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1.
Using interlayer tunneling spectroscopy we studied anomalous magnetoresistance state in graphite in pulsed magnetic fields up to 55 T. At low temperatures we found the opening of a pseudogap on tunneling spectra at fields above 17 T. The gap value is saturated above 30 T to 2Δ=70 mV. The gap feature is gradually smearing out with temperature but is still observed up to temperatures of ∼250 K. We discuss possible origin of the pseudogap as being related with the field induced charge density wave (CDW) state in analogy with that recently observed in NbSe3 above Peierls transition temperature.  相似文献   

2.
A potential of superconductivity of pure graphite has been theoretically examined. At normal pressure, the carrier concentration is too low to exhibit superconductivity. On applying pressure, the band dispersion along the c-axis is significantly enhanced, resulting in an increase in the carrier concentration; 1020 cm-3 at p=30 GPa. This is favorable to observe superconductivity. Accurate Fermi surfaces are illustrated: a new Fermi surface appears around K point at p=25 GPa.  相似文献   

3.
We present a room temperature high-pressure X-ray diffraction study of the layered compound 1T-TaS2 up to 20 GPa. This material is known to exhibit a variety of structural phase transitions that are ascribed to the stabilization of charge density wave states. It has been recently shown that at pressures larger than 3 GPa and up to 25 GPa, 1T-TaS2 becomes superconductor below about 5 K. It was suggested that this superconductivity coexists with different CDW states, an hypothesis that can be tested by X-ray diffraction. Our first results at room temperature show that at around 1.9 GPa, the nearly-commensurate (NCCDW) phase transforms into a phase similar to the high temperature incommensurate phase (ICCDW). Above 9 GPa, we show the existence of another IC phase, still discernable up to 20 GPa despite the pressure-induced crystal damage above 13 GPa. These results are consistent with resistivity measurements, but call for a complete exploration of the PT phase diagram of 1T-TaS2.  相似文献   

4.
The structure and equation of state of CsCl-type sodium chloride have been determined using high-pressure powder X-ray diffraction from 32 to 134 GPa. The CsCl-type phase remains stable over this entire pressure range. Pressure-volume data can be fitted with a Vinet equation of state with K30 GPa=135.1 GPa, K30 GPa=3.9, and V30 GPa=27.70 Å3. The nearest-neighbour distance between sodium and chlorine atoms decreased as pressure increased. Significant discrepancies of nearest-neighbour distance between previous theoretical predictions and this study were observed at pressures higher than 70 GPa.  相似文献   

5.
Magnetic properties and the magnetocaloric effect of the compound TbPdAl are investigated. The compound exhibits a weak antiferromagnetic (AFM) coupling, and undergoes two successive AFM transitions at TN=43 K and Tt=22 K. A field-induced metamagnetic transition from AFM to ferromagnetic (FM) state is observed below TN, and a small magnetic field can destroy the AFM structure of TbPdAl, inducing an FM-like state. The maximal value of magnetic entropy change is −11.4 J/kg K with a refrigerant capacity of 350 J/kg around TN for a field change of 0-5 T. Good magnetocaloric properties of TbPdAl result from the high saturation magnetization caused by the field-induced AFM-FM transition.  相似文献   

6.
We report synthesis of quasi-one-dimensional conductor NbS3, TEM studies and transport properties of the prepared samples. The grown NbS3 whiskers are Peierls conductors known as phase II with the transitions at TP1=365 K and TP2=150 K. CDW1 is stable and not so sensitive to growth conditions. It can slide and be synchronized by the external microwave irradiation up to 16 GHz. CDW2 strongly depends on growth conditions. Nevertheless, it also shows sliding and synchronization.  相似文献   

7.
In this paper, we report 30 THz measurements of ammonia in its excited, ν2 = 1, inversion state. These measurements include transition frequencies and pressure shifts (where significant enough to be observed). Included in the data are two forbidden and three ro-inversion transitions, as well as 18 transitions never directly measured before. The measurements were made with an all-solid-state, diode-laser, difference-frequency spectrometer. Using an innovative laser-frequency locking scheme, this spectrometer provided accurately determined and continuously tuned THz-frequencies without requiring accurate knowledge of the absolute laser frequencies. The details of the spectrometer’s frequency calibration is discussed. A global analysis of NH3 based on the available ground state, ν2 = 1 state, and ν2-band transitions was carried out, and the resulting set of recommended molecular effective-Hamiltonian parameters for ammonia is presented. In addition, calculated center frequencies and intensities for all possible transitions up to J = 20 between rotation, inversion, and vibration levels in the ground and ν2 = 1 states are included as supplementary material.  相似文献   

