Normal-state Magnetoresistance Research Articles

Comparative study of Kondo effect in Vanadium dichalcogenides VX2 (X=Se & Te)

We report on the electrical transport, magnetotransport, and magnetic properties studies on the transition metal dichalcogenides VSe2 and VTe2 and draw a comprehensive comparison between them. We observe Kondo effect in both systems induced by the exchange interaction between localized moments and conduction electrons at low temperature, resulting into resistance upturn at 6 K for VSe2 and 17 K for VTe2. From the field dependent resistance measurements we find that the data is fitted best with modified Hamann equation corrected by the quantum Brillouin function for VSe2, while the data is fitted best with modified Hamann equation corrected by the classical Langevin function for VTe2. Interestingly, we observe a contrasting magnetoresistance (MR) property between these systems across the Kondo temperature. That means, negative MR is found in both systems in the Kondo state. In the normal state MR is positive for VSe2, while it is negligible for VTe2. In addition, both systems show weak ferromagnetism at low temperature due to intercalated V atoms.

Rashba spin-orbit coupling enhanced magnetoresistance in junctions with one ferromagnet

We explain how Rashba spin-orbit coupling (SOC) in a two-dimensional electron gas (2DEG), or in a conventional $s$-wave superconductor, can lead to a large magnetoresistance even with one ferromagnet. However, such enhanced magnetoresistance is not generic and can be nonmonotonic and change its sign with Rashba SOC. For an in-plane rotation of magnetization, it is typically negligibly small for a 2DEG and depends on the perfect transmission which emerges from a spin-parity-time symmetry of the scattering states, while this symmetry is generally absent from the Hamiltonian of the system. The key difference from considering the normal-state magnetoresistance is the presence of the spin-dependent Andreev reflection at superconducting interfaces. In the fabricated junctions of quasi-2D van der Waals ferromagnets with conventional $s$-wave superconductors (Fe$_{0.29}$TaS$_2$/NbN) we find another example of enhanced magnetoresistance where the presence of Rashba SOC reduces the effective interfacial strength and is responsible for an equal-spin Andreev reflection. The observed nonmonotonic trend in the out-of-plane magnetoresistance with the interfacial barrier is an evidence for the proximity-induced equal-spin-triplet superconductivity.

Transport properties and phase diagrams of FeSe1−S (0 ≤ x ≤ 1) single crystals

Based on the high quality FeSe1−xSx (0 ≤ x ≤ 1) single crystals synthesized via a hydrothermal method, we carried out systematic measurement of the normal state magnetoresistance and Hall effect. Through the maximum entropy mobility spectrum analysis (MEMSA), we investigate the evolution of mobility spectrum, which reflects the multi-band structures for different doping content x and temperature T. The relationships between Tc and carrier properties calculated by MEMSA, as well as bond angles and anion height calculated by first-principle method are investigated. Nonlinear to linear transition of Hall resistivity(ρxy(B)) is mainly attributed to the decreasing mobilities and merging of mobility peaks with the reductive x or increasing T. The relationship between Tc and carrier density n show weak interaction for the orthogonal phases when x ≤ 0.2, but nearly positive correlation for the tetragonal phases when x > 0.2. Bond angle as well as the Se/S anion height change almost linearly with x within the tetragonal phases, and the V-shaped relationships with Tc indicate that structural factors play a key role in the regulation of superconductivity. Our researches are helpful for understanding the multi-band and complex phase diagram of iron-based superconductors.

Observation of Coexisting Weak Localization and Superconducting Fluctuations in Strained Sn1-xInxTe Thin Films.

Topological superconductors have attracted tremendous excitement as they are predicted to host Majorana zero modes that can be utilized for topological quantum computing. Candidate topological superconductor Sn1-xInxTe thin films (0 < x < 0.3) grown by molecular beam epitaxy and strained in the (111) plane are shown to host quantum interference effects in the conductivity coexisting with superconducting fluctuations above the critical temperature Tc. An analysis of the normal state magnetoresistance reveals these effects. A crossover from weak antilocalization to localization is consistently observed in superconducting samples, indicating that superconductivity originates dominantly from charge carriers occupying trivial states that may be strongly spin-orbit split. A large enhancement of the conductivity is observed above Tc, indicating the presence of superconducting fluctuations. Our results motivate a re-examination of the debated pairing symmetry of this material when subjected to quantum confinement and lattice strain.

