bluuebunny/crossref_metadata_2025 huggingface dataset:
Gene perturbation and intervention in probabilistic Boolean networks
Authors: Shmulevich Ilya , Dougherty Edward R. , Zhang Wei
Published: October 2002

Motivation: A major objective of gene regulatory network modeling, in addition to gaining a deeper understanding of genetic regulation and control, is the development of computational tools for the identification and discovery of potential targets for therapeutic intervention in diseases such as cancer. We consider the general question of the potential effect of individual genes on the global dynamical network behavior, both from the view of random gene perturbation as well as intervention in order to elicit desired network behavior.

Results: Using a recently introduced class of models, called Probabilistic Boolean Networks (PBNs), this paper develops a model for random gene perturbations and derives an explicit formula for the transition probabilities in the new PBN model. This result provides a building block for performing simulations and deriving other results concerning network dynamics. An example is provided to show how the gene perturbation model can be used to compute long-term influences of genes on other genes. Following this, the problem of intervention is addressed via the development of several computational tools based on first-passage times in Markov chains. The consequence is a methodology for finding the best gene with which to intervene in order to most likely achieve desirable network behavior. The ideas are illustrated with several examples in which the goal is to induce the network to transition into a desired state, or set of states. The corresponding issue of avoiding undesirable states is also addressed. Finally, the paper turns to the important problem of assessing the effect of gene perturbations on long-run network behavior. A bound on the steady-state probabilities is derived in terms of the perturbation probability. The result demonstrates that states of the network that are more ‘easily reachable’ from other states are more stable in the presence of gene perturbations. Consequently, these are hypothesized to correspond to cellular functional states.

Availability: A library of functions written in MATLAB for simulating PBNs, constructing state-transition matrices, computing steady-state distributions, computing influences, modeling random gene perturbations, and finding optimal intervention targets, as described in this paper, is available on request from is@ieee.org.

Contact: is@ieee.org

* To whom correspondence should be addressed.
Quantification of protein surfaces, volumes and atom–atom
contacts using a constrained Voronoi procedure
Authors: McConkey B.J. , Sobolev V. , Edelman M.
Published: October 2002

Motivation: Geometric representations of proteins and ligands, including atom volumes, atom–atom contacts and solvent accessible surfaces, can be used to characterize interactions between and within proteins, ligands and solvent. Voronoi algorithms permit quantification of these properties by dividing structures into cells with a one-to-one correspondence with constituent atoms. As there is no generally accepted measure of atom–atom contacts, a continuous analytical representation of inter-atomic contacts will be useful. Improved geometric algorithms will also be helpful in increasing the speed and accuracy of iterative modeling algorithms.

Results: We present computational methods based on the Voronoi procedure that provide rapid and exact solutions to solvent accessible surfaces, volumes, and atom contacts within macromolecules. Furthermore, we define a measure of atom–atom contact that is consistent with the calculation of solvent accessible surfaces, allowing the integration of solvent accessibility and inter-atomic contacts into a continuous measure. The speed and accuracy of the algorithm is compared to existing methods for calculating solvent accessible surfaces and volumes. The presented algorithm has a reduced execution time and greater accuracy compared to numerical and approximate analytical surface calculation algorithms, and a reduced execution time and similar accuracy to existing Voronoi procedures for calculating atomic surfaces and volumes.

Availability: Source code and executable programs are available from the authors.

Contact: brendan.mcconkey@weizmann.ac.il

* To whom correspondence should be addressed.
Subtle motifs: defining the limits of motif finding algorithms
Authors: Keich U. , Pevzner P.A.
Published: October 2002

Motivation: What constitutes a subtle motif? Intuitively, it is a motif that is almost indistinguishable, in the statistical sense, from random motifs. This question has important practical consequences: consider, for example, a biologist that is generating a sample of upstream regulatory sequences with the goal of finding a regulatory pattern that is shared by these sequences. If the sequences are too short then one risks losing some of the regulatory patterns that are located further upstream. Conversely, if the sequences are too long, the motif becomes too subtle and one is then likely to encounter random motifs which are at least as significant statistically as the regulatory pattern itself. In practical terms one would like to recognize the sequence length threshold, or the twilight zone, beyond which the motifs are in some sense too subtle.

Results: The paper defines the motif twilight zone where every motif finding algorithm would be exposed to random motifs which are as significant as the one which is sought. We also propose an objective tool for evaluating the performance of subtle motif finding algorithms. Finally we apply these tools to evaluate the success of our MULTIPROFILER algorithm to detect subtle motifs.

Contact: keich@cs.ucsd.edu

* To whom correspondence should be addressed.
Measurement of the physical and electrical thickness of ultrathin gate oxides
Authors: Chang H. S. , Yang H. D. , Hwang H. , Cho H. M. , Lee H. J. , Moon D. W.
Published: October 2002

To evaluate the reliability in measurements of the thickness of ultrathin gate oxides in the range of 2–9 nm, various techniques based on different methodologies were used for comparison. The physical thickness was determined with medium energy ion scattering spectroscopy (MEIS), high-resolution transmission electron microscopy (HRTEM), and spectroscopic ellipsometry (SE). The physical thickness was compared with the electrical thickness measured with current–voltage (I–V) and capacitance–voltage (C–V) measurements with quantum effect corrections. The physical thickness of amorphous SiO2 layers in the range of 2–9 nm determined with MEIS and HRTEM is in a good agreement with the corresponding electrical thickness from C–V and I–V measurements within 0.3 nm. For SE, which is the main technique used for in-line monitoring, we observed that it can be used for 2–9 nm ultrathin gate oxides but is more sensitive to the details of the oxide characteristics.
Development of the ultra-high-sensitive Kr adsorption technique to evaluate the pore-size distribution of thin-film materials
Authors: Yanazawa Hiroshi , Mastunaga Hironori , Itoh Hitoshi , Nakai Kazuyuki , Suzuki Isao
Published: October 2002

