moret,synth.Met.27(1988)B301,Fig.1(B))

areon‑

aband

川「

296

AdναπCesiπSyπtheiicUιtals

π,=π'↑+11d.1ntheaCωaltreatmentofthecoulombinteractionstheHartree‑Fock(HF)approximationwiⅡbeemployed,whereuandy.tennsareapproximatedasf0ⅡOWS,Uπn<π'↓>→UπKπΠ↓>+Uπ'↓くπΠ)‑UくπΠ<1し↓>,V"π'凡→V,πK11j>+vii凡くπD‑V'πⅨ川ThisHFapproximationisthedurableandsystematicproceduresforthesearch
Ofthepossiblegroundstates.(Theaboveapproximationtothevtermisthe
Hartreeapproximation,butweusethetermHFbelowforsimplicity.)1tshould
bekeptinmind,however,thatthequantitativeaspectsofresultsofthis
approximationshouldnotbetakenliteraⅡybutthattheresultswi11beabasisfor
thefurtherdetailedtheoreticalstudies.

3.2.1hガ0usgr0鹿πdstatessojarrealiZιdiπorgαπicc,ystals

1πtelP1αyof'SかUcturα1α11dΞleCか0πicproperlies

297

3.2.4.chargedisproporti0παti0πイCD)orchar8eorderiπgrcoj
Themeta11icstatesinmolecularcrystalsarerealized6rstofaⅡbythecharge杠ansfer(CT)betweensegregateddonorsandacceptors.Theactualconduction
Canbealongeitherdonorsoracceptors:inthecaseofET2×,e.g.onETdonors.
血血epresenceofseveralinequivalentsitesinaunitce11,somesite(S)canhaveadi廿erentamountofcatderdensiwfromtheothersite(S),whichistermedas
CDorco.
3.2.5.uotti加U1αtors

In血epresenceofstrongon・sitecoulombrepulsive血teraction,U,(compared
Withthekineticenergytypica11yrepNsentedbythebandwidth,XV)athalf丘11ing
innon・degenerateband,i.e.ifthetotalnumberofelectronslvisequaltothe
totalnumberoflatticesites,N,electronsarelocalizedateachlatticesitetoavoid

3.2.1.peierlsstate血ecoulombrepulsion.ThisistheMottinsulator.1nthisstateoneBohr

ThisisthestatethathasbeennrstrealizedinTTF‑TCNQandtnggeredthe
Scientincactivitytowardthemolecularcrystals[871.Duetotheelectron‑phonon
interactionandstrongnestingoftheone・dimensionalityofthelr、bandthe
Origina11atticegetsunstabletowardtotheformationofthesuperlatticewiththemagnetonofspinispresentineachsitetostartwithasshowninFig.26.such
acasequiteoftenresultsinmagnetica11yorderedstate,e.g.N6elorderedstatebecauseofthesuperexchangeinteractions,ノ',betweenspins.suchmagnetic
mteractionprocesses,whichgovemthelowenergyexcitations,areproperly

Penodicityof2kRkFbeingtheFermimomenωm[88].1nthisstatethecDwwith
Samewavevector,2kRisestablishedandtheenergygapiscreatedattheFen11i

descdbedbytheHeisenbergsP加model.NotethatinthisMott血Sulatotthe

Chargeexcitationsarepusheduptotheregionoftheveryhighenergy(oftheenergyofelectronsasinord血aryinsulatorsandthenspinsusceptibilityx,isSuppressed:i.e.thepeierlsstateisnon・magnetic.Theconductivityatlow
frequencies,however,survivesbecauseoftheslidingdegreeoffreedomofthe

Orderofの.HencespinandchargeareseparatedinlheMottinsulatorS血thelow
卸ergyreglon. Thisis Ca11edthespin・chargeseparation.

PhaseofcDW,i.e.phasons,especia11y廿2kFisinc011血ensuratewiththe

3.2.6.wigπersolidイWS)

reciproca11atticevector[89].1nthestaticli血t,however,theconductivity
VanishesbecauseofthepinningofcD圦lbyimpurities[90].Thecom、
mensurabilityalsoactsasapinn血gmechanism.
3.2.2.SDwstate霞'︻ー︑i1

Thisstate,whichhasaspatia11ypedodicmagnetizationwithwavevector2kis

realized血thepresenceofhighdegreeofthenestingoftheFen11isudaceonone
handandthesh0π一rangedrepulsiveinteractionsbetweenelectronsonthe0血er

1,【

」1!11

Becauseoftheli血teddimensionalityandanisotropyofthesystems,壮leSy11血etryofscorderparametersisnotalwaysthesameasthatofBcsbutca11
beanisotropicwithlinenodes[92].