Extended Kohler’s Rule of Magnetoresistance

A notable phenomenon in topological semimetals is the violation of Kohler$^,$s rule, which dictates that the magnetoresistance $MR$ obeys a scaling behavior of $MR = f(H/\rho_0$), where $MR = [\rho_H-\rho_0]/\rho_0$ and $H$ is the magnetic field, with $\rho_H$ and $\rho_0$ being the resistivity at $H$ and zero field, respectively. Here we report a violation originating from thermally-induced change in the carrier density. We find that the magnetoresistance of the Weyl semimetal, TaP, follows an extended Kohler$^,$s rule $MR = f[H/(n_T\rho_0)]$, with $n_T$ describing the temperature dependence of the carrier density. We show that $n_T$ is associated with the Fermi level and the dispersion relation of the semimetal, providing a new way to reveal information on the electronic bandstructure. We offer a fundamental understanding of the violation and validity of Kohler$^,$s rule in terms of different temperature-responses of $n_T$. We apply our extended Kohler$^,$s rule to BaFe$_2$(As$_{1-x}$P$_x$)$_2$ to settle a long-standing debate on the scaling behavior of the normal-state magnetoresistance of a superconductor, namely, $MR$ ~ $tan^2\theta_H$, where $\theta_H$ is the Hall angle. We further validate the extended Kohler$^,$s rule and demonstrate its generality in a semiconductor, InSb, where the temperature-dependent carrier density can be reliably determined both theoretically and experimentally.

Robust superconductivity against the antiferromagnetic ordering in Pr2Pt3Ge5

The polycrystalline sample of Pr2Pt3Ge5 have been prepared successfully. The temperature and magnetic field dependence of resistivity as well as heat capacity has been carried out. The resistivity reveals the robustness of superconductivity against intrinsic antiferromagnetic ordering in the system. The onset of superconducting phase transition, TCon ≈ 6.7 K and the two antiferromagnetic transitions at TN1 ≈ 3.5 K and TN2 ≈ 4.3 K are almost similar to its single crystal counterpart. Important superconducting as well as normal state parameters have been estimated and discussed. Hc2(0) ≈ 2.5T as estimated using Werthamer-Helfand-Hohenberg (WHH) formalism while Hc2(0) ≈ 2.1T by using Ginzburg-Landau (GL) theory. The Hc2(0) value is quite high as compared to its single crystal counterpart. The reason could be the formation of pinning centers hitherto unknown. The broadening of resistivity curves with the increment of magnetic field has been analyzed using the thermally activated flux flow of vortices (TAFF) model whereas the zero field broadening is analyzed in terms of Aslamazov-Larkin (AL) predictions. From TAFF analysis the activation energy (U0) has been determined. The normal state magnetoresistance at different isothermal conditions doesn't obey the Kohler's rule, which reveals the possibility of an anisotropic Fermi surface and/or the multiband nature of the system. The specific heat measurement doesn't show substantiate jump at superconducting temperature similar to the single crystal counterpart. As we increase the magnetic field, the antiferromagnetic peaks get suppressed and completely vanishes around 10T.

Dimensional Crossover and Its Interplay with In-Plane Anisotropy of Upper Critical Field in β-(BDA-TTP)2SbF6

Resistance measurements have been performed to investigate the dimensionality and the in-plane anisotropy of the upper critical field (Hc2) for β-(BDA-TTP)2SbF6 in fields H up to 15 T and at temperatures T from 1.5 to 7.5 K, where BDA-TTP stands for 2,5-bis(1,3-dithian-2-ylidene)-1,3,4,6-tetrathiapentalene. The upper critical fields parallel and perpendicular to the conduction layer are determined and dimensional crossover from anisotropic three-dimensional behavior to two-dimensional behavior is found at around 6 K. When the direction of H is varied within the conducting layer at 6.0 K, Hc2 shows twofold symmetry: Hc2 along the minimum Fermi wave vector (maximum Fermi velocity) is larger than that along the maximum Fermi wave vector (minimum Fermi velocity). The normal-state magnetoresistance has twofold symmetry similar to Hc2 and shows a maximum when the magnetic field is nearly parallel to the maximum Fermi wave vector. This tendency is consistent with the Fermi surface anisotropy. At 3.5 K, we found ...