To meet the increased demand of porosity evaluation of low-k materials, the ultra-high-sensitive Kr gas adsorption technique was developed. A homemade differential-type adsorption apparatus enables us to detect Kr adsorption as small as 2 μg. This corresponds to the sensitivity improvement by four orders of magnitude compared to the conventional nitrogen adsorption method. Established analytical procedures for the pore-size distribution, Brunauer–Emmett–Teller theory, Kelvin equation, and Dollimore–Heal method, are applied to Kr adsorption. The results were calibrated with one obtained from nitrogen adsorption using standard samples. Applicability of the Kr adsorption method to a low-k thin-film sample was demonstrated.
Molecular dynamics simulation of copper reflow in the damascene process
Authors: Su Ming-Horng , Hwang Chi-Chuan , Chang Jee-Gong , Ju Shin-Pon
Published: October 2002

This article presents a molecular dynamics simulation of the copper reflow process for the recently developed damascene process, in which copper replaces aluminum as the interconnect material. A deposition simulation is performed, and one of the results from this simulation, namely a morphology with a void defect within the filling trench, is used as the initial morphology for the annealing process. The influence of variations in the annealing process parameters on void filling within the trench and on the copper microstructure is investigated. The article establishes a three-dimensional trench model and also provides deposition and reflow models. The annealing procedure is modeled by employing the Langevin technique to simulate heating and cooling of the thermal layer located beneath the Ti barrier layer which covers the trench. The many-body, tight-binding potential model is adopted to simulate the interatomic force between atoms. The results of this study indicate that the duration for which a constant annealing temperature is maintained plays an important role in determining the success of the reflow process. A short duration fails to produce motion of the atoms located in the trench above the void, and this motionless region of atoms prevents atoms from flowing into the trench to fill the void. The motion trace of trench atoms during the reflow process shows that circular motion is evident in the atoms that are located in the region surrounding the void, while atoms in the region above the void migrate for a long distance in the direction of the void. Finally, it is determined that a longer heating duration is beneficial in improving the microstructure of the interconnects.
Argon and oxygen ion chemistry effects in photoresist etching
Authors: Greer Frank , Van L. , Fraser D. , Coburn J. W. , Graves David B.
Published: October 2002

Photoresist (PR) etch rate kinetics were investigated for Olin i-line PR using a high-vacuum chamber with independent beams of argon ions, oxygen ions, and fluorine atoms. Etch yields were measured as a function of the fluorine atom/ion flux ratio for both ions. The etch yield of PR with oxygen ions was always higher than with argon, but the difference in yield between the two cases decreased as the F atom/ion flux ratio was increased. The effect of ion energy and ion angle of incidence on PR etching was also investigated. It was observed that the PR sputter yield increases linearly with the square root of ion energy for both oxygen and argon ions. The extrapolated threshold energies for sputtering were determined for both ions: EO2+th(θ=45°)=6 eV, EAr+th(θ=45°)=36 eV. The angular dependence of the PR etch yield (EY) was consistent with that typically observed for physical sputtering, even for F atom/Ar ion flux ratios as high as 150. These EY results were incorporated into a generalized site balance model of the PR etching process.
Molybdenum/aluminum stacked metal taper etching for high-resolution thin-film transistor liquid-crystal display
Authors: Tsujimura Takatoshi , Makita Atsuya
Published: October 2002

Instability in molybdenum/aluminum taper etching for large, high-resolution thin-film transistor (TFT) liquid-crystal display (LCD) gate line was analyzed for stable TFT LCD manufacturing. The gate line edge shape is influenced by the passivity period that decreases the overetching period that is necessary for taper creation. Passivity is mainly created soon after molybdenum just-etching time when the aluminum/molybdenum area ratio is huge and the electric current concentrate to the molybdenum edge. Bubbles on the substrate play an important role to decrease the cathode/anode area ratio to decrease the electric current in molybdenum to suppress passivity formation. Wire breakdown measurement is effective to maintain substrate-wide taper gentle shape profile by sampling inspection.
Copper drift in methyl-doped silicon oxide film
Authors: Cui Hao , Bhat Ishwara B. , Murarka Shyam P. , Lu Hongqiang , Hsia Wei-Jen , Catabay Wilbur
Published: October 2002

Drift of copper (Cu) ions into methyl-doped silicon oxide film, a promising low-dielectric-constant (low-κ) material for inter-layer dielectrics applications in ultralarge scale integrated circuit (ULSI) interconnects, was investigated using bias temperature stressing (BTS)/capacitance–voltage (C–V), current–voltage (I–V) at elevated temperatures and time-dependent dielectrics breakdown (TDDB) tests. Standard and control metal–insulator–semiconductor capacitor test samples with “sandwich” dielectric layer structures were used in the tests. BTS in nitrogen (N2) ambient at an electric field magnitude up to 1.5 MV/cm and with temperatures between 175 and 275 °C was used to accelerate the Cu ion drift. By studying flat band voltage shift in C–V tests, leakage current magnitude in I–V tests and time to fail in TDDB tests, it is demonstrated that Cu ions readily drift into methyl-doped silicon oxide under electric fields at elevated temperatures. Results obtained using different techniques correlate well to each other. The activation energy of Cu drift is determined to be 0.76 eV in N2 ambient. The film is found to have poorer resistance to Cu drift than silicon oxide (SiO2) deposited using plasma-enhanced chemical vapor deposition (PECVD). Therefore, good Cu drift barrier layers are required for reliable ULSI interconnects using this low-κ material. A thin layer of PECVD SiC is proven to be a good Cu drift barrier layer.
Growth and electron field emission characteristics of nanodiamond films deposited in N2/CH4/H2 microwave plasma-enhanced chemical vapor deposition
Authors: Wang S. G. , Zhang Qing , Yoon S. F. , Ahn J. , Wang Q. , Zhou Q. , Yang D. J.
Published: October 2002