υ.Typica11ythosebetweentheneighboringsites,vplayparticularrolesat
quarter丘11ingasiseasilyunderstoodasinFig.27.Thisstatecanbeconsidered

asaMndofcostateandisca11edthexvignersolidespecia11ywhentheelectrons
笹elocalized血regularinterval.

,無3.2.3.S1ιPerc011ducti11gイSC)state

、,

1十j

[9U.ThemagneticpropeniesofthisstateisessentiaⅡythesameasthoseofNeel
State,butwithsma11eramountofthemagnetization,whilethetransp0丘
Propertiesareingeneralsi血lartothoseofcDxvstate,i.e.insulating.

血non・degeneratenarrowbandsthescreeningofcoulombinteractionisnot
necessarilystrongandt11enthelongerrangedpanofthecoulombintel'actionv.
Could血Pnncipleplayimportantrolesbesidestheon・sitecoulombinteraction

1曹Ure26

It↓1↓
AschematicviewoftheMottinsulatin菖State.circlesandanowsTepresent
Sitesandelectronspins,respectively.

舮1"

1πtιIP1αyql'structura1απdΞlectr0πicproperties

Adva,1CesiπSyπtheticUιtals

298

FigⅡre27Aschematicviewof仕lewsstate
3.2.フ.Aπtiferro"1αgπeticlVどe1イAF)statι

Q5

、
Y

EF

C
V

元＼>小﹂d. C 山

10↓0↑0299

S血Cethesuperexchangeinteraction,j,血theMottinsulatorswithnon・

玲

0
r

X

‑Q5

degenerateorbitalsisantifenomagneticbetweenneighboringsites,theN6elstateisusua11ythegroundstatewithmagneticmomentclosetooneB011rmagnetonon
eachsitewithpossiblereductionsduetothequantumauctuationsintdnsict010W
dimensionalityThemagneticfNstrationsalsocausesuchreductionofspins,and
insomecases,totaldestNctionofthemagnetlcmoments.

11

r1

[」

.}ル!

F地UN29

CVY

Band

「

XV

StNctureandFen11isurfaceof(TMTrF)2Xcalculatedwithjnthe
extendedHuckelapproximation.(takenfromref.1の3.2.8.spiπ・gappedイSG)state

3.3.イTUTCF)2X

EvenintheMottinsulators,wherelocalizedspinsarepresentateachsite,non・

TMTCFdenotesTMTTForTMTSFmolecule.Theamountofchargetransferin

magneticgroundstatesaresometimesrealizedwherethespinsusceptibilityis,1」「

Vanishingtowardabsolutezero.Anexampleistheformationofthesingletstaterealizedbythedimenzations.Thefrustrationcanalsoleadtothenon、magnetic
groundstate.

血isfa血lywithone・dimensionalstructuresissuchTMTCFυ2゛andxl‑.Theb即dstructureandtheFer血SurfaceareopenasshowninFig.29forthecase
Of(TMTTF).X[941.Thephasediagramofthisfa血lyontheplaneofpressureφ)andtemperature(乃isshowninFig.30proposedbyJerome[95].Themajor

3.2.9.spiπ・peierlSイSP)state

T(K)Spins=ν2,thenon、magneticsingletgroundstateisrealizedbythealtemating
Superexchangeinteractioncausedt11rough壮ledimerizationsoflatticeasinthe

＼

＼

100

Inthef0110wingtheoreticaldiscussionsaremadeoneachcaseoftypicalexamplesoforganicconductorssofarstudiedindetail.
̲^^、、、'^'ーー"^^..^^^ー1.,1.,ゆ.
^^ー^Figure28Aschematicviewofthespstate Thewhitearrowsshowthelamce
dist0丘ion.

＼Tρ

Metal

＼

＼

＼

Peierlsstateasschematica11yshowninFig.28.Thisdimerizedstate,whichis

CaⅡedthespin、peierlsstate,istheoretica11ydescnbedasthepeielsstateof
Spinlessfen11ionsintroducedbytheJordan・wignertransfonnationofspins[93]・

P
王SS
PT

Insomeofquasionedimensionalantife寳omagneticHeisenbergsystemswith

10

＼

TN

C・AF

SDW

IC・SDW

TSC
SC

Pressure、M、1氏1"1町,'1(TMTTF)2Br(TMTSF)2CI04

町曹Ⅱre30phase diagramof(TMTCF).xproposedbyJerome[95].PI,SRC・AF,1C・

匂Dwandsc representparamagneticinsulating,spin‑peierls,commensuratean廿fenomagnetic, incommensuratespindensitywaveandsuperconductingphases,
respectively.Tdenotesthetemperaturewheret11eresistivityshowstheminimum

r一
300

AdναπCeS加Syπtheticuetals

1πterP1αyofSかUdιιralaπd五lectr0πicproPιrtiιS

e丘ectsofpressureinthiscaseareconsideredtobetheincreaseoftheinterchaintransferintegralsresultinginthemorewarpingoftheFermisurfacesononehand

1

1

2

2

ノ

1

301

2

2

andthereductionofthee丘ectivestrengthofcoulomb血teractionontheother.

