Phase transitions in N—n-propylpyridinium-TCNQ2 and N—n-butylpyridinium-TCNQn at high pressures

The electrical resistivity of N-n-propylpyridinium-TCNQ₂ (NPPy-TCNQ₂) and N-n-butylpyridinium-TCNQ_n (NBPy-TCNQ_n) has been measured as a function of temperature and pressure. Phase transitions in these salts have been studied at high pressures. The transition temperature (T_c) in NPPy-TCNQ₂ at atmospheric pressure increased with increasing pressure at the rate of dT_c/dP = + 12.0 degkbar^?1. The value of volume change calculated from the Clapeylon-Clausius relation was + 4.4 cm³ mol^?1. The electrical resistivity along the a- and c-axis increased with increasing pressure below 7 kbar. This anomalous electrical behaviour is closely related to the crystal structure of NPPy-TCNQ₂. The resistivity dropped sharply at about 11 kbar. This abrupt change may be due to a new pressure induced phase transition.The T_c of the NBPy-TCNQ_n increased remarkably with increasing pressure up to 0.7 kbar, above which the phase transition disappeared. The phase transitions of N-n-alkyl-substituted pyridinium TCNQ salts depend strongly on the nature of cations.     

Electrical conductivity in HMTSF-TNAP under pressure

The resistivity of the organic conductor HMTSF-TNAP has been measured at pressures up to 25 kbar and at temperatures down to 1.2 K. Under pressure the increase in the stacking axis resistivity of HMTSF-TNAP (Δ^2,2 -Bi(4,5-trimethylene-1,3-diselenole)-11,11′,12,12′-tetracyano-2m6- naphtoquinodimethane) below 47 K is reduced, although the transition temperature T_p falls at only 0.7 K kbar^?1. This weak pressure dependence compared to that of HMTSF-TCNQ correlates with the larger resistivity anisotropy in the TNAP salt.     

The D.C. voltage-dependent resistivity of CuCl powder compacts under pressure

The CuCl powder compacts are found to exhibit d.c. voltage dependent resistivity. The resistivity drops by six orders of magnitudes when the applied voltage exceeds a critical value V_c.V_c is found to decrease with the increase in pressure. The high conducting phase shows a positive temperature coefficient of resistance at 1 atm of 0.004°C^-1 which increases with pressure at a rate 0.0004°C^?1 GPa^?1. It is suggested that the pressure induced transition reported at 4 GPa could arise from the application of d.c. voltage.     

A study of the temperature and pressure dependence of the electrical resistivity of a dilute Cr-Ge system

The electrical resistivity of a series of dilute Cr-Ge alloys containing up to 1.5 atm % Ge, was measured as a function of temperature and pressure. The measurements clearly demonstrate the existence of resistivity anomalies at the incommensurate-commensurate spin density wave transition temperature (T_IC) in contrast with recently reported results. The complete magnetic phase diagram, determined for the first time from electrical resistivity measurements, contains a triple point in contrast with previous neutron diffraction results but in agreement with thermal expansion measurements. It was found that the incommensurate spin density wave state is absent in alloys with more than 1 atm % Ge. The Néel temperatures and incommensurate-commensurate transition temperatures are affected differently by pressure. Pressure decreases T_N in all the alloys while it increases T_IC for those alloys in which the incommensurate-commensurate transition occurs. The decrease of T_N with pressure is much larger for the commensurate-paramagnetic than for the incommensurate-paramagnetic transition. The electrical resistivity of the alloys at room temperature behaves anomalously with applied pressure. This anomalous behaviour is attributed to an antiferromagnetic-paramagnetic phase transition that is induced in the alloys by applied pressure.     

High pressure phase transition in PbSnTe

The high pressure structural phase transition in the alloy series Pb_{1 ? x}Sn_xTe has been investigated using resistivity and Hall coefficient measurements. With increasing of tin content the critical pressure decreases linearly and the phase transition becomes less drastic. The anomalous behavior of the resistivity in the phase transition region is explained by band structure changes. It is suggested that the high pressure phase transition in Pb_{1 ? x}Sn_xTe is a metallic-covalent transition.     