The growth and field emission properties of nanodiamond films have been investigated. Uniform nanodiamond films with an average grain size of 10–60 nm have been successfully prepared by microwave plasma-enhanced chemical vapor deposition technique using a gas mixture of nitrogen-methane-hydrogen. Field emission results show that the threshold electric field increases, whereas the emission current density and the number of emission sites decrease with increasing mean diamond grain size. The film with a small grain size of 10 nm has a threshold electric field of 1.5 V/μm, and reaches an emission current of 780 μA/cm2 at electric field of 3.5 V/μm, demonstrating that small grain size nanodiamond films are promising material for low-field electron emitters.
Characterization of Ti-based nanocrystalline ternary nitride films
Authors: Aouadi S. M. , Chladek J. A. , Namavar F. , Finnegan N. , Rohde S. L.
Published: October 2002

Ternary nitride coatings of Ti–X–N, where X= Cr, Zr, or B, were deposited at low temperatures (<200 °C) on silicon substrates using ion beam assisted deposition (IBAD). The deposited films were characterized postdeposition using x-ray diffraction (XRD), atomic force microscopy, Rutherford backscattering (RBS), x-ray photoelectron spectroscopy (XPS), spectroscopic ellipsometry (SE), and nanoindentation. The elemental and phase composition of the films were investigated using XRD, RBS, and XPS measurements. The TiZrN coating was found to consist of a single phase in contrast with TiCrN and TiBN which consisted of three phases, namely, Cr+TiN+Cr2N and TiB2+TiN+BN, respectively. Their optical constants were determined from SE measurements and the phase compositions were deduced from the analysis of these SE data. The elemental compositions deduced from the optically determined phase compositions correlated well with the results obtained from the more direct chemical analysis techniques, namely RBS and XPS. The mechanical properties of the coatings were evaluated using nanohardness testing. The hardness and elastic modulus were measured and were found to be the highest for TiBN (hardness=40 GPa and E = 325 GPa).
Etchless fabrication of photonic crystals in silicon
Authors: Murakowski Janusz , Pustai David , Prather Dennis
Published: October 2002

We present a method for fabricating two-dimensional photonic crystal structures in high index materials, including amorphous silicon. The method is a two-step process, in which a perforated polymer membrane is first created using multilayer electron beam lithography, and then a high index material is deposited by sputtering. The result is a suspended dielectric slab with an array of holes, which acts as a planar photonic crystal. We present the fabrication process and structures realized with this method.
Laser interferometry as a diagnostic tool for the fabrication of reactive ion etching-edge-emitting lasers
Authors: Aperathitis E. , Hatzopoulos Z. , Georgakilas A. , Richeboeuf L.
Published: October 2002

Reactive ion etching (RIE) has been combined with laser interferometry at 905 nm as an in situ diagnostic technique for the fabrication of laser mirrors for GaAs/AlxGa1−xAs laser diodes. The laser structure was a graded-index separate confinement multiple quantum well heterostructure and the mirrors were formed with an optimized BCl3 RIE process. The laser interferometry was used for real-time identification of heterointerfaces, monitor of the etch rate, end point control and detection of the development of roughness during the dry etching process of the GaAs/AlxGa1−xAs laser diodes. It has been demonstrated that the relatively simple technology of RIE assisted by laser interferometry at 905 nm for in situ etching control is capable of the engineering of complex processes and the fabrication of advanced photonic circuits.
Plasma etching of thick polynorbornene layers for electronic packaging applications
Authors: Li Yiming , Hess Dennis W.
Published: October 2002

The etch characteristics of thick (>20 μm) polynorbornene films used as insulators in chip-to-board interconnection structures were studied in O2/CHF3-based plasmas. Two polynorbornene materials were investigated in this study: one with an epoxy substituent and the other with a silicon substituent. The effects of processing parameters such as radio-frequency (rf) power, chamber pressure, flow rate, gas composition, and mask material on polymer etch rate and via sidewall angle were investigated. The etch rate increased with rf power and chamber pressure. An increase in the CHF3/O2 ratio gave a maximum etch rate at ∼10% CHF3 for the epoxy-containing polynorbornene and at ∼20% CHF3 for the Si-containing polynorbornene. The etch rate decreased with total flow rate in O2/CHF3 plasmas but did not change substantially with flow rate in O2 plasmas. The sidewall angle was strongly affected by rf power and chamber pressure.
Electrical characterization of micro-organisms using microfabricated devices
Authors: Chang H. , Ikram A. , Kosari F. , Vasmatzis G. , Bhunia A. , Bashir R.
Published: October 2002

This article describes the electrical characterization of live and heat-inactivated micro-organisms within microstructures. Electrophoretic movement of live and heat-inactivated Listeria innocua on interdigitated fingered electrodes confirmed the presence of negative charges on the live micro-organisms and demonstrated positive or neutral charges on the heat-inactivated micro-organisms. A micropore was fabricated in an oxide coated silicon diaphragm, which was placed between two chambers containing ionic buffer solutions. When the micro-organism electrophoretically traversed the pore, the background ionic current was blocked and a decrease in the current was observed. As an initial test case, negatively charged polystyrene beads, which were 2.38 μm in diameter, were electrophoretically driven across the pore. Then, Listeria innocua, suspended in tris-glycine buffer, were also moved through the pore and their electrophoretic mobility was determined. In addition, the values of mobility of the micro-organisms through the micropore were found to be about 3 orders of magnitude smaller than their mobility without the pore, which can be explained by the higher hydrostatic pressures experienced by the micro-organisms when traversing through the pore.
Drastic changes in the field emission characteristics of a Mo-tip field emitter array having PH3-doped a-Si:H as a resistive layer material throughout vacuum packaging processes in a field emission display
Authors: Ha J. K. , Chung B. H. , Han S. Y. , Choi J. O.
Published: October 2002