InordertounderstandtheoveraⅡfeaturesofthisphasediagnm,itistobenoted

thatasseenfromFig29,thebandishalfn11edinsteadofthequaner・nⅡing
Whichisexpectedifa11TMTCFsareequivalent.Thishalf・n11ingisduetothe
existenceoftheweakdimerizationduetothealternationofthetransferintegrals

11
1J,ー .JJ

alongthechaindirection.Howeveritistobenotedthatthisdimedzationisvery
Weakcomparedwith壮lemutualcoulombinteractionsaswiⅡbeeasilyinfened
bythesma11nessoftheband、gapseeninFig.29,andthenhaH・n11血gnessofthis
Systemshouldbetakenwithcare.ActuaⅡytheoriginoftheinsulatingstateinthe

regionoflowpressurewiⅡbethew'sdrivenbytheintersitecoulomb:,一,''劃コtり'rJ一リ︑."哩﹂1M,t

interaction,V[961,ratherthantheMottinsulatorsexpectedforcasewithone
C町ierpersite.onceC卸iers(holesinthiscase)getlocalizedandthesystemis
insulatingandthenthereexistlocalizedspins.Theirspatialpattemwi11again
reaectrelativeimportanceofthedimerizations:inthewsthespinswiths=ν2
Wi11bepresenuneveryothersite,whilespins=ν2Wi11beonthedimersifthe
dimedzationisstrong.1nthepresenceofbothvanddimerizationthe

吉:.J1

11

1
1

bytheresultsofHFcalculationsasshowninFig.31[97].Theantifenomagnetic
Pattemobserved血SomeTMTTFcompounds[98]isactuaⅡyofthetypeshown

inthiS丘gure,andthepartialcohasbeenexpedmenta11yobservedindi丘erent
butsi加1雛Systems(DI‑DCNQD.Ag[99].Thespindegreeoffreedominsucha
dbh'、、dnsionalantiferroma・

neticHeisenbergmodelwiths=ν2工Vithboththecouplingtothelattice,入,andinterchainsuperexchangeinteractionノ↓. Iftheformerisdominantthespstateis
realizedbutasthelattergetsimponant,e.g.bytheexternalpressure,itis
believedthattherewi11bea6rstorderphasetransitionfromthespstatetothe

門"
1,.,1

"1・111
'i,N

"、1.1
゛、1^1

11i^11,

FigⅡre32

AschematicrepresentationoftheHFcalculationconespondingtothe2kF
CDwand2k,SDwcoexistingstate血(TMTSF)2PF6

Wherethereisonlyonebandwithhighdegreeofnesting.(Notethatthesystem
maystayinmetaⅡicstateifthereexistotherFerlmsurfacesleftasinthecases
insomeoftheheavyelectronsystems[10U.)AnotherinterestingfeaturefoundOnlyrecentlyisthecoexistenceof2k,CDwandsDwin(TMTSF)JF6[102].A
血eoreticalmodelhasbeenproposed[103]forthisunexpectedphenomenon,
Whichstressestheimportanceoflongrangecoulombinteractionnotonly
betweenthenearestneighbors,vbutalsothosebetweenthenextnearest
neighbors,V'.Theexpectedchargeandspindensitymodulationsareasshown血Fig.32.AsseeninFig.30,thescsetsinconc0血tantlywith山ec011apseofthe
SDwstate.Thereexist血dicadonsthatthesy11血etryoforderparameterofthis
ScstateisnotBcslikebutwithlineofnodes[104].1fthisisthecase,theCoulomb血teractionsleadingtosD圦Ishouldbeplayingessentialrolestothe
Stabilityofthisscstate.
ノ︑

1
1卜

intermediatesiωationsofthepartialcoarerealized,ashasbeendemonstnted

VV'V

3.4.伍τ).X

Afstate[100].onfurtherincreaseofthepressurethedegreeofthelocalization
WiⅡbereducedasdeducedfromthesuppressionofTpinFig.30,andtheground

Thisfa血lyhasmanypolywpeswithavanetyofgroundstate,someofwhich
Wi11bediscussedinthef0110wing.Thecrystalstructuresofthesearebasica11ythe
Salnei.e.composedoftwo・dimensionalE「1ayers,butspatialanangementsof
ETmoleculesinaunitce11aredi貸erentwhichresultindrastica11ydi丘erent
groundstatesasdescnbedbelow.