8.
We measured the heat capacity of CeIrSi3 (100 mK<T<6 K) under high pressure up to P=1.38 GPa. The measurements have been used a quasiadiabatic method utilizing a CuBe piston-cylinder pressure cell in a dilution refrigerator. At 0 GPa, a sharp anomaly which indicates the antiferromagnetically transition is observed at TN=5 K. TN decreases monotonically with increasing pressure up to P=1.38 GPa. The magnetic entropy is released below TN only 19% of R ln 2 at 0 GPa. And the magnetic entropy decreases with increasing pressure up to 1.38 GPa, 64% compared to that at 0 GPa.  相似文献   

9.
TaO2F, with a ReO3-type structure, has been studied at up to 12.8 GPa using monochromatic synchrotron powder diffraction and diamond anvil cells. Two-phase transitions at ∼0.7 and 4 GPa were observed on compression. Below ∼0.7 GPa the cubic material was found to have a bulk modulus (K0) of 36(3) GPa (Kp fixed at 4.0), similar to that reported for NbO2F but much smaller than that of ReO3. Immediately above 0.7 GPa on compression, the diffraction data were not fully consistent with a VF3-type structure as previously proposed for NbO2F. On decompression, the data between 8 and 4 GPa could be satisfactorily attributed to a single R-3c phase with a VF3-type structure and an average bulk modulus of 60(2) GPa.  相似文献   

10.
We present a synchrotron X-ray diffraction study of pressure-induced changes in nanocrystalline anatase (with a crystallite size of 30-40 nm) to 35 GPa. The nanoanatase was observed to a pressure above 20 GPa. Direct transformation to the baddeleyite-TiO2 polymorph was seen at 18 GPa. A fit of the pressure versus volume data to a Birch-Murnaghan equation yielded the following parameters: zero-pressure volume, V0=136.15 Å3, bulk modulus, KT=243(3) GPa, and the pressure derivative of bulk modulus, K′=4 (fixed). The bulk modulus value obtained for the nanocrystalline anatase is about 35% larger than that of the macrocrystalline counterpart.  相似文献   

11.
In this work, we report the behavior of electrical resistivity of SmB6 at temperatures between 2.2 and 70 K in pulsed magnetic fields up to 54 T. A strong negative magnetoresistance was detected with increasing magnetic field, when lowering the temperature in the range T<30 K. We show that the amplitude of negative magnetoresistance reaches its maximum dR/R~70% at B=54 T, in the vicinity of phase transition occurring in this strongly correlated electron system at TC~5 K. The crossover from negative magnetoresistance to positive magnetoresistance found at intermediate temperatures at T>30 K is discussed within the framework of exciton-polaron model of local charge fluctuations in SmB6 proposed by Kikoin and Mishchenko. It seems that these exciton-polaron in-gap states are influenced both by temperature and magnetic field.  相似文献   

12.
We investigated the behavior of the structure of titanium hydride (TiH2), an important compound in hydrogen storage research, at elevated temperatures (0-120 °C) and high pressures (1 bar-34 GPa). Temperature-induced changes of TiH2 as indicated in the alteration of the ambient X-ray demonstrated a cubic to tetragonal phase transition occurring at about 17 °C. The main focus of this study was to identify any pressure-induced structural transformations, including possible phase transitions, in TiH2. Synchrotron X-ray diffraction studies were carried out in situ (diamond anvil cell) in a compression sequence up to 34 GPa and in subsequent decompression to ambient pressure. The pressure evolution of the diffraction patterns revealed a cubic (Fm-3m) to tetragonal (I4/mmm) phase transition at 2.2 GPa. The high-pressure phase persisted up to 34 GPa. After decompression to ambient conditions the observed phase transition was completely reversible. A Birch-Murnaghan fit of the unit cell volume as a function of pressure yielded a zero-pressure bulk modulus K0=146(14) GPa, and its pressure derivative K0=6(1) for the high-pressure tetragonal phase of TiH2.  相似文献   

13.
The pressure-volume-temperature behavior of osmium was studied at pressures and temperatures up to 15 GPa and 1273 K. In situ measurements were conducted using energy-dispersive synchrotron X-ray diffraction in a T-cup 6-8 high pressure apparatus. A fit of room-temperature data by the third-order Birch-Murnaghan equation-of-state yielded isothermal bulk modulus K0=435(19) GPa and its pressure derivative K0=3.5(0.8) GPa. High-temperature data were analyzed using Birch-Murnaghan equation of state and thermal pressure approach. The temperature derivative of bulk modulus was found to be −0.061(9) GPa K−1. Significant anisotropy of osmium compressibility was observed.  相似文献   