Low temperature magnetoresistance in La1.32Sr1.68Mn2O7 layered manganite under hydrostatic pressure

The La_1.32Sr_1.68Mn₂O₇ layered manganite system has been studied by the low temperature electrical resistance and magnetoresistance under hydrostatic pressure up to 25 kbar. We have observe both, a Curie temperature (T_C) and a metal-insulator transition (T_MI) at 118 K in the ambient pressure. The applied pressure shifts the T_MI to higher temperature values and induces a second metal-insulator transition (T²_MI) at 90 K, in the temperature dependence of resistivity measurements. Also, the pressure suppresses the peak resistance abruptly at T_C. When an external field of 5 T is applied, we have observed a large negative magnetoresistance of 300% at the transition temperature and a 128% at 4.5 K. However, the increased pressure decreases the magnetoresistance ratio gradually. When the pressure reaches its maximum available value of 25 kbar, the magnetoresistance ratio decreases at a rate of 1.3%/kbar. From our experimental results, the decrease of magnetoresistance ratio with pressure is explained by the pressure induced canted spin state which is not favor for the spin polarized intergrain tunneling in layered manganites.     

Vortex states in mesoscopic single crystals Bi2Sr2CaCu2O8+δ in high magnetic fields

The c-axis resistivity measurements were performed in the vicinity of the ab-plane in order to investigate the interaction between Josephson vortices and pancake vortices in Bi₂Sr₂CaCu₂O_8+δ mesoscopic single crystals. It was found that the angular dependence of the c-axis resistivity drastically changes in high magnetic field regime. The vortex lock-in transition becomes considerably broad in high magnetic fields, while the angular dependence of resistance exhibits the sharp lock-in features in low magnetic field region.     

Electric and magnetic anomalies at the charge-density-wave transition in niobium-substituted vanadium diselenides

A systematic study has been made of the effect of niobium substitution on the CDW transition in VSe₂, especially in the low-doping region. Transitions were monitored by measurements of static magnetic susceptibility, which has been found to be particularly sensitive for both onset and lock-in discontinuities, and by measurements on resistivity and Hall effect. Results, discussed in terms of the model of Chan and Heine and in terms of McMillan's phenomenological model, suggest that structural changes due to the presence of the niobium dopant lead to enchanced electron-phonon coupling thus raising the CDW onset temperature.     

LDA+DMFT study of magnetic transition and metallization in CoO under pressure

The magnetic and spectral properties of the paramagnetic phase of CoO at ambient and high pressures have been calculated within the LDA+DMFT method combining local density approximation (LDA) with dynamical mean-field theory (DMFT). From our results CoO at ambient pressure is a charge transfer insulator in the high-spin t _2g ⁵ e _g ² configuration. The energy gap is continuously decreased, and finally a transition into metallic state occurs with the increase in pressure that is consistent with experimental behavior of electrical resistivity. Notably, the metal-insulator transition in CoO is found to be accompanied by the high-spin to low-spin (HS-LS) transition in agreement with XES data. The metal-insulator transition is orbital selective in the t _2g states of cobalt only, whereas the e _g states become metallic after the spin transition at higher pressures.     

Thermal expansion of the organic conductor β-(BEDT-TTF)2I3

We report hiogh resolution thermal expansion measurements between 1.2 and 300 K along the c¹-axis in the organic conductor β-(BEDT-TTF)₂I₃ where the superconducting properties are strongly dependent on the way the system is prepared in the pressure-temperature diagram. We clearly observed a strong anomaly at 172 K related to the structural phase transition and the resistivity anomalies reported by several groups. The temperature analysis in the low temperature regime is coherent with a phonon dominant contribution. Finally, the presence of small hysteresis in a large region around the transitions seem to indicate why both the pressure and the temperature cycling may stabilize small regions of the high-T_c phase of the system, as observed.     

Pressure dependence of superconductivity in the pseudo-one-dimensional compound Tl2Mo6Se6

Measurements of the temperature and pressure dependences of the resistivity of the pseudo-one-dimensional ternary compound Tl₂Mo₆Se₆ are presented. We find that the conductivity parallel to the highly conducting c-axis is enhanced by pressure and the superconducting transition temperature T_c is suppressed by pressure at a rate $\text{?T}{}_{\mathrm{c}}\text{?P}\text{=?7.6×10}{}^{\mathrm{?5}}$ kbar^?1. These results are discussed in relation to the current models of transport in one-dimensional conductors.     