StateistransformedtosDxvstate.Thedi任erencebetweenAFstateandsDW
Statewi11becharactedzedasf0ⅡOWS:intheAFstatethelocalizationofthe

3.4.1.αイΞT).1,

ChargesetS血independentlyofthemagneticordehng,whilesDwstateemerges

OutofthemetaⅡicstatebutcanbeinsulatingoncesDX八Istatesetsin,becauseof

theopeningofthegapeverywhereovertheFermisudaceasinthepresentcase,1

2

1

Expedmenta11ythetemperaturedependenceofresistivityofthissystemhave
血terest血gfeatures,i.e.atambie址Pressuretheresistivitysharplynsesataround
50KasTisloweredbutwithapparendy丘niteli111itingvaluetowardsT=0(i.e.
五mteconductivity),whileatp=12kbarthetemperaturedependenceofthe
tesistivityismetaⅡicto、vardsT=0[105].1nthealmostinsulatingstateatlow
tb'h'bld'+δ一δ+δ一δ

Figure31AschematicrepresentationoftheHFresultfor(TMTCF)2×.Transfer

integralsalongtheTMTCFchains,tlandt2,andtheon・siteandintersitecoulom
energies,uandvareindicated,whileδdenotesthecoandthelengthofthea杠OWS
Schematica11yrepresenttheamountofmagneticmomentonthec0杠espondingsite'

廿OpicaⅡy血dicatingnon・magneticgroundstate[106].ThispolywpehaS4ETS加aunitceⅡ,whosespatia1町angementisshown血Fig.33SchematicaⅡywheN
eachcirclerepresentsETmoleculeandthevaluesoftheoverlaP血tegrals

beNeenthesameHOMoorbitalsofneighbodngmoleculesareindicated.The
tesultingbandstNCωresandFer加Surfacesareshown血Fig.34.Asseen,aⅡof

Adνα11CesiπSyπtheticuetals

302

1πie1ア1αyQf'Sかιιdιιrα1απdElectr0πicproPιrtiιS

303

1

0.8

b4

b

3

」

3

1

a3

3

b

2

a1
11

11

L。

1

b

4

Stackl3.O×10、3(>Φ)あーーー︑1一11玉

a1
a2
a3
b1
b2
b3
b4

a1

2

4.9
・1.8
・12.3
・14.2
・62
2.3

0.フ

0.6

0.5

0.4

a2

0.3

Ⅱ

0

'︑q号

Figure33AschematicviewoftheS註'uctureoftheETplaneina・(EI)213.

thefourb釦dsareweaklyseparatedandtheFen11ilevelc0丘espondingt0壮le

0.04

0・01

CⅡ(N(CN)ユ】氏【N(CN)ユ】CI

CU(NCS)ユ

thetransferintegrals.TheHFcalculationswithonlyupredict廿lephasediagram
asshowninFig.35[10刀,wheretheabscissaisthevalueofthetransferintegral

CausedbycTbetweencolU血IS.Hencethecomparisonbetween血eoreucal
Predictionsandexpedmentalresultsmayleadtothepossibilityofthespground,(BEDT・1TF)2X
100
夕1

Parama部etic

壱毛ン1

仏j

ヘ巡&

[一゛ー

insulating(AH)statehasone・dimensionalarraysofspin(withclosetos=ν2)

0.14"gure35Thel(Fphasediagramofa・(ET)213・typestruCωreontheplaneoftMandu

thereca11bea丘niteoverlapbetweenthesetwobandsdepend血gonthechoiceof

]1
1

0.12

t嗣(ev)

α・(ET)1ET゛ν2islocatedroughlybetweentop・mostandthenexttopbands.1nrealittι4,whichtumsouttobe壮lekeyfactortodeterminethedegreeofbandoverla
ItiS血fenedthatthissystemislocatedinaverysubderegionoft11eparameters.
Besidethepossibilityofantifenomagneticmetals(AFM),t11eantifenomagnetic

0.060.080.1

Insulator

Par且τrla即et河1
MetalStateatambientpressureaccompanyingtheformationofthelocalizedspins

10alongthec・axiscausedbytheco.

