Dome-shaped magnetic phase diagram of thermoelectric layered cobaltites

Using muon spin spectroscopy we have found that, for both NaxCoO(2) (0.6相似文献   

Galvanomagnetic properties of polycrystalline manganese selenide Gd<Subscript>0.2</Subscript>Mn<Subscript>0.8</Subscript>Se

The electrical and galvanomagnetic properties of the Gd_0.2Mn_0.8Se solid solutions are investigated in zero magnetic field and in a field of 13 kOe in the temperature range of 80–400 K. The negative magnetoresistance below room temperature and hysteresis of the I–V characteristics are found. The change in the magnetoresistance sign and thermopower with increasing temperature is established. The carrier type is determined from the Hall constant; the difference between the thermopower and Hall coefficient signs at high temperatures is established. The experimental data are explained using the model of orbital ordering and spin-orbit interaction.     

Anomalous quasiparticle renormalization in Na0.73CoO2: role of interorbital interactions and magnetic correlations

We report an angular resolved photoemission study of NaxCoO2 with x approximately 0.73 where it is found that the renormalization of the quasiparticle (QP) dispersion changes dramatically upon a rotation from GammaM to GammaK. The comparison of the experimental data to the calculated band structure reveals that the quasiparticle renormalization is most pronounced along the GammaK direction, while it is significantly weaker along the GammaM direction. We discuss the observed anisotropy in terms of multiorbital effects and point out the relevance of magnetic correlations for the band structure of NaxCoO2 with x approximately 0.75.     

Magnetic anomalies in Gd2PdSi3

The results of ac and dc magnetic susceptibility, thermopower and Hall effect measurements of a compound, Gd₂PdSi₃, establish that this compound orders magnetically below T_n = 20 K. Though the ordering appears to be of an antiferromagnetic-type, the paramagnetic Curie temperature is positive with the magnitude being nearly the same as that of T_N, suggestive of the existence of ferromagnetic correlations. The thermopower at 300 K is large, apparently due to Pd 4d electrons, decreasing monotonically with temperature. There is a change in the sign of Hall constant well below T_N Also considering the observation of Kondo-like characteristics above 21K earlier by us, the overall thermal, transport and magnetic behaviour of this compound is interesting.     

Electrical and thermal transport properties of dilute Pr in Pd

We have measured susceptibility, electrical resistivity thermopower, thermal conductivity, specific heat, thermal lattice expansion and Hall effect of dilute alloys of Pr (x = 0.014, 0.03) and of Lu (x = 0.014) in Pd. We find a rather strong valence instability at room temperature from L_III X-ray absorption (v = 3.1), which manifests itself by a concentration-independent characteristic temperature, by a resistivity minimum and by a strong anomally of the thermopower. The susceptibility also indicates strong magnetic polarisation of the surrounding Pd-atoms through the excited crystal field levels but not through the ground state of the impurity. The neutron scattering shows several sharp crystal field excitations between 0 and 1 meV and a very broad quasi-elastic scattering at higher temperatures with the 4f-form factor. We take this also as evidence for a weak coupling of valence instabilities to the low lying crystal field levels of the impurity and a strong coupling to the higher ones.     

Magnetic properties of spin-orbital polarons in lightly doped cobaltates

We present a numerical treatment of a spin-orbital-polaron model for NaxCoO2 at small hole concentration (0.7 < x 1). We demonstrate how the polarons account for the peculiar magnetic properties of this layered compound: They explain the large susceptibility; their internal degrees of freedom lead both to a negative Curie-Weiss temperature and yet to a ferromagnetic intralayer interaction, thereby resolving a puzzling contradiction between these observations. We make specific predictions on the momentum and energy location of excitations resulting from the internal degrees of freedom of the polaron, and discuss their impact on spin-wave damping.     