High-precision measurements of the compressibility and the electrical resistivity of bulk <Emphasis Type="Italic">g</Emphasis>-As<Subscript>2</Subscript>Te<Subscript>3</Subscript> glasses at a hydrostatic pressure up to 8.5 GPa

High-precision studies of the volume and the electrical resistivity of g-As₂Te₃ glasses at a high hydrostatic pressure up to 8.5 GPa at room temperature are performed. The glasses exhibit elastic behavior in compression only at a pressure up to 1 GPa, and a diffuse structural transformation and inelastic density relaxation (logarithmic in time) begin at higher pressures. When the pressure increases further, the relaxation rate passes through a sharp maximum at 2.5 GPa, which is accompanied by softening the relaxing bulk modulus, and then decreases, being noticeable up to the maximum pressure. When pressure is relieved, an unusual inflection point is observed in the baric dependence of the bulk modulus near 4 GPa. The polyamorphic transformation is only partly reversible and the residual densification after pressure release is 2%. In compression, the electrical resistivity of the g-As₂Te₃ glasses decreases exponentially with increasing pressure (at a pressure up to 2 GPa); then, it decreases faster by almost three orders of magnitude in the pressure range 2–3.5 GPa. At a pressure of 5 GPa, the electrical resistivity reaches 10^–3 Ω cm, which is characteristic of a metallic state; this resistivity continues to decrease with increasing pressure and reaches 1.7 × 10^–4 Ω cm at 8.1 GPa. The reverse metal–semiconductor transition occurs at a pressure of 3 GPa when pressure is relieved. When the pressure is decreased to atmospheric pressure, the electrical resistivity of the glasses is below the initial pressure by two–three orders of magnitude. Under normal conditions, both the volume and the electrical resistivity relax to quasi-equilibrium values in several months. Comparative structural and Raman spectroscopy investigations demonstrate that the glasses subjected to high pressure have the maximum chemical order. The glasses with a higher order have a lower electrical resistivity. The polyamorphism in the As₂Te₃ glasses is caused by both structural changes and chemical ordering. The g-As₂Te₃ compound is the first example of glasses, where the reversible metallization under pressure has been studied under hydrostatic conditions.     

Pressure induced phase transition in PbSnTe

Hall coefficient R_H and resistivity ? for Pb_0.56Sn_0.44Te crystal were measured as a function of temperature and pressure in the region of the ferroelectric phase transition. It enabled to obtain the pressure dependence of the phase transition temperature. Analysis of the experimental data showed that the lattice anharmonicity gives the dominant contribution to the phase transition mechanism.     

Compositionally controlled semimetal to superconducting transition in NaF doped LaOFeAs: Enhancement in Tc due to Na-doping

We report the synthesis of LaOFeAs based oxypnictide superconductors (La_1?xNa_xO_1?xF_xFeAs) using sodium fluoride as a fluorinating agent. NaF doping leads to a systematic decrease in both the a and c lattice parameters. Resistivity measurements show the onset resistivity transition temperature at 30.9 (±0.05) K and corresponding Meissner transition at 28 (±0.05) K in La_0.85Na_0.15O_0.85F_0.15FeAs which is highest in LaOFeAs type superconductors synthesized at ambient pressure. Further increase of NaF content in LaOFeAs leads to suppression of T_c. The above superconductors show a negative value of the Seebeck coefficient which indicates that electrons are the dominant charge carriers.     

Electrical transport and crystallization studies of glassy semiconducting Si20Te80 alloy at high pressure

The effect of pressure on the electrical resistivity of bulk Si₂₀Te₈₀ glass is reported. Results of calorimetric, X-ray and transmission electron microscopy investigations at different stages of crystallization of bulk Si₂₀Te₈₀ glass are also presented. A pressure induced glass-to-crystal transition occurs at a pressure of 7 GPa. Pressure and temperature dependence of the electrical resistivity of Si₂₀Te₈₀ glass show the observed transition is a pressure induced glassy semiconductor to crystalline metal transition. The glass also exhibits a double T_g effect and double stage crystallization, under heating. The differences between the temperature induced crystallization (primary crystallization) and pressure induced congruent crystallization are discussed.     

