Essay

談 話 室Interplay between magnetism andsuperconductivity in orthorhombic compounds
UCoGe, URhGe and UIrGeJiří Pospíšil

Research Group for Materials Physics for Heavy Element Systems

I came to JAEA from Charles University in Prague in 2014. My scientific field of interest is single crystals
growth and research of the magnetism and strongly correlated electron phenomena. I continued my work here
in JAEA and I would like to thank all my colleagues for outstanding support in my research. We have prepareda series of new actinide materials and gained results revealing fine details of interplay between ferromagnetismand superconductivity. We also spent here very nice time with my wife Alena and sons Jirka and Martin. We
have found new friends in Tokai and it was also a great opportunity for us to travel around Japan and visit many
beautiful places.
The research of quantum phase transitions is
unique part of the physics exploring novel electronic
phenomena in materials. Uranium intermetallic
compounds excel in this field due to the specificuranium 5f electrons on the boundary of localized anditinerant characters which makes them extremely
sensitive on external influences. The boundary between
the localized and itinerant characters was empiricallyestablished at the uranium ions distance d U-U = 3.5 Åcalled as a Hill criterion [1]. Materials with larger d U-U
than that criterion are usually magnetically ordered,superconductivity often appears in the materials
with shorter d U-U than the limit. The most interesting
phenomena can be elicited in the crossover regime
in particular cases understood in the scenario of aquantum phase transition.
Many uranium materials can be pushed to thecrossover regime typically by external pressure.
However, there is a narrow group of compounds
which fulfil the Hill criterion naturally. Isostructuralcompounds URhGe and UCoGe crystallizing
in TiNiSi-type structure are such cases andFig.1
Magnetic phase diagram of the orthorhombic TiNiSi-type
compounds UCoGe, URhGe and UIrGe.

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基礎科学ノート Vol. 23

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February 2017

fundamental coexistence of ferromagnetism (FM) andsuperconductivity (SC) was discovered as a result of
the critical ferromagnetic fluctuations [2, 3].
The H-T phase diagram of URhGe is famous also due
to SC re-entrance enforced by external magnetic field
H R [4]. Similar scenario was also detected in UCoGe [5].
The subject of our research is FM state of thesecompounds because in comparison to URhGe, the H R
does not coincide with reentrant SC dome in UCoGe.
We have used the opportunity in JAEA and grown aseries of the high quality single crystals of composition
UCo 1-x Rh x Ge. We have performed high magnetic
field experiments in cooperation with International
MegaGauss Science Laboratory in ISSP, University ofTokyo and studied the evolution of the value of H R asa function of temperature through whole concertation
range. We have found the H R following Curie
temperature T C between URhGe and UCo 0.7Rh 0.3Ge
having almost constant H R /T C ratio. However,the H R /T C ratio suddenly increases in the region
UCo0.7Rh0.3Ge - UCoGe and H R follows different criticaltemperature so called T max (Fig. 1). T max is a mysteryof many compounds with strongly correlated electron
phenomena. T max appears as a broaden maximum inthe temperature dependent magnetic susceptibilityand is connected with a broad metamagnetic transition
typically in high magnetic fields. In the case of UCoGe
T max = 40 K and related metamagnetic transition
appears at magnetic field almost 50 T. The effectwhich stands behind of T max is so far not resolved andwill be subject of our further research. It is however
surprising finding that the T max is most likely the key
to the features of the re-entrant SC dome in FM SC
UCoGe and URhGe.
By systematic explorations of the UCo1-xRhxGe, wehave recognized that next to this system may exist
another candidate UIrGe compound. UIrGe compoundis intriguing by d U-U also in the proximity to Hillcriterion. Another important feature is that UIrGe
is isoelectronic to UCoGe and URhGe, because all
three transition metals belong to the same group [6].
It is known that UIrGe orders antiferromagnetically(AFM) at T N = 16.5 K. Then, FM/AFM boundary ispresented in URh1-xIrxGe system although both parent
compounds have almost the identical value of d U-U.
UIrGe clearly represents a promising candidate whereelectron correlation phenomena can be raised when arequisite external variable will be applied to reach thetransformation FM/AFM boundary.

Fig.1
Magnetic phase diagram of the orthorhombic TiNiSi-type
compounds UCoGe, URhGe and UIrGe.

University of Tokyo on so far the best available single
crystals prepared here in JAEA.
We have successfully constructed the first p -T phase
diagram of UIrGe compound. The T N seems to suddenly
vanish by first order transition at critical pressure p c ≈12 GPa.
Within the group of three isoelectronic compounds,UIrGe is second one together with UCoGe where
magnetism can be suppressed by hydrostatic pressure.
The advance experiment down to temperature 20
mK and pressure 15 GPa is progress to search theSC in UIrGe. The URh1-xIrxGe system also revealed theimportance of the characteristic T max temperature
which again suddenly appears at AFM side of the phase
diagram URh1-xIrxGe system (Fig. 1).
The trio of UCoGe, URhGe and UIrGe compounds
represents a unique g roup for research of thetheory of magnetic phase transitions extended
about consequences to theory of unconventional
superconductivity with many unresolved details. Wewill continue the research in this field.

[1]H.H.Hill, ed. W. N.Miner, page 2, AIME, New York,1970.
[2]D. Aoki, A. Huxley et al., Nature, 413, 613 (2001).
[3]N.T. Huy, A. Gasparini et al., Physical Review Letters,

We have tried to vanish the AFM order in the UIrGe
compound by hydrostatic pressure up to 15 GPa andexamined the presence of a potential non-Fermi liquidstate (NFL) associated to potential quantum phasetransition at critical pressure. The research was realizedin cooperation with The High-Pressure Group in ISSP,

99, 067006 (2007).
[4]F. Levy, I. Sheikin et al., Science, 309, 1343 (2005).
[5]D. Aoki, T.D. Matsuda et al. Journal of the PhysicalSociety of Japan, 78, 113709 (2009).
[6]M. Valiska, J. Pospisil et al. Physical Review B, 92,045114 (2015).

Left photo: Hokkaido in 2015. Bicycle expedition around lakes Mashu and Kussharo and Mt. Io.
Right photo: Spring at Mt. Fuji.

基礎科学ノート Vol. 23

No.1

February 201720