Hall coefficient and conduction mechanism in intermediate concentration n-Ge at helium temperatures

Hall coefficient measurements for intermediate concentration n-type Ge were carried out at liquid helium temperatures. The measurements show that the Hall coefficient and mobility increase with decreasing temperature down to 1.7 K and with increasing magnetic field up to 25 KG. These behaviours are opposite to what was observed in low concentration samples. We conclude that the thermal activated localised hopping motion does not exist in our concentration level, 6 × 10¹⁶ cm^?3, but rather the delocalised quasi-free carriers still dominate the overall conduction for temperature as low as 1.7 K. A model is suggested to explain the Hall mobility behaviour. The model based on the decrease of the dominant scattering mechanism, ionised impurity scattering in our case, as the temperature is lowered and when the magnetic field is increased. From the Hall coefficient behaviour at 4.2 and 1.7 K as well as the resistivity measurements, we found no effect of magnetic field on the unique activation energy existing in this concentration level.     

Single crystal growth,structural and transport properties of bad metal RhSb_2

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 10¹⁸ cm^-3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb_2. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.     

Hall effect in the normal phase of the organic superconductor (TMTSF)2PF6

We report accurate Hall effect measurements performed in the normal phase of the quasi-one-dimensional organic conductor (TMTSF)2PF6 at ambient pressure. The Hall coefficient is found to be strongly temperature dependent all the way from 300 K down to the spin density wave onset arising around 12 K. These new results emphasize the existence of a high temperature regime above 130 K where the Fermi liquid model is not satisfactory.     

Magnetotransport and superconductivity of α-uranium

We have measured the electrical resistivity, magnetoresistance and Hall effect on several new single-crystal samples and one polycrystalline sample of α-U. The residual resistivity ratios of these samples vary from 13 to 315. Matthiessen's law appears to hold above the onset of the charge-density wave phase transitions that begin near 43?K, but not below this temperature. Sharp features at all three charge-density wave transitions are observed and the effects of high magnetic fields on them are presented and discussed. The magnetoresistance is anisotropic, reaches 1000% at 2?K and 18?T and does not exhibit Kohler scaling. The Hall coefficient is positive, independent of magnetic field and slightly temperature dependent above about 40?K in agreement with earlier studies. Below 40?K the Hall coefficient changes sign as the temperature falls, varies with field and becomes much more strongly negative at the lowest temperatures than has been reported. Some of our results suggest that a spin-density wave may coexist with the charge-density wave states. Superconductivity is observed in two of our samples; we argue that it is intrinsic to α-U and suggest that it is consistent with a two-band model. Several parameters characterizing the transport and superconductivity of α-U are estimated.     

23Na NMR evidence for charge order and anomalous magnetism in NaxCoO2

Oriented powder samples of NaxCoO2 are studied by 23Na NMR and SQUID magnetometry. In nominal 0.50相似文献   

Cascade of bulk magnetic phase transitions in NaxCoO2 as studied by muon spin rotation

Using muon spin rotation, well-defined bulk approximately 100% magnetic phases in NaxCoO2 are revealed. A novel magnetic phase is detected for x=0.85 with the highest transition temperature ever observed for x>or=0.75. This stresses the diversity of x>or=0.75 magnetic phases and the link between magnetic and structural degrees of freedom. For the charge-ordered x=0.50 compound, a cascade of transitions is observed below 85 K. From a detailed analysis of our data, we conclude that the ordered moment varies continuously with temperature and suggest that the two secondary transitions at 48 and 29 K correspond to a moderate reorientation of antiferromagnetically coupled moments.     

The Hall effect in the organic conductor TTF-TCNQ: choice of geometry for accurate measurements of a highly anisotropic system

We have measured the Hall effect on recently synthesized single crystals of the quasi-one-dimensional organic conductor TTF-TCNQ (tetrathiafulvalene-tetracyanoquinodimethane), a well known charge transfer complex that has two kinds of conductive stacks: the donor (TTF) and the acceptor (TCNQ) chains. The measurements were performed in the temperature interval 30 K < T < 300 K and for several different magnetic field and current directions through the crystal. By applying the equivalent isotropic sample approach, we have demonstrated the importance of the choice of optimal geometry for accurate Hall effect measurements. Our results show, contrary to past belief, that the Hall coefficient does not depend on the geometry of measurements and that the Hall coefficient value is approximately zero in the high temperature region (T > 150 K), implying that there is no dominance of either the TTF or the TCNQ chain. At lower temperatures our measurements clearly prove that all three phase transitions of TTF-TCNQ could be identified from Hall effect measurements.     