The field emission characteristics of a field emitter array (FEA) with a new resistive layer structure both before and after annealing processes was measured. The gate voltage for emission current of 10 nA per tip increased from 70 to 93 V. The drastic changes in the field emission characteristics of a Mo-tip FEA with PH3-doped a-Si:H after a vacuum packaging process is attributed to the increase in the resistivity of the resistive layer. From the results of elastic recoil detection, secondary ion mass spectrometry, Raman spectroscopy, the increase in the resistivity of PH3-doped hydrogenated amorphous silicon is assumed to be related to the evolution of hydrogen in the thin film. Taken into consideration the fact that the hydrogen evolution occurs at 300–400 °C, the increase in the resistivity of PH3-doped a-Si:H is inevitable if typical frit glass would be used for the hermetic sealing. The resistive layer material with a constant resistivity through all the processes in the field emission display would be desirable. But, if the resistivity would change a lot during the processes, as in the case of doped hydrogenated amorphous silicon, the resistive layer should be designed to target the value after the vacuum packaging processes.
Submicron technology for III-nitride semiconductors
Authors: Palacios T. , Calle F. , Monroy E. , Muñoz E.
Published: October 2002

The optimization of III-nitride submicron technology based on e-beam lithography is described. On AlGaN alloys, procedures must be completely different than on Si, due to the poor electrical conductivity and low chemical reactivity of these materials. To overcome these problems, several metal/resist multilayers have been studied, both theoretically and experimentally, the most successful being the resist/metal/semiconductor and the resist/metal/resist/semiconductor schemes. The applicability of the optimized procedure was demonstrated in the fabrication of some AlGaN-based devices: surface acoustic wave filters, metal-semiconductor-metal photodiodes, and high electron mobility transistors. All these submicron devices showed a clear improvement of their performance, as expected from size shrinkage.
Inductively coupled plasma etching for arrayed waveguide gratings fabrication in silica on silicon technology
Authors: Bretoiu S. , Di Mola D. , Fioravanti E. , Visona S.
Published: October 2002

Arrayed waveguide gratings (AWGs) in silica on silicon technology were fabricated and tested in our laboratory. The silica optical layers were all deposited in low-pressure chemical vapor deposition furnaces, and waveguide core was etched using a photoresist mask in an inductively coupled plasma source with C4F8/O2/He gas mixture. This article reports the dependence of important process parameters, including aspect ratio dependent etch rate (ARDE) effect, selectivity, and waveguide sidewall angle, on rf power, chamber pressure, and gas flow rates. In particular, the effects of He and O2 addition on ARDE effect and waveguide sidewall angle were investigated. Based on these results, a reliable and high throughput process was set up to etch silica waveguides suitable for AWG fabrication, with etch rate higher than 300 nm/min, selectivity on photoresist higher than 5:1, and waveguide sidewall angle higher than 88.5°. Measurements of the first 16 channel 200 GHz AWGs fabricated with this process are presented and discussed. To conclude, preliminary results are reported about etching of high aspect ratio (>3:1) trenches with depths greater than 15 μm, to be used for stress releasing grooves in AWGs.
Edge termination design and simulation for bulk GaN rectifiers
Authors: Baik K. H. , Irokawa Y. , Ren F. , Pearton S. J. , Park S. S. , Lee S. K.
Published: October 2002

GaN bulk rectifiers show excellent on-state resistances (in the mΩ cm−2 range), forward turn-on voltages of ∼1.8 V and reverse-recovery times of <50 ns. A key requirement is to develop effective edge termination techniques in order to prevent premature surface-induced breakdown. We have performed a simulation study of the effects of varying the dielectric passivation material (SiO2, SiNx, AlN, Sc2O3, or MgO), the thickness of this material, the extent of metal overlap onto the dielectric and the ramp oxide angle on the resulting reverse breakdown voltage (VB) of bulk rectifiers. We find that SiO2 produces the highest VB of the materials investigated, that there is an optimum metal overlap distance for a given oxide thickness and small oxide ramp angles produce the highest VB.
TUNNELING ZERO-BIAS ANOMALY IN THE ULTRA-QUANTUM LIMIT
Authors: MASLOV D. L. , TSAI S.-W , GLAZMAN L. I.
Published: October 2002

Effects of the electron-electron interaction on tunneling into a metal in a very strong magnetic field (ultra-quantum limit) are considered. It is shown that for short-range repulsive interactions, tunneling is dominated by the renormalization of the coupling constant leading eventually to the CDW instability. Near the gap, the conductance gT assumes a universal form. For the long-range interaction, there exists an intermediate energy EPL such that for Δ ≪ E ≤ EPL the conductance obeys a power-law scaling form, similar to that of a 1D Luttinger liquid. For smaller energies, scaling breaks down and gT is again dominated by the CDW-instability. It is also shown that the energy dependence of gT may be non-monotonic: whereas gT is always suppressed by the interaction at lower energies, at higher energies it may also be enhanced above its non-interacting value. Such an unusual behavior gets a natural explanation in terms of multiple scattering from the Friedel oscillation near the surface.
DOES THE HEAVY ELECTRON MAINTAIN ITS INTEGRITY AT QUANTUM CRITICAL POINT?
Authors: COLEMAN P.
Published: October 2002