IA.F.1nsulator

3.4.2.KイΞr).X

S!C

Thissystem,whichalsohaS4ETsinaunitce11exhibitsaphasediagramas
ShowninFig.36WithaninterestingfeaωreofhavingboundariesbetweenAFI
andsc[108].Thereexistindicationsthatthesy11血etryofthisscstateis

1(weakFeπ0.)介

0.4

Uofdw(orpressure)(ueff/＼V)crkical

イ

(>Φ)

0.2

Figure36>︑窃Φ﹂仁Φ

口

0

Y

、0.4

「

M

Figure34

Y

「

X

M

Ⅲ丘erentfromthatofBCS[109].ThelocationsofETmoleculesinaunitceⅡare

asinFig.37Wheretheoverlapintegralsbetweenmoleculesarealsoindicated.AS
Seen仕leoverlapintegralblislargerthananyothersimplyingtheformationsof

じ0.2

PhasediagramofK・(ET).xproposedbyKanoda[108]X

BandstructureandFer血Sudaceofa・(ET)213.

M

血edimerizationsbetweentwomoleculesconnectedbythisbl.Actua11y,asseen
ImFig.38thebandstructuresintheparamagneticstateissuchthatthereexists@clearband・gapbetweensecondandthirdbandsindicatingthathighertwoand

10wertwobandsareduetotheanti・bondingandbondingorbitalswithintheAdνα11CesiπSyπtheticuetals

304

1πtelP1αyqf'SかιιCt互rα1απd五leCか0πicproperties
︑︑

0.95

degreeofdimedzationrepresentedbybl.The1圷Phasediagramontheplaneof

0.9

'ー'^

antifenomagnetismofspins=ν20neachdimer.oncethee什ectivevalueofu
i.e.U/wisreducedbypressure,Fig.39PredictsthetransitionfromAFlto

、^

、.

0.85

Paramagneticmetals(PM),whichwiⅡbescasint11ecaseof(TMTCF)2×,Fig.
30.

0.8

.

＼

＼

一＼

thisblanduisshowninFig.39[11田.TheAFlinthiscaseissameasthe＼

dimers,respectively.Thecrucialparameter,then,charactedzingthesystemisthe

305

＼
、、

＼

AFI

AF

＼

U

、、
、、
、、

U。2
3.4.3.αイΞτ).UHgイSCN)'

0.75

ThespatialarrangementofETsinthissystemisshowninFig.40togetherwith
theoverlapintegralsleadingtotheextendedHuckelbandstructuresshownin
Fig.41.Asseenthissystemisintermediatebetweena・(ET)213andK・(ET)2×;ifthedegreeofdimerizationinK・(ET)2Xandthedegreeoftheband・overlaP血α、(ET)213arevaried,thissystemisrealized[11U.Thisisschematica11yshownin
Fig.42(a).Thisa丘ordsauni丘edviewonthesethreedi丘erentpolytypes.Atthe

P財
20

25A

30

Itl(×10'2)(ev)、'筒).0゛船)。
F地Ure39The11FphasediagramofK・(ET)2×・typestructureontheplaneofltland
U.

SametimethephasediagramexpectedfromthisHFcalculationsisshowninFig.
42(b)[11U.

1231234
CCCPPPP

4

P

2

゛

、

、

4

C3

2

25.フ×10、3

P3

3

10.5
11.4

d

P3

10.0
・1.フ

P

2

C3

f+゜
f+。

・2.9

%

・,、1,11

加

2

b2

bppqq

q2

3q

q

11

P

P1

4

、1.9×10、3
6.8
・1.1 ・1.4
・10.0
・9.フ
13.3
132

C2

Stacklq

Fi客Ure401AschematicviewofthestructureoftheETplaneina、(ET)2MHg(SCN)4Fi倉Ure37Aschematicviewofthest皿CωreoftheETplaneinK・(ET)2×.
0.6
ノ^、、

、、

0.6

、、

ノ

＼

、、

ノ

0.4

、、

0.2

02

「

Y

0

「

X

0

0.2

>励Φ﹂にΦ0tr'ー(>Φ)>励﹂Φ武Φ

(>Φ)

、

0.4

"ノ

0.4

ノ'
、、ー

.ノ

・0.4

'一

「

M

Figure38

Y

r

Z

M

BandstructureandFermisurfaceofK・(ET)2×.

M

す

11

Z

06

「

Fi套Ure41

M

X

「

Z

M

Z

BandS訂'uctureandFermisudaceofa、(ET)2MHg(SCN)4.