14.
The compression behavior of a natural haüyne has been investigated to about 8.1 GPa at 300 K using in situ angle-dispersive X-ray diffraction and a diamond anvil cell at High Pressure Experiment Station, Beijing Synchrotron Radiation Facility (BSRF). Over this pressure range, no phase change or disproportionation has been observed. The isothermal equation of state was determined for the first time. The values of V0, K0, and K0 refined with a third-order Birch-Murnaghan equation of state are V0=751.6±0.4 Å3, K0=49±1 GPa, and K0=3.3±0.3, respectively.  相似文献   

15.
Results of electrical resistance measurements on MgB2 at ambient temperature up to 25 GPa are presented. An abrupt reduction of nearly 30% in resistance around 18 GPa is observed. Band structure calculations in the presence of a frozen-in distortion of the E2g phonon mode reveal that one of the closed Fermi sheets corresponding to the σ-band opens along the Γ-A direction at this pressure. It is suggested that the anomaly observed in the resistance is due to this phonon mediated electronic topological transition (ETT).  相似文献   

16.
High pressure behavior of CaB6 with cubic crystal structure is investigated by means of energy dispersive X-ray diffraction and by employing in situ resistance measurement in a diamond anvil cell. Two newcome high pressure phase transitions are found with pressure ranging from ambient to 26 GPa. The first one at 12 GPa is a structural phase transition from CsCl-type structure to orthogonal structure, which is reflected by both the X-ray diffraction and the resistance variation. The other one at 3.7 GPa is suggested to be an electronic transition, which is observed only in resistance measurement. The diffraction pattern recovered while the pressure is released to 0 GPa with a pressure hysteresis over 11 GPa, which implies the reversibility of the two phase transitions. Bulk moduli of the cubic and orthogonal phases are estimated by fitting the data to a Brich-Murnaghan equation of state equal to 169.9 and 48.2 GPa, respectively.  相似文献   

17.
The cubic CsCl type compound YbPd is known to be a valence fluctuating compound and to undergo four phase transitions at 0.5, 1.9, 105 and 125 K. In the present study, we focus on pressure effect on the transitions. The transitions at and , which are accompanied by thermal hysteresis, are of first order. With increasing pressure, the transitions shift toward lower temperature direction. The transition at T1 satisfies the Clausius-Clapeyron's relation, which also supports the first-order transition. T1 disappears at around 1.5 GPa. Above 1.5 GPa, some new ground state would be realized. We can observe an anomaly at 2.8 K which would be associated with magnetic ordering at 1.9 K. The anomaly becomes more distinct with pressure. Since applying pressure stabilizes magnetic Yb3+, the magnetic ordering becomes more conspicuous.  相似文献   

18.
The crystal structural, magnetic and electrical transport properties of double perovskite CeKFeMoO6 have been investigated. The crystal structure of the compound is assigned to the monoclinic system with space group P21/n and its lattice parameters are a=0.55345(3) nm, b=0.56068(2) nm, c=0.78390(1) nm, β=89.874(2). The divergence between zero-field-cooling and field-cooling M-T curves demonstrates the anisotropic behavior. The Curie temperature measured from Cp-T curve is about 340 K. Isothermal magnetization curve shows that the saturation and spontaneous magnetization are 1.90 and 1.43 μB/f.u. at 300 K, respectively. The electrical behavior of the sample shows a semiconductor. The electrical transport behavior can be described by variable range hopping model. Large magnetoresistance, −0.88 and −0.18, can be observed under low magnetic field, 0.5 T, at low and room temperature, respectively.  相似文献   

19.
The purple molybdenum bronze KMo6O17 is a quasi-two-dimensional compound which shows a Peierls transition towards a commensurate metallic charge density wave (CDW) state. High magnetic field measurements have revealed several transitions at low temperature and have provided an unusual phase diagram “temperature-magnetic field”. Angular studies of the interlayer magnetoresistance are now reported. The results suggest that the orbital coupling of the magnetic field to the CDW is the most likely mechanism for the field induced transitions. The angular dependence of the magnetoresistance is discussed on the basis of a warped quasi-cylindrical Fermi surface and provides information on the geometry of the Fermi surface in the low temperature density wave state.  相似文献   

20.
We report on the preparation and characterization of the variation of B′-site transition metal in Sr2CrMO6 (M=Mo, W) with double perovskites structure. The magnetic susceptibility shows that Sr2CrMoO6 and Sr2CrWO6 are antiferromagnets with TN=40 and 30 K at H=1 T, respectively. In addition, a large magnetoresistance ratio (MR) of ∼38% (H=3 T) at 5 K was observed in the Sr2CrWO6 compound. However, the Sr2CrMoO6 compound does not show any significant MR even at high fields (MR∼4%; H=3 T and 5 K). The measured O K-edge X-ray absorption is in agreement with the calculated O p-density of states for both compounds.  相似文献   

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