This talk will discuss non-Fermi liquid and quantum critical behavior in heavy fermion materials, focussing on the mechanism by which the electron mass appears to diverge at the quantum critical point. Is the quantum critical point merely a case of electron diffraction off a quantum critical spin density wave, or does it involve a fundamental break-down in the composite nature of the heavy electron place at the quantum critical point? We discuss the nature of the critical langrangian and show that the Hall constant changes continuously in the first scenario, but may "jump" discontinuously at a quantum critical point where the composite character of the electron quasiparticles changes.
OBSERVATION OF A SECOND ENERGY SCALE IN YbAl3 ABOVE 40 T
Authors: CORNELIUS A. L. , EBIHARA T. , LAWRENCE J. M. , RISEBOROUGH P. S. , THOMPSON J. D.
Published: October 2002

YbAl 3 is an intermediate valent compound with a Kondo temperature T K in excess of 500 K and a rather low conduction electron density of ~0.5/atom. Recent measurements are suggestive of a second energy scale T coh of order 50 K that dominates the low temperature ( T ≪ T coh ) thermodynamic properties. Previous de Haas-van Alphen (dHvA) measurements on YbAl 3 in magnetic fields to 17 T reveal a fairly simple Fermi surface with 6 branches having effective masses m* ranging from 8 to 24 m0 (see Refs. [2-3]). We report magnetization and dHvA results on YbAl 3 in pulsed magnetic fields up to 60 T. For T ≪ T coh we indeed find that the magnetization 'crosses' over from the zero field energy scale T coh to the high temperature energy scale T K at a magnetic field B * ≈ 40 T (≈ K B T coh /μB). For B > B *, we find dHvA oscillations when magnetic field is applied along the <100>, <110> and <111> directions. For magnetic field applied along <111>, the Fermi surface changes very little for B > B*, and the effective masses are all reduced by a factor of 2-3 relative to their low field values. This is due to the large change in the characteristic temperature, which goes from T coh below B * to T K above B *. We believe this is the first work to directly observe the two energy scales and to observe the crossover in the dominant energy scale as a function of magnetic field.
SPECIFIC HEAT ANOMALY FOR H ≥ 28.5 T IN CeIrIn5
Authors: KIM J. S. , ALWOOD J. , KUMAR P. , STEWART G. R.
Published: October 2002

Recently Takeuchi reported a weak, 'metamagnetic-like' increase in the magnetization around 42 T in single crystals of CeIrIn 5 for field in the c-direction. We report specific heat measurements on single crystal CeIrIn 5, H parallel c-axis, measured in the dc hybrid magnet at NHMFL in Tallahassee between 1.4 and 10 K. A clear anomaly in C/T in 35 T is observed to peak at 1.8 K, with an entropy of 6% of RIn2. This anomaly grows in size and shifts upwards in temperature (both monotonically) with increasing H until at 45 T T peak =4.1 K , with the entropy associated with the transition equal to 14% of RIn2. C/T data at 32 T show3 only the high temperature side of the peak occurring above 1.4 K, while C/T data at 28.5 T show no anomaly down to 1.4 K. This is consistent with our Tpeak vs H data, which imply T peak → 0 at 26 T.
THERMODYNAMIC STUDIES OF THE FIELD-INDUCED GAP IN THE QUASI-ONE-DIMENSIONAL S = 1/2 ANTIFERROMAGNET Yb4As3
Authors: LANG M. , ZHERLITSYN S. , WOLF B. , AOKI H. , CICHOREK T. , GEGENWART P. , SCHMIDT B. , STEGLICH F. , OCHIAI A.
Published: October 2002

The quasi-one-dimensional S = 1/2 antiferromagnet Yb 4 As 3 is studied by using low-temperature measurements of the specific heat C(T,B), thermal expansion α(T,B) and longitudinal elastic mode c 11(T,B). As has been previously shown [M. Köppen et al., Phys. Rev. Lett. 82, 4548 (1999)], finite magnetic fields perpendicular to the spin chains induce a gap in the spin-excitation spectrum (reminiscent of massive, soliton-like excitations) which manifests itself in distinct anomalies in the specific heat and thermal expansion. In this paper, we present an extension of the above work placing special emphasis on the lattice response and the evolution of the gap at higher fields. The main observations are: (i) the field-induced gap causes a minimum in the c 11 elastic constant both as a function of temperature and field. Applying a simple two-level model allows for a determination of the gap value Δ(B) as well as the constant G(B) = ∂Δ/∂ε introduced to account for the spin-lattice coupling. (ii) At B ≤ 9 T , the Δ(B) values derived from the various quantities are consistent with Δ (B) ∝ B 2/3 as predicted by the quantum sine-Gordon model. (iii) Measurements of C(T,B = const) in dc-fields up to 18 T and c 11(T = const, B) in pulsed fields up to 50 T,however, reveal deviations from this behavior at higher at field. (iv) Isothermal measurements of c 11 (B) show a sharp increase above 35 T which is almost T-independent for T ≤ 10 K and whose origin is unknown.
ULTRASONIC MEASUREMENTS AT THE METAMAGNETIC TRANSITION IN URu2Si2
Authors: SUSLOV A. , DASGUPTA D. , FELLER J. R. , SARMA B. K. , KETTERSON J. B. , HINKS D. G. , JAIME M. , BALAKIREV F. , MIGLIORI A. , LACERDA A.
Published: October 2002