M

AdναπCesiπSy,1thιticuetals

K・(ET)2X

C

dimerizationltb11

bandwidth

AFa

鄭"詣
U
4^

゛

])/P

α

α

ノー1

"'"

D

(ET)2MHg(SCN)4(1)AF凡EU

Itb41(M゜tl、(ET)2X

S

r

0.274ev

tp・0.021ev

0.111ev

tq・0.040ev
t,0.034ev
t$0.049ev
tto.0065ev

FigⅡre43Aschematicviewofthe StructureoftheBETsplanein入、(BEIS)2Gax̲

bandoverlap

Uexpectedforthesesystems.Thesespinsarealignedantifenomagnetica11in

theHFapproximationaS血F電.44.HoweverthisHFapproximationinoresthequanωmauctuationsofthesespins,whichinsomecasesplaycNcialroles.such

e丘ectsofquantumauctaationswiⅡbeproperlyincorporatedbymappingthisSiωationtotheHeisenbergsP血Ha加ltonianwherethelocalizedS血Swith

dmerlzation

S=ν2areonthedimersa11dthesespinsare血teractingbythesuperexchaneinteractionsbetweenthem.1nthepresentcasethesesuperexcha11geinteractions

Itb11

1ι1

'N

.

q

P

S
bandwidth

1:

A

Z4‑Z.

P

紅eestimatedbyuseoftheodginaltransfer加tegralsbetweenmolecules.The
resultsareshowninFig.45.ThereexistthedlmedzationS血theresultant

Superexchangeinteractions,i.e./B=jc,togetherwithnon、negligible/.The.1゛^'工三じ・・

ーー.ーー
"NI"
.‑111、父.
11.シ̲0、、1111)

FiguN421heHF(a)andexpected(b)phasediagramforK・(ET)2×,α・(ET)21.a11da、(ET)2MHg(SCN)4・wpes.AF,andAf。aretheAForderedphasesfortheKanda・type
Stuctures,respectively.(1)and(M)implyinsulatingandmeta11icphases,respectively
Anowswi仕Ipdesignatethee任ectoftheappliedpressure
3.4.4.入イ召五TS)2X

Figure44Aschematicrepresentationof血eHFresultsfor入、(BETS)GaxY辛

J

＼

﹂

＼S

＼J

︑︑

＼

﹂一

←十

B

J

J

＼

＼t

＼

﹂一

＼J

J

C

JC

＼

＼S

︑︑J

SpaualarrangementofBETsmoleculesisschematica11yshowninHg43
togetherwiththetransferintegrals.Asseenthetransferintegral,tA,islargerth飢

十←

JB

←十

Thisfa血lywithx=GaxY4̲haS4BETsmoleculesinaunitce11asinthecase
Of(ET)2×.Expedmentshavedisclosedthatthephasediagramofthissystem
Sharesc011血onfeatureswitl)K・(ET)2Xbutwithremarkablediaerence,i.e.theinsulatingstatenexttothescstateisnon・magnetichereincontrasttotheAFI
StateinK・(ET)2×[112].Thiscanbeunderstoodtheoretica11yasf0ⅡOWS'The

十←

加

r

一δ十δ

(b)

B

2

0.080ev

P

W'・

Bt
C
tAt

a
P

bandoverlap

1柘41

P

t

P
P

A

307

P

UET

F

".ー

‑Fa

C

︑K1ー

(

︑)

.ー゛

q

﹁﹂︑ー

U

α・(ET)213

1πtιIplayql'strudιιrα1απdΞ1ιCtr0πicproperliιS(ET)2MHg(SCN)4

二

(a)

恒S

rン一1じじーじ1舮一ン*

306

OthersleadingtothepossiblefonnationofthedimersbetweenBETsland
1]!

BETS2.ActuaⅡytheHFcalculationsleadstotheantifenomagneticgroundstate,
WheremagnetizationonBETslandBETS2areparaⅡelbutantiparaⅡelt0血Ose
OnBETS3andBETS4[113].ThemagnitudeofsP血momentsisdependenton
UandthesumofthoseonBETslandBETS2isclosetos=ν2forthevalueof

11

恥倉Ⅱre45aE丘'ectivetwo・dimensionalHeisenbergmodelforlocalizedsP血Sinthedimer
modelfor入・(BETS)2Gax.Y4̲.

AdναπCesiπSyπtheticuetals

308

]1

CP

121234

3P
P
P1

1PP

m‑Nt

2

(2)

23
C
C14

P2

4.1×10‑2ev
2.9
11.5
8.2
14.4
9.1

L。

86

1

Phasediagramofsuchaspinsystem血two・dimensionhasbeenstudied[114],
WherethegroundstatesareeitherAFstateorsinglet'statewitha丘niteenergygap

ER・

DucrcR

120

壮ansfonnationtotheAFstatewiⅡbepossiblebysma11Perturbations.Especia11y

100

ahedralAngle(田(degree)

1

、夕
、夕

C辻

r︑︑.一一N.一一如.0‑・φ.0‑のの・寸.の

QO‑

U).0

ハ.ト

の.゛

の入

+十

LS
AT(汎

←十

P白
1N

←←←

forthespinexcitations,i.e.thesGstate.Fromthis,itwi11bearguedthat入、BETS
SystemislocatedveryclosetotheboundarybetweentheAFstateandsGstate
血elatterofwhichisthecandidatehere[113].1fthisisthecase,the

SPI
2

C2

F地Ure47AschematicviewoftheS加CtureoftheETplaneinθ、(ET).Rbzn(SCN)in
thehigh・temperature(a)andtheslowlycooledlow、tempeTature(b)phases.

