‘Electron collapse’ and the resistivity of liquid cesium

The electron collapse in cesium is pictured as the formation of a ‘virtual bound state’, the tunneling process accounting for the 6s-5d dynamic conversion. The anomalous variation of the resistivity of liquid cesium with pressure has been explained satisfactorily.     

Thermoelectric power of cerium at high pressures

The electronic phase transition in cerium occurring near 7 kbar pressure at room temperature which is attributed to the 4f–5d electron promotion has been studied using thermoelectric power as a tool. The important results that have emerged out of this work are: (a) the relatively large variation in the absolute thermoelectric power ofγ-cerium (normal fcc phase) with pressure prior to the phase transition (in contrast to the rather small resistivity change with pressure in this region); (b) a sharp decrease in the thermoelectric power accompanying the iso-structuralγ-α phase transition; and (c) the continuous decrease in the thermoelectric power ofα-cerium (collapsed fcc phase) with pressure, ultimately changing sign at higher pressures. An explanation based on the “virtual bound state” model is proposed to account for these results.     

Enhancement of thermoelectric performance with pressure in Ce0.8Fe3CoSb12.1

Transport properties (resistivity, thermal conductivity, and Seebeck coefficient) and sound velocities have been determined for the skutterudite Ce_0.8Fe₃CoSb_12.1 with pressure up to 14 GPa. From these measurements, high pressure anomalous features were found in all transport properties. By correlating these with results from previous x-ray work, it has been determined that there is likely an electronic topological transition in this material induced by pressure. This is possibly due to the known pressure variation of valence in the void-filling Ce atom and has been found to induce an improved figure of merit at higher pressures, which shows a nearly two-fold increase with applied pressure. At higher pressures, it was determined that this anomalous behavior is suppressed and is possibly induced by insertion of Sb from the cage into the remaining central voids of the structure, similar to that seen in the CoSb₃ parent compound.     

Relaxation of the normal electrical resistivity induced by high-pressure in strongly underdoped YBa2Cu3O7–δ single crystals

We investigate the relaxation of the normal electrical resistivity, induced by high-pressure in YBa₂Cu₃O_6.45 single crystals. It is determined that the pressure affects to the phase composition of the sample. Under pressure phases with different (but similar) critical temperatures form. It is determined that the application-removal pressure process is completely reversible. Above T_c the temperature dependence of the resistivity in the layers' plane at different hydrostatic pressures can be approximated with high accuracy with the scattering of electrons by phonons model. With increasing pressure, the residual resistance is reduced and the contribution of intraband s–s scattering increases. Additionally, the role of the interband s–d scattering and the Debye temperature is enhanced.     

Nonlinear simulations of peeling-ballooning modes with anomalous electron viscosity and their role in edge localized mode crashes

A minimum set of equations based on the peeling-ballooning (P-B) model with nonideal physics effects (diamagnetic drift, E×B drift, resistivity, and anomalous electron viscosity) is found to simulate pedestal collapse when using the new BOUT++ simulation code, developed in part from the original fluid edge code BOUT. Nonlinear simulations of P-B modes demonstrate that the P-B modes trigger magnetic reconnection, which leads to the pedestal collapse. With the addition of a model of the anomalous electron viscosity under the assumption that the electron viscosity is comparable to the anomalous electron thermal diffusivity, it is found from simulations using a realistic high-Lundquist number that the pedestal collapse is limited to the edge region and the edge localized mode (ELM) size is about 5%-10% of the pedestal stored energy. This is consistent with many observations of large ELMs.     

Enhancement of the anomalous Hall effect and spin glass behavior in the bilayered manganite La2−2xSr1+2xMn2O7

The Hall resistivity and magnetization have been investigated in the ferromagnetic state of the bilayered manganite La_2−2xSr_1+2xMn₂O₇ (x=0.36). The Hall resistivity shows an increase in both the ordinary and anomalous Hall coefficients at low temperatures below 50 K, a region in which experimental evidence for the spin glass state has been found in a low magnetic field of 1 mT. The origin of the anomalous behavior of the Hall resistivity relevant to magnetic states may lie in the intrinsic microscopic inhomogeneity in a quasi-two-dimensional electron system.     

Pressure-induced colossal piezoresistance effect and the collapse of the polaronic state in the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn2O7

We have investigated the effect of hydrostatic pressure as a function of temperature on the resistivity of a single crystal of the bilayer manganite (La(0.4)Pr(0.6))(1.2)Sr(1.8)Mn(2)O(7). Whereas a strong insulating behaviour is observed at all temperatures at ambient pressure, a clear transition into a metallic-like behaviour is induced when the sample is subjected to a pressure (P) of ~1.0 GPa at T < 70 K. A huge negative piezoresistance ~10(6) in the low temperature region at moderate pressures is observed. When the pressure is increased further (5.5 GPa), the high temperature polaronic state disappears and a metallic behaviour is observed. The insulator to metal transition temperature exponentially increases with pressure and the distinct peak in the resistivity that is observed at 1.0 GPa almost vanishes for P > 7.0 GPa. A modification in the orbital occupation of the e(g) electron between 3d(x(2)-y(2)) and 3d(z(2)-r(2)) states, as proposed earlier, leading to a ferromagnetic double-exchange phenomenon, can qualitatively account for our data.     