Ultrasonic velocity measurements were performed on a single crystal of URu 2 Si 2 at various temperatures in the 20 ms 50T pulsed magnets at the NHMFL, Los Alamos, NM. At 4 K a three-fold splitting of the metamagnetic transition is seen; corresponding features were observed earlier in magnetization studies. As the temperature is raised, the three-fold splitting evolves first into two features, then into a single broad dip, after which it disappears entirely.
MAGNETIC-FIELD-INDUCED SUPERCONDUCTIVITY IN LAYERED ORGANIC MOLECULAR CRYSTALS WITH LOCALIZED MAGNETIC MOMENTS
Authors: CEPAS O. , MCKENZIE R. H. , MERINO J.
Published: October 2002

The synthetic organic compound λ (BETS) 2 FeCl 4 undergoes successive transitions from an antiferromagnetic insulator to a metal and then to a superconductor as a magnetic field is increased. We use a Hubbard-Kondo model to clarify the role of the Fe 3+ magnetic ions in these phase transition. In the high-field regime, the magnetic field acting on the electron spins is compensated by the exchange field He due to the magnetic ions. This suggests that the field-induced superconducting state is the same as the zero-field superconducting state which occurs under pressure or when the Fe 3+ ions are replaced by non-magnetic Ga 3+ ions. We show how Hc can be extracted from the observed splitting of the Shybnikov-de Haas frequencies. Furthermore, we use this method of extracting He to predict the field range for field-induced superconductivity in other materials. We also show that at high fields the spin fluctuations of the localized spins are not important.
PERSISTENT CURRENTS AT FIELDS ABOVE 23T
Authors: HARRISON N.
Published: October 2002

Experimental studies made on organic conducting salts of the composition α- (BEDT-TTF) 2M Hg(SCN) 4 (where M = K , Tl and Rb) indicate that they exhibit persistent currents at magnetic fields exceeding 23T. The presence of currents cannot be explained by the quantum Hall effect, while superconductivity seems unlikely. All indications point towards a new type of dissipationless current flow involving relative gradients in the pinning of a CDW and quantized orbital magnetism.
MAGNETIC FIELD-INDUCED DENSITY WAVE TRANSITION IN A τ-PHASE ORGANIC CONDUCTOR
Authors: GRAF D. , BALICAS L. , BROOKS J. S. , MIELKE C. , PAPAVASSILIOU G. C.
Published: October 2002

Pulsed magnetic fields to 53 tesla reveal a new high field phase transition in τ-[ P-(S,S) - DMEDT-TTF ]2 ( AuBr 2)( AuBr 2) y (where y ~.75). This transition appears as an abrupt increase in the magnetoresistance at a threshold field ( B Th ~44 tesla for T = 0.55 K). B Th shifts toward higher magnetic fields with increasing temperature, and there is evidence of higher field sub-phases. Measurements of Shubnikov de Haas oscillations below 33 tesla have provided compelling evidence as to the two-dimensional (2D), metallic nature of this compound. In the following report we discuss this new unexpected state, reminiscent of the field-induced spin density wave (FISDW) behavior in the quasi one-dimensional (Q1D) Bechgaard salts.
INVESTIGATION OF MULTIPLE-SPIN EXCHANGE IN 2D FILMS OF 3He: NMR STUDIES
Authors: PARKS C. , SULLIVAN N. S. , STACHOWIAK P.
Published: October 2002

Results of NMR studies of the nuclear spin-spin relaxation times at low temperatures are reported for commensurate monolayers of 3 He physisorbed on hexagonal boron nitride. Temperature independent relaxation rates are observed for 0.11< T <0.56 K, and are interpreted in terms of the quantum exchange motions of the 3 He atoms. The relaxation times and the inferred effective exchange rates were observed to change significantly on replacing the 3 He atoms with relatively immobile Ne atoms. This result is understood in terms of a large 3-spin exchange in addition to 2-spin exchange.
MAGNETISM AND SUPERCONDUCTIVITY IN YBa2Cu3O6+x SUPERCONDUCTORS
Authors: DAI P. , MOOK H. A. , HAYDEN S. M. , HIESS A. , LEE S.-H. , DOGAN F.
Published: October 2002

Perhaps the most puzzling attribute of the high-temperature cuprate superconductors is that for underdoped materials the energy gap normally associated with superconductivity is observed well above the superconducting transition temperature Tc. Neutron scattering measurements of YBa 2 Cu 3 O 6.6 have identified small moments that increase in strength as the temperature is reduced below the pseudogap temperature T*. Polarized beam neutron scattering measurements demonstrate that the signal is magnetic in origin. Furthermore, the pattern of neutron intensities found in the experiment is unusual as it is not readily understandable in terms of conventional spin magnetism. Although we have considered the origin of the moment in terms of the orbital bond currents, new experimental data suggest that only part of the signal is consistent with this interpretation. We present detail analysis of polarized neutron data and systematic studies of oxygen chain ordering. Finally, we discuss our most recent results on spin-glass phase of Ca-doped YBa 2 Cu 3 O 6.
MAGNETIC FIELD TUNING OF CHARGE AND SPIN ORDER IN THE CUPRATE SUPERCONDUCTORS
Authors: POLKOVNIKOV A. , SACHDEV S. , VOJTA M. , DEMLER E.
Published: October 2002

Recent neutron scattering, nuclear magnetic resonance, and scanning tunneling microscopy experiments have yielded valuable new information on the interplay between charge and spin density wave order and superconductivity in the cuprate superconductors, by using a perpendicular magnetic field to tune the ground state properties. We compare the results of these experiments with the predictions of a theory which assumed that the ordinary superconductor was proximate to a quantum transition to a superconductor with co-existing spin/charge density wave order.
MIXING OF SINGLET AND TRIPLET PAIRING FOR SURFACE SUPERCONDUCTIVITY
Authors: GOR'KOV L. P. , RASHBA E. I.
Published: October 2002