嶋

4

1」、1

・9.4

C

rヨPidC側ing

140

2.4×10‑2ev

C

(b)

ME「刈̲

θ

309

L.

2

10

PP
C

PARA八IAa、1E「1CINSいLAT(汎

4

C

P

C

(こ・N(弓夏SN寸(ZUのさ.U一↑Nト廿ΞUの)仁N一↑N↑

86

F全)Φぢ一邸﹂Φ厶EΦ↑1y!
L1(= ) ・ N ( 石 遵 1 N

t
諾
.︑ヨ
声
t

ー﹂ー゛ー︑{,Ⅱう,"ー︑'︑ー!ー﹂;ーしー"γヒ"︑.一1ー",︑

1L

「1

P

SI^C側ing

2100NΞUの)φφ.0でり↑」.
」

TN.8︑乏一寸(ZUの)︑乏のりN﹄

1・

1πtιIP1αyQf'strιιdιιrα1απdΞ1ιCか011icproPιrties(a)︻仁N.8".Σ一寸(ZUU))"乏厶止N↑1、N(誘)奇岳.N︻N冒)Z一(弓)NδNr'、"、1,1・1

Transferlntegral(り(10"2ev)
Figure46Phasediagramforθ・typeETsaltsproposedbyMorietal.[10刀,asafunCⅡon
Ofa訂'ansferinthetransversedirection(りandthedihedralangleofdonorcolumns
酬套Ⅱre48〆1

Aschematicrepresentationof血eHFresultsforθ、(ET)2Rbzn(SCN)4.The
e11ipsesdenotethedimers.、

府"'11

1πteJア1αyqj'strudιιrα1α11dΞleCか011icproperties

AdvaπCesiπSyπtheticuetals

310

、ノ

ノ

ノ

、

、

/

inasthetemperatureisloweredslowlyasshowninFig.47andthegroundstateiS血Sulatingandnon・magnetic[11刀.Theresultsoft11eHFcalculationsforthis
Systemareshown血Fig48[118].AS血入・BETsthereexistsspins=V20neachdimer,whichwiⅡbedescdbedbyⅡleHeisenbergspinmodelwiththee丘ective
SuperexchangeinteractionsasshowninFig.49.Asseentheexistenceofthenon、
magneticgroundstateinthiscaseisobviouslynotduetothedimedzationsof
Superexchangeinteraction.1nstead,itwi11beduetothef皿Stration.Actua11ythe^JI

1J2 0.21JI

ーーJ3 0.19JI
ノ＼

/＼
ノ

＼
＼ノ

ノ

Figure49

ノ

ノ＼
＼

ノ

＼ノ

/
＼

ノ
＼ノ

PhasediagramofsuchfNstratedspinmodelsareexpectedtohaverichstructures

E丘ectivetwo、dimensionalHeisenbergmodelforlocalizedspinsinthedimeTmodelforθ・(ET)2Rbzn(SCN)'

asshowninFig.50[1191.Thissystemisexpectedbe血theshadedregion血the丘gure.

Recentresultsofsinlilarcalculationsfor(ET)2Xindudingbothintersiteand
Onsitecoulombinteraction[12Upredictthatthegroundstateofa・(ET)?13isa
Costatewiththe"stripe"pattemalongtheh・axis,ratherthanthatalongthec、
axisasshowninaninsetofFig.35deducedwhenonlyonsiteuistaken血toaccountononehand,andthatthegroundstateofθ・typeisalsocostatewiththe
Stripe"pattemalongt4,ratherthant11eMottinsulatingstateasshowninFig.
47(b)suggestedwhenonlyuisinduded.TheseindicatethatthecowiⅡbe
rathergeneralinthepresenceofcoulombinteractionof6niterangewhich

2DAFH(AFLRO)^

IDAFHch祉那σ3)

ノ

ノノ
ノ

,ノ、

Vac

θ・(ET)2Rbzn(SCN)4
一{＼角︻

ノ

SG

2
(MG)

,/
TLAFH(nosG)

IY,リ

,/
/
ノ

deservesfurtherdetailedstudies.