Phase-fluctuating superconductivity in overdoped La2−xSrxCuO4

Superconducting phase fluctuations are often associated with the pseudogap phase of the copper-oxide superconductors. However, the same fluctuations exist in the overdoped part of the phase diagram where the pseudogap is absent, suggesting that phase fluctuations are independent of the pseudogap In underdoped cuprate superconductors, phase stiffness is low and long-range superconducting order is destroyed readily by thermally generated vortices (and anti-vortices), giving rise to a broad temperature regime above the zero-resistive state in which the superconducting phase is incoherent1,2,3,4. It has often been suggested that these vortex-like excitations are related to the normal-state pseudogap or some interaction between the pseudogap state and the superconducting state5,6,7,8,9,10. However, to elucidate the precise relationship between the pseudogap and superconductivity, it is important to establish whether this broad phase-fluctuation regime vanishes, along with the pseudogap11, in the slightly overdoped region of the phase diagram where the superfluid pair density and correlation energy are both maximal12. Here we show, by tracking the restoration of the normal-state magnetoresistance in overdoped La2−xSrxCuO4, that the phase-fluctuation regime remains broad across the entire superconducting composition range. The universal low phase stiffness is shown to be correlated with a low superfluid density1, a characteristic of both underdoped and overdoped cuprates12,13,14. The formation of the pseudogap, by inference, is therefore both independent of and distinct from superconductivity.

Superconductivity and normal state magnetoresistance in superconducting FeSe:Sb

We prepared a series of β-FeSe samples with a nominal composition of Fe1.11Se1−x Sb x (0⩽x⩽0.5). The X-ray diffraction, transport and magnetic measurements were performed on these samples to investigate the structure, the superconducting properties and the normal state transport and magnetic properties. Although the X-ray diffraction data suggested that Sb atoms were not incorporated into the β-FeSe phase, the transport data showed observable changes of superconductivity, normal state resistivity and magnetoresistance. This was represented by the increase in the superconducting transition temperature and the upper critical field. Also, for the samples with a low level of Sb content, a clear decrease of the normal state resistivity and a substantial increase of the residual resistance ratio were observed. Furthermore, the samples showed a significant increase of the normal state magnetoresistance that appeared not to follow the Kohler’s rule. The results were discussed in the frame of reduction of excess Fe at interstitial sites of β-FeSe.

The magnetoresistance of a PrFeAsO1−xFy superconductor

We have measured the temperature and magnetic field dependence of resistivity of polycrystallinePrFeAsO0.60F0.12 samples in the normal and superconducting state. The superconducting onset transition temperatureTcon shiftsby ∼4 Kas H is increased from 0 to 14 T. The zero-temperature upper critical fieldHc2(0) estimated by using the Ginzburg–Landau theory and theWerthamer–Helfand–Hohenberg equation exceeds 100 T. The resistivity belowTcon exhibits Arrhenius behavior due to thermally activated flux flow of vortices. The activation energyU0, determined from the Arrhenius plot of the resistivity, shows a power-law decrease () with magnetic field. The normal-state magnetoresistance varies quasilinearly withmagnetic field and does not obey Kohler’s rule, suggesting a multiband feature of thematerial.

Magnetoresistance and Hall effect in e-doped superconducting SrLaCuO thin films

We have epitaxially grown c-axis oriented SrxLa1-xCuO2 thin films by rf sputtering on KTaO3 substrates with χ = 0.12. The as-grown deposits are insulating and a series of superconducting films with various Tc(ρ=0) up to 26 K have been obtained by in-situ oxygen reduction. Transport measurements in the ab plane of these samples have been undertaken. We report original results on the temperature dependence of the Hall effect and on the anisotropic magnetoresistance (T ≥ Tc). We discuss the magnitude of upper critical fields and anisotropy, the Hall effect, which presents changes of sign indicative of the existence of two types of carriers, the normal state magnetoresistance, negative in parallel magnetic field, a possible signature of spin scattering. These properties are compared to those of hole-doped cuprates, such as BiSr(La)CuO with comparable Tc.

Normal-state Nernst effect in electron-dopedPr2−xCexCuO4−δ: Superconducting fluctuations and two-band transport

We report a systematic study of normal state Nernst effect in the electron-doped cuprates Pr$_{2-x}$Ce$_x$CuO$_{4-\delta}$ over a wide range of doping (0.05$\leq x \leq$0.21) and temperature. At low temperatures, we observed a notable vortex Nernst signal above T$_c$ in the underdoped films, but no such normal state vortex Nernst signal is found in the overdoped region. The superconducting fluctuations in the underdoped region are most likely incoherent phase fluctuations as found in hole-doped cuprates. At high temperatures, a large normal state Nernst signal is found at dopings from slightly underdoped to highly overdoped. Combined with normal state thermoelectric power, Hall effect and magnetoresistance measurements, the large Nernst effect is compatible with two-band model. For the highly overdoped films, the large Nernst effect is anomalous and not explainable with a simple hole-like Fermi surface seen in photoemission experiments.