We consider structure of the Cooper wave function for superconductivity in a surface layer. Broken space inversion at the surface results in lifted spin degeneracy and in two branches of the gapped energy spectrum as caused by the spin-orbit interaction. The pair wave function consists of a mixture of both singlet and triplet components. Anisotropy of the Knight shift measurable in the NMR experiments is calculated in the whole temperature regime. Implications for a few known experimental situations is briefly discussed.
METAL-INSULATOR CROSSOVER IN HIGH Tc CUPRATES: GAUGE FIELD THEORY VERSUS EXPERIMENTS
Authors: MARCHETTI P. A. , SU Z.-B. , YU L.
Published: October 2002

The U(1)×SU(2) Chern-Simons gauge field theory, proposed by the authors to explain in a unified fashion the metal-insulator crossover of the in-plane resistivity upon temperature decrease in heavily underdoped cuprates without magnetic field and a similar phenomenon, observed in several classes of superconducting cuprates, when a strong magnetic field suppresses the superconductivity, is briefly outlined and confronted with recent experiments.
THE MILLIMETRE-WAVE MAGNETO-OPTICAL RESPONSE OF Sr2RuO4
Authors: ARDAVAN A. , RZEPNIEWSKI E. , EDWARDS R. S. , SINGLETON J. , MAENO Y.
Published: October 2002

We report a study of the angle-dependent millimetre-wave magnetoconductivity of the p-wave triplet-paired perovskite superconductor Sr 2 RuO 4. We find two harmonic series. We assign the first to interlayer cyclotron resonance of the β-Fermi surface and its harmonics, yielding a cyclotron effective mass of [Formula: see text]. We assign the second series, which contains only odd harmonics, to cyclotron resonance of the γ-Fermi surface, yielding a cyclotron effective mass of [Formula: see text]. In addition, we find a very strong absorption mode in the presence of a magnetic field component parallel to the quasi-two-dimensional (Q2D) planes of the sample. Its dependence on the orientation of the magnetic field cannot be described in the context of conventional Q2D cyclotron resonance, and the origin of this mode is not yet clear.
SPIN DENSITY WAVE ORDER AND FLUCTUATIONS IN (TMTSF)2PF6 AT VERY HIGH MAGNETIC FIELDS
Authors: CLARK W. G. , VONLANTHEN P. , GOTO A. , TANAKA K. B. , ALAVI B. , MOULTON W. G. , REYES A. , KUHNS P.
Published: October 2002

We report extremely high field and frequency NMR measurements of the proton spin-lattice relaxation rate (1/T1) in (TMTSF)2PF6. They probe the spin density wave (SDW) fluctuations in the critical regime near the SDW transition and in the ordered phase at lower temperatures. The highest field measurements were made using the NHMFL hybrid magnet at 44.7 T and a proton NMR frequency of 1.9 GHz. A quadratic increase in the transition temperature is observed that agrees with transport measurements made at lower fields. As the field is increased above 40 T, a change in the SDW critical dynamics is seen. Also, the behavior of 1/T1 vs. the alignment of the magnetic field suggests a magnetic transition at moderate to high fields with the characteristics of a sharp change in the orientation of the SDW polarization, but with parameters that differ from the spin-flop transition seen at low field.
MAGNETISM AT THE SPATIAL LIMIT
Authors: MANOHARAN H.
Published: October 2002

In this talk I will survey our recent experimental results in the detection and manipulation of magnetism at the spatial limit. Our experiments rely on atom manipulation techniques and scanning tunneling microscopy at low temperatures to enable atomic-scale imaging and control.

We have observed "quantum mirages" in focusing devices of order 10 nanometers in size, built by assembling structures out of individual atoms. We have directly imaged the spin perturbations due to isolated magnetic moments on a metal surface. The detection of this localized magnetism can then be utilized in a type of teleportation experiment, in which the spectroscopic signature of an atom is sampled and projected to a remote location by means of a surrounding sea of electrons confined in an engineered nanostructure. The quantum mirage thus cast by a single magnetic atom can be coherently refocused at a distinct point where it is detected as a phantom atom around which the electronic structure mimics that at the real atom. Once materialized, this phantom can interact with real matter in intriguing ways.

We have constructed other nanoscale magnetic structures which either elucidate the coupling between isolated moments or provide a mechanism for controlling and exploiting spin coupling over long distances. We have also been developing novel communication methods based on the fundamental effects we have discovered.
FERROMAGNETIC AND STRUCTURAL INSTABILITIES IN Ca2-xSrxRuO4
Authors: NAKATSUJI S. , MAENO Y.
Published: October 2002

Ca 2 - x Sr x RuO 4 is a unique quasi-two-dimensional multi-band system that connects the Mott insulator Ca2RuO4 with the spin-triplet superconductor Sr2RuO4. The isovalent Sr substitution changes the crystal distortions and thus modifies the hybridization and degeneracy of t2g orbitals. Such effective change of electron correlation induces the Mott transition and rich evolution of itinerant magnetism. One of the striking features in this system is the appearance of ferromagnetic critical behavior in the metallic phase near a zero-temperature structural instability (x c ≈ 0.5). The low temperature susceptibility shows Curie-like diverging behavior, and the electronic specific heat (Ce) divided by temperature, Ce/T, enormously increases up to 250 mJ/mol-Ru at x = 0.5. Moreover, critical enhancement of Wilson ratio is attributable to a ferromagnetic instability developed near the structural instability.
EFFECTS OF PARALLEL MAGNETIC FIELDS ON THE UNUSUAL METALLIC BEHAVIOR IN TWO DIMENSIONS
Authors: POPOVIC D.
Published: October 2002