Sr2RuO4的热电势

Sr2RuO4是第一个无CuO面的层状强关联氧化物超导体.测量了9至260K温度范围内Sr2RuO4的热电势,观测到在此温度范围内其热电势为正值.用两种载流子模型对实验数据进行了拟合,并且与Hall系数的实验结果进行了比较,发现低温下两种载流子对热电势和Hall系数的贡献比较类似,但在高温区空穴对热电势的贡献很大而相应地对Hall系数的贡献不占主要地位. 关键词： Sr2RuO4 热电势     

Finite-temperature density instability at high landau level occupancy

We study here the onset of charge density wave instabilities in quantum Hall systems at finite temperature for Landau level filling nu>4. Specific emphasis is placed on the role of disorder as well as on an in-plane magnetic field. Beyond some critical value, disorder is observed to suppress the charge density wave melting temperature to zero. In addition, we find that a transition from perpendicular to parallel stripes (relative to the in-plane magnetic field) exists when the electron gas thickness exceeds approximately 60 A. The perpendicular alignment of the stripes is in agreement with the experimental finding that the easy conduction direction is perpendicular to the in-plane field.     

Physical properties of Cr5−XVXSi3 Solid solutions at low temperatures

Conclusions We have studied the temperature dependence of the electrical resistivity, absolute thermo-emf, magnetic susceptibility, and Hall coefficient of Cr_5–XV_XSi₃ solid solutions over the temperature range 78–300K. The results suggest that all the samples studied are Pauli paramagnetics, their conductivity arising from both holes and electrons.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, Vol. 16, No. 7, pp. 134–136, July, 1973.     

Thermopower for a molecule with vibrational degrees of freedom

We study a simple model to describe resonant tunneling through an organic molecule between two conducting leads, taking into account the vibrational modes of the molecule. We solve the model approximately analytically in the weak coupling limit and give explicit expressions for the thermopower and Seebeck coefficient. The behavior of these two quantities is studied as function of model parameters and temperature. For a certain regime of parameters a rather peculiar variation of the thermopower and Seebeck coefficient is observed.     

Bulk band gap and surface state conduction observed in voltage-tuned crystals of the topological insulator Bi2Se3

We report a transport study of exfoliated few monolayer crystals of topological insulator Bi2Se3 in an electric field effect geometry. By doping the bulk crystals with Ca, we are able to fabricate devices with sufficiently low bulk carrier density to change the sign of the Hall density with the gate voltage V(g). We find that the temperature T and magnetic field dependent transport properties in the vicinity of this V(g) can be explained by a bulk channel with activation gap of approximately 50 meV and a relatively high-mobility metallic channel that dominates at low T. The conductance (approximately 2×7e2/h), weak antilocalization, and metallic resistance-temperature profile of the latter lead us to identify it with the protected surface state. The relative smallness of the observed gap implies limitations for electric field effect topological insulator devices at room temperature.     

Magnetic ordering and spin waves in Na0.82CoO2

NaxCoO2, the parent compound of the recently synthesized superconductor Na(x)CoO(2):yH(2)O, exhibits bulk antiferromagnetic order below approximately 20 K for 0.75相似文献   

Evidence of diffusion regions at a subsolar magnetopause crossing

On 1 April 2001, the Polar satellite crossed a subsolar magnetopause associated with antiparallel magnetic fields. Over a width approximately 6 magnetosheath ion skin depths (approximately 3 magnetospheric ion skin depths), perpendicular ion flows different from E x B/B(2) as well as Hall magnetic and electric field signatures were observed. At a smaller scale, the electron flow decoupled from the magnetic field near a deep minimum in the magnetic field strength. Separatrices were identified as boundaries of low frequency electric field turbulence associated with density minima and parallel electric fields. The reconnection rate was less than 2% of the asymptotic Alfvén speed.