Equation of state and electrical resistivity of the heavy fermion superconductor CeCoIn<Subscript>5</Subscript> to 51 GPa

We have used X-ray diffraction to study the structural phase of CeCoIn₅ in external pressure. Using high-pressure X-ray diffraction, we find that the crystalline phase is stable in the P4/mmm phase for pressures ≤51.2 GPa. From our measured equation of state, we find a bulk modulus given by B ₀ = 72.8 ± 2.9 GPa and a first pressure derivative of B ^′ = 5.1 ± 0.3. Measurement of the electrical resistivity of CeCoIn₅ to pressures as high as 34.4 GPa shows the existence of a peak in resistivity at p ^? = 8.2 ± 0.2 GPa.     

Anomalous electrical properties of linear chain mercury compounds

The electrical resistivity of Hg_2.86AsF₆ has been studied as a function of temperature. At room temperature, the resistivity along the chain direction is 10^?4 Ω-cm with an anisotropy of about 10². This incommensurate linear chain system remains metallic at low temperatures with resistance ratio ?_ab(300 K)/ ?_ab(1.4 K) ? 3000 and still increasing with no apparent sign of residual resistivity. A large anisotropic magnetic field dependence of the resistivity is observed below 30 K. Near 4 K, the c-axis resistance drops abruptly more than three orders of magnitude, apparently to zero, while ?_ab is continuous. The c-axis transition is suppressed in a small magnetic field.     

Kinetic effects in manganites La1 − x Ag y MnO3 (y ≤ x)

We have measured the resistivity, magnetoresistance, and thermopower of ceramic manganite samples La_{1 ? x} Ag _y MnO₃ (y ≤ x) doped with silver as functions of temperature (4.2–350 K) and magnetic field (up to 26 kOe). A metal-insulator phase transition is observed in all investigated samples at temperatures close to room temperature. The behavior of the resistivity and thermopower in the high-temperature paramagnetic region is interpreted using the concept of small radius polaron; the activation energy decreases with increasing doping level. The resistivity in the low-temperature ferromagnetic region is approximated by the expression ρ_FM(T) = ρ₀ + AT ² + BT ^4.5 presuming the existence of electron-electron and electron-magnon interactions. A resistivity minimum and a strong magnetoresistive effect are observed at low temperatures. The latter effect is associated with scattering of charge carriers at grain boundaries, which are antiferromagnetically ordered relative to one another. The temperature dependence of thermopower in the magnetically ordered phase is described in the framework of a model taking into account the drag of charge carriers by magnons.     

Superconductivity of In-doped Sn0.62Pb0.33Ge0.05Te alloys

A study of the Hall and Seebeck coefficients and of resistivity has been carried out on an Sn_0.62Pb_0.33Ge_0.05Te alloy doped by 5 and 10 at. % In. A superconducting transition with the maximum critical temperature T _C～4 K has been discovered in samples with hole concentrations p≥1×10²¹ cm^?3. The dependence of T _C on hole concentration has been established to be of a threshold nature. The onset of superconductivity is accompanied practically simultaneously by a growth of the resistivity and a sharp drop of the Seebeck coefficient. These features in the experimental data indicate the existence of a band of In resonance states within the allowed valence-band spectrum and strong resonance hole scattering to impurity states. The threshold character of the T _C(p) dependence is connected with the holes filling the resonance states. A positive correlation between the resonance scattering intensity and the critical temperature is observed.     

Dependence of temperature-dependent electrical resistivity of SrIrO3 on hydrostatic pressure to 9.1 kbar

Non-Fermi-liquid behavior and close proximity to a quantum critical point in the 5d transition metal iridate SrIrO₃ at ambient pressure motivate our search for possible anomalous behavior in its transport properties under pressure. The electrical resistivity in the ab-plane of a single crystal of SrIrO₃ has been measured over the temperature range 1.35–285 K at both ambient and 9.1 kbar hydrostatic pressure. The resistivity decreases slightly over the entire temperature range, but no superconducting transition or changes in the non-Fermi-liquid behavior are observed under pressure. It is estimated that significantly higher pressures are likely required before sizable changes in the properties of SrIrO₃ will occur.