The fate of the metallic phase in parallel magnetic fields represents one of the major open issues in the studies of dilute, strongly interacting systems in two dimensions (2D). In this experiment, the temperature dependence of conductivity σ(T) of a 2D electron system in silicon has been studied in parallel magnetic fields B. At B = 0, the system displays a new and unexpected kind of metallic behavior with dσ/dT > 0, and a novel type of a metal-insulator transition,1 which occurs at a critical electron density nc(0). At low fields (B ≤ 2 T ), nc increases as nc(B) - nc(0) ∝ Bβ(β ≈ 0.9), and the zero-temperature conductivity scales as σ(ns, B, 0)/σ (ns, 0, 0) = f(Bβ/δn) (where δn = (ns - nc(0))/nc(0), and ns is electron density) as expected for a quantum phase transition. The metallic phase persists in fields up to 18 T, consistent with the saturation of nc at high fields. These results strongly suggest that the 2D metal may exist even for spinless electrons.
HOPPING CONDUCTIVITY IN ONE-DIMENSIONAL Ca3Co2O6 SINGLE CRYSTAL
Authors: BROTO J. M. , RAQUET B. , RAKOTO H. , BAIBICH M. N. , LAMBERT S. , MAIGNAN A.
Published: October 2002

We studied the electronic conductivity of the quasi-one dimensional Ca3Co2O6 single crystal. The results evidence a VRH conductivity with temperature-induced crossover between 1D (intra-chain) and 3D transport and the opening of a Coulomb gap in the d bands. At low temperatures, an applied magnetic field induces a large negative magneto-resistance (MR) independent from the 3D magnetic ordering. Both spin-dependent hopping and field-induced suppression of the Coulomb gap are discussed.
55Mn NMR AND MAGNETIZATION STUDIES OF LaxSr1 - xMnO3
Authors: CALDWELL T. , KUHNS P. L. , MOULTON W. G. , REYES A. P.
Published: October 2002

The 55Mn NMR has been measured in LaxSr1 - xMnO3 for x = 0.2 and 0.3 as a function of temperature, in zero field and in field. The samples had a room temperature resistivity of 10-4 ohm-cm, and the magnetizarion showed a sharp FM transition 312 K. The zero field FM frequency follows T3/2 power law at both x values consistent with the simple magnon model. A second feature at lower frequency of uncertain origin is observed.
TAMM-TYPE OF STATES AT THE INTERFACE IN La1-xSrxMnO3 (x=0.4, 0.55) SUPERLATTICES
Authors: DZERO M. , GOR'KOV L. P.
Published: October 2002

We address some of the results of the recent experiments by Izumi et al. where the properties of heterostructures composed of La 1 - x Sr x MnO 3 (x = 0.4, 0.55) layers have been investigated. Anomalies in conductivity and large magnetoresistance measurements for several heterostructures with thin constituent ferromagnetic (x = 0.4) and antiferromagnetic (x = 0.55) layers are explained in terms of existence of the localized (Tamm-type) of states at the interface.
MAGNETIC RESONANCES OBSERVED IN THE HIGH-FIELD MAGNETO-OPTICAL ABSORPTION OF THE QUANTUM ISING FERROMAGNET LiHoF4
Authors: EDWARDS R. S. , NARDUZZO A. , LYONS E. , CHILDRESS L. , BLUNDELL S. J. , SINGLETON J. , WARD R. C. C.
Published: October 2002

The magneto-optical absorption of the quantum Ising ferromagnet LiHoF4 has been measured in the frequency range 38 to 140 GHz, the temperature range 0.5 to 20 K and magnetic fields of up to 15 T. The sample was studied in two orientations, with the external magnetic field aligned perpendicular or parallel to the uniaxial direction, i.e., along the quantum and classical directions. In each case, several families of resonances are observed, including linear EPR branches and a series of non-linear fast moving resonances. We present this data and propose an interpretation for the origin of these resonances.
PRESSURE DEPENDENT MAGNETIZATION AND MAGNETIC ORDERING IN RARE EARTH RUTHENATES, Sm2RuO5, Gd2RuO5, Tb2RuO5, AND Nd3RuO7
Authors: GUERTIN R. P. , McCALL S.
Published: October 2002

The antiferromagnetic ordering temperatures of Sm2RuO5, Tb2RuO5 and the ferromagnetic ordering temperature of Nd3RuO7 are reduced by the application of high hydrostatic pressure, whereas that of antiferromagnetic Gd2RuO5 remains unchanged. The results are discussed in terms of the enhancement of the crystalline electric field interaction on the Ln cation constituents. A versatile method of measuring magnetization in high fields and high pressure is presented.
NEW NON-COOPERATIVE QUANTUM PHENOMENON IN A FERRIMAGNET WITH ANTIFERROMAGNETIC IMPURITY
Authors: LAGUTIN A. S. , SEMENO A. , VANACKEN J. , BRUYNSERAEDE Y.
Published: October 2002

The differential magnetic susceptibility of Gd0 01Y2.99Fe5O12 polycrystalline compounds is studied at the temperature range from 0.6 to 4.2 K in pulsed magnetic fields up to 50 T. It was observed that the magnetization of the compound with x = 0.01 shows a quantum (stepwise) behavior at T = 0.6 K in magnetic fields around 33 T. The Heisenberg model of a ferrimagnet with weakly coupled antiferromagnetic impurities is used to explain the obtained results. Following this model, the energy levels of the impurity ions become not equally spaced due to the magnon-magnon interaction between the iron spins and the rare earth spins. Increase of the external field leads to the crossover of the excited states with the ground one, which takes place in different fields. In result, several peaks of the magnetic susceptibility appear under a variation of magnetic field in the vicinity of a field, which is equal to the effective field of the Gd-Fe exchange interaction.