Isotropic graphite melting under pulsed heating

Experiments on pulsed heating (few microseconds) of graphite with measurements of the liquid carbon resistivity are described. It is confirmed that heating in water at atmospheric pressure do not allow production and study of liquid carbon; in the best case, the liquid state region beginning is achieved. Heating in sapphire tubes results in pulsed pressure (to ten of kbar) as expanding graphite bears against the tube wall. This increasing (during few microseconds) pressure makes it possible to study the carbon liquid state in a limited volume. Isochoric heating resulted in the possibilities ofmeasuring the liquid carbon resistivity at high specific energies (to ～32 kJ/g) and high pressures. Such measurements are extremely expensive at stationary studies.     

一种新的恶二唑衍生物在金刚石对顶砧中的电阻率随压力和温度的变化关系

利用金刚石对顶砧测量了恶二唑衍生物微晶, 1,4-bis[(4-methyloxyphenyl)-1,3,4-oxadiazolyl]- 2,5-bisheptyloxyphenylene (OXD-2), 电阻随压力和温度的变化关系，并利用有限元分析方法计算了样品的电阻率。实验中，测量压力和温度达到了16 GPa和150℃。样品的电阻率随着温度的升高而降低，说明样品表现出半导体传导特性。在90-100 ℃之间，样品的电阻率有一明显的下降，说明这时发生了温度诱导的相变。随着压力的增加，样品的电阻率在6GPa左右达到最大值，此后随着压力的增加而下降。结合原位x光数据，在6GPa左右的电阻突变应该是由于样品在压力的诱导下发生了无序化的相变。     

Material ablation and plasma state for single and collinear double pulses interacting with iron samples at ambient gas pressures below 1 bar

The influence of the ambient gas pressure on ablated material and the plasma state of laser-inducedplasmas using single and collinear double pulses is studied. A Q-switchedNd:YAG laser pulse is focused on iron samples in air at pressures ranging from 0.1 to 1013 mbar. At 1013 mbar double pulses ablate up to a factor of 4.6 more material than single pulses of the same energy. Whereas between 1013 mbar and 100 mbar a strong variation of the ablation ratio is found, the pressure dependence vanishes below 100 mbar. Electron densities in the plasma are always higher for single pulses and increase monotonously with higher ambient pressures. While the electron density for single and double pulses at 1013 mbar is close to the same value of about 10¹⁷ cm^-3, the electron temperature is more than 1500 K higher for double pulses. The plasma state achieved with double pulses at atmospheric pressure is similar to that obtained with single pulses of the same energy at a pressure of 100 mbar. Thus the effect of double pulses can be interpreted as a “laser-generated vacuum” in terms of the formation of a transient localized low particle density region in the interaction region. PACS 52.38.Mf; 78.47.+p; 42.62.Fi     

Anomalous Hall effect and electron transport in ferromagnetic MnBi films

The electron transport properties of highly c-axis oriented MnBi thin films of various thicknesses have been investigated. Samples are metallic but the low temperature resistivity shows an unusual T(3) dependence. Transverse Hall effect measurements show that both the ordinary and anomalous Hall coefficients decrease with decreasing temperature below 300 K, but the ordinary Hall coefficient (R(0)) undergoes a sign reversal around 105 K, where the magnetic anisotropy also changes sign. Analysis of the Hall data for various samples shows that the anomalous Hall coefficient (R(s)) exhibits a strong ρ(2) dependence, where ρ is the longitudinal resistivity.     

Pressure dependence of the resistivity of zinc oxide

The change of resistivity of n-type ZnO with hydrostatic pressure has been found to be 2.2 × 10^?6/kg cm^?2 and is nearly linear for pressures up to 28000 kg/cm². This result can be shown to be incompatible with a simple model of electron mobility which assumes that the scattering due to acoustic modes arises from a deformation potential associated with lattice dilatation.     

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.     

Novel synthesis of NixZn1?xFe2O4 (0?≤?x?≤?1) nanoparticles and their dielectric properties

Nanocrystalline Ni _x Zn_1−x Fe₂O₄ (0 ≤ x ≤ 1) ferrite powders with average particle size 15–20 nm have been successfully prepared at a very low temperature (180 °C) by a novel auto combustion process using citric acid and ethylenediamine as a coordinating agent and bridging ligand, respectively. Phase purity of the solid solutions has been confirmed by X-ray diffraction. Morphological characterizations of the prepared samples were performed by high resolution transmission electron and field emission scanning electron microscopy. Extensive Fourier transformed infrared spectroscopic characterization has been carried out to identify the plausible mechanism of the synthesis process. Composition-dependent electrical properties (resistivity and dielectric constant) of the synthesized solid solution have been investigated. Interestingly, a non-linear variation of dielectric permittivity with respect to composition has been observed. The room temperature electrical resistivity as well as the dielectric permittivity of Ni_0.5Zn_0.5Fe₂O₄ was found to decrease with the decrease of particle size.     