Total Suppression of Superconductivity by High Magnetic Fields inYBa2Cu3O6.6

We have studied the variation of transverse magnetoresistance of underdoped YBCO(6.6) crystals, either pure or with reduced T(c) down to 3.5 K by electron irradiation, in fields up to 60 T. We find evidence that the superconducting fluctuation contribution to the conductivity is suppressed only above a threshold field H(c)'(T), which is found to vanish at T(c)' > T(c). In the pure YBCO(6.6) sample, H(c)' is already 50 T at T(c). We find that increasing disorder weakly depresses H(c)'(0), T(c)', and T(nu), the onset of the Nernst signal. Thus, these energy scales appear more characteristic of the 2D local pairing than the pseudogap temperature which is not modified by disorder.

Large Anisotropic Normal-State Magnetoresistance in CleanMgB2Thin Films

We report a large normal-state magnetoresistance with temperature-dependent anisotropy in very clean epitaxial MgB2 thin films (residual resistivity much smaller than 1 microOmega cm) grown by hybrid physical-chemical vapor deposition. The magnetoresistance shows a complex dependence on the orientation of the applied magnetic field, with a large magnetoresistance (Delta(rho)/(rho)0=136%) observed for the field H perpendicular ab plane. The angular dependence changes dramatically as the temperature is increased, and at high temperatures the magnetoresistance maximum changes to H||ab. We attribute the large magnetoresistance and the evolution of its angular dependence with temperature to the multiple bands with different Fermi surface topology in MgB2 and the relative scattering rates of the sigma and pi bands, which vary with temperature due to stronger electron-phonon coupling for the sigma bands.

TRANSPORT IN MgB2 IN HIGH MAGNETIC FIELDS

Magnetotransport measurements are presented on polycrystalline MgB 2 samples. The 'resistive' upper critical magnetic field reveals a temperature dependence with positive curvature from Tc = 39.3 K down to about 20 K, then changes to slightly negative curvature reaching 26 T at 1.5 K. The 26-Tesla upper critical field is much higher than what is known so far on polycrystals of MgB 2 but it is in agreement with the recent data obtained on epitaxial MgB 2 films.1 The deviation of Hc2(T) from standard BCS might be due to the proposed two-gap superconductivity in this compound.2 Recently we have found evidence for the existence of two superconducting energy gaps in this system using Andreev reflection spectroscopy.3 The temperature dependence of the resistivity and the normal state magnetoresistance are analyzed.

Magnetotransport and the upper critical magnetic field in MgB 2

Magnetotransport measurements are presented on polycrystalline MgB 2 samples. The `resistive' upper critical magnetic field reveals a temperature dependence with a positive curvature from T c=39.3 K down to about 20 K, then changes to a slightly negative curvature reaching 25 T at 1.5 K. The 25-T upper critical field is much higher than what is known so far on polycrystals of MgB 2 but it is in agreement with recent data obtained on epitaxial MgB 2 films. The deviation of B c2( T) from standard BCS might be due to the proposed two-gap superconductivity in this compound. The observed quadratic normal-state magnetoresistance with validity of Kohler's rule can be ascribed to classical trajectory effects in the low-field limit.

Is the c-Axis Charge Transport in Tl2Ba2CuO6 Consistent with the Cold-Spot Model?

We show that the experimentally observed temperature dependence of the resistivity anisotropy in overdoped Tl2Ba2CuO6 is in qualitative agreement with the predictions of the cold-spot model. However, the angular dependence of the c-axis normal state magnetoresistance in the same compound cannot be described within the cold-spot model.

Superconductivity and Hall effect of YBa/sub 2/Cu/sub 3/O/sub y//La/sub 0.7/Ca/sub 0.3/MnO/sub y/ multilayers

We measured the transport properties of a-axis oriented YBa/sub 2/Cu3O/sub y//La/sub 0.7/Ca/sub 0.3/MnO/sub y/ (YBCO/LCMO) multilayers to study the effects of layer coupling on the superconductivity, magnetoresistance and anomalous Hall effects. The decrease of flux pinning in the YBCO/LCMO multilayer causes the diminishing of sign reversal of Hall coefficients. The normal state magnetoresistance was found to depend on the coupling strength of ferromagnetic layers.

Negative magnetoresistance and Hc2 