IDA則ChainSσ2)

3.5.S記"1"1α,ya,1dDiscussio,1S

SG

00
IDAFHch毎nSσ1)

311

Theresultsofrecenttheoreticalstudiessearchingforsystematicunderstandingof
血evadetyofgroundstatesrealizedin(TMTCF)2Xand(ET)2Xsaltsare
in廿oduced.BasedonthespindependentHartree‑Fockapproximationforthe
CoulombinteractiontogetherwiththeextendedHuckeltypeoft11eband
S杠'ucturecalculations,thekeyparameterscharactedzingeachfa血ly(polytype)笹eexuactedresultinginthecoherentundersta11din80ftheapparentlyunrelated
Wpesofthegroundstates.Thecalculationssofar,however,havebeenlilmtedto
Caseswhereonlyonemolecularorbital,eitherLUMoorHOMO,isrelevant.
ThereexistanotherinterestingcaseswheretheoverlapbetweenLUMoand
HOMoorbitalsappearstobeimportantasindmitcompounds[12田orthe
血Xingbetween訂・orbitalandd・orbitalplayscrucialrolesasin(DCNQI)2CU.1n
血esecasesonecanexpectdcherpossibilities,WMcharejustbeingexplored.

12(MG)vacM
J2/JI

Fi琴Ⅱre50Theschematicphasediagramof仕leHeisenbergspinmodeldescdbedinFig.
49.1tissy卿netncwithrespecttothedotdashedline.SG,nDAFH,TLAFH,AFLRoand
MGrepresentspingap,π、dimensionalantifenomagneticHeisenbergmodel,tdangU1紅IatticeantifenomagneticHeisenbergmodel,antifenomagneticlong・rangeorderandthe

Majumdat‑Ghoshpoint,respectivelyTheregionswhereaspingapisknowntoexistandWhereaspingapisexpectedtoexistareshownbythethicklinesandtheshaded紅ea,
respectively

thee丘ectsofsma11amountofche血Calimpudtiesareofinterest,ashasbeen
demonstratedin山ecaseofdisorderedspin・peierlssystemsn15].1nan0Ⅱler

memberofthisfamilywithx=FexY4πtheexuafactorsduetothelocalized
SpinsofFe3‑introduceanotherinterestinginterplaybetweenscandmagnetism,
WhoselmcroscopicmechanismhasnotyetbeenfUⅡyexplored[116].
3.4.5.θイ五T).X

ThisfalmlywhichhaS2ETsinaunitce11,hasaphasediagramasshowninHg

46[11刀.Abovea11,inthecaseofθ・(ET)2Rbzn(SCN)小Whichissensitivet0血e
thennalvariationsthedimedzationresultinginthedoublingoftheunitceⅡSets

詑

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RobertNlelvi11eNletzger

PhysicaB);H.seo,prepdnt,sub血ttedtoJ.phys.SOC.Jpn

DιPαπ1πeπtqf'che形istか,Uπiversi砂qi'A1αhα柳α,rιιSCα10osa,Aι35487‑0336,
USA

血memorium:pierLuigiNordioa936‑1998)andAaronN.Blocha942‑1995):
血eyhelpeduS丘ndtheway.

1.1ntroductionandEarlyHistory
Dudngthe丘rstsixdecadesofthetwentiethcenωryinterestinthesolid・state
Prope丘iesoforganicmoleculesandtheirionswaslimited.conventionalorganic
Solidshavecovalentche血Calbondswithineachmoleculea11donlyvander
WaalsorLondoninteractionsbetweenmolecules.1tisrelativelyunusualtonnd
Partialorcompleteformalchargesonconstituentcationsandanions,and血ereforecoulombinteractionsbetweenorganicmoleculesincrystals.
HowevermetaⅡicconductioninorganicsystemshadbeendreamtofasearly^

軍1911[U.untilthemid1960sthesedousresearchersoftheorganicsolidstate'werefew:HideoAkamatsu[2],Noels.BaylissB],Melvincalvin[4],DanielD.
Eley[5],HelmuthKainer[6],Aleksandrl.KitaigorodsMi[フ],JanKommandeur[8],YoshioMatsunaga[9],HardenM.McconneⅡΠ田,Albenszen地y6rgyinu,

翻入N.Terenin[12]andA.T.vartanyan[12].
Thelandmarkresults,whichprope11edlow・dimensionalorganicsystemstothe
部eatlevelofpresentinteresthavebeenamixofexperimentalbreakt11roughs

如dtheoreticalideas:a)superconductivity血Potassium・intercalatedgraphitea965)[13];317