Electrical conductivity and thermal EMF of CsI at high pressures

Abstract

The pressure dependence of thermal EMF and the resistivity-temperature dependence of CsI has been measured at pressures 20-50 GPa. In CsI non-monotonous change of resistivity, thermal EMF and activation energy of charge carriers has been observed at pressures above 40 GPa. The sign of thermal EMF corresponds to the electron conductivity. At pressures below 47 GPa the resistivity-temperature dependence is of the type characteristic of non-degenerate semiconductors, at pressure above 49 GPa it is characteristic of degenerate semiconductors (or metals). The observed properties are connected probably with the continuous distortion of B2 to an hcp-like phase.     

Unexpected behaviour of YbCu4.5 at very high pressures

We report resistivity measurements of the heavy fermion compound YbCu_4.5 for pressures up to 23.5 GPa. Although the temperature dependence of the resistivity in general does not change compared to previous results at lower pressures, surprisingly the temperature of the Kondo resistivity maximum increases with pressure at pressures exceeding 12.5 GPa. At the highest pressure of 23.5 GPa the resistivity was measured down to 50 mK. No trace of magnetic order, i.e. any anomaly in the resistivity behaviour has been found at this highest pressure reached.     

Thermoelectric behaviour of SmS and Sm0.84Gd0.16S at high pressures and temperatures

The 4f→ 5d electronic phase transition in SmS has been studied using thermoelectric power as a probe. The variation of thermopower with pressure in Sm_0.84Gd_0.16S and in the high pressure phase of SmS is anomalous, characterized by a rather large pressure coefficient. The temperature coefficient of thermopower in Sm_0.84Gd_0.16S is large and negative at low pressures leading to a change of sign at higher temperatures. The anomalies can be understood on the basis of the ICF model.     

Resistivity changes of evaporated Ti films caused by sorption of O2, CO2 and H2

Resistivity changes of Ti films evaporated in UHV were measured during sorption of O₂, CO₂ and H₂ at very low pressures (p 1·6×10^–6–1·6×10^–5Pa). During the sorption process the pressure was kept constant. For O₂ and CO₂ an increase of the resistivity with a tendency to saturation was observed. For H₂ anomalous curves with a resistivity maximum and a subsequent decrease of the resistivity were obtained. Possible mechanisms of the processes are discussed. For the Ti/O₂ interaction a simple model describing the time dependence of the resistivity changes is proposed.We are indebted to dr. L. Pátý for consultations in vacuum technology and his permanent help in the experimental work and to dr. Z. Knor for a valuable discussion and for kindly extending us many references.     

Collapse of 5 f -Electron Ferromagnetism in UPtAl Under High Pressures

UPtAl exhibits a ferromagnetic ordering of U magnetic moments at temperatures below T C =42.5 K. The magnetic-ordering transition is accompanied by an anomaly in the temperature dependence of electrical resistivity. This allows us to determine the value of Curie temperature from 𝜌 vs. T data that can be measured at very high pressures, at which reliable magnetization measurements are difficult. We report on resistivity measurements performed on an UPtAl single crystal under hydrostatic pressures p h 8 GPa. It was observed that the initial increase of T C with p becomes gradually reduced for p >2 GPa until the maximum T C value of , 48 K is reached between 4 and 6 GPa that is followed by a progressively developing downturn of the T C ( p ) curve. The latter result is attributed to the approaching collapse of the U 5 f -moment ferromagnetism. Low-temperature resistivity data point to a rapidly reduced magnetic anisotropy at highest pressures.     

Electrical properties of thin films of Eu2O3 substituted compounds

Europium oxide (Eu₂O₃) substituted compound has been prepared by solid-solid reaction of the powders of Eu₂O₃, BaCO₃ and CuO at 950°C for 16 hours. The thin films have been deposited by high vacuum evaporation technique (vacuum ≈ 10⁻⁶ torr). The variation of current (I) with voltage (V) at room temperature (RT) i.e. 294 K and in ice (273 K) are found to be linear. The variation of electrical resistivity (ρ) with temperature (T) by heating the sample above RT has been determined. Resistivity is found to decrease with increase in temperature. Further the variation of electrical resistivity (ρ) with temperature (T) from 77 K, liquid nitrogen temperature (LNT), to 270 K has also been determined. It is observed that resistivity suddenly becomes zero at around 87 K. Thus the prepared material has superconducting properties with superconducting transition temperature, T _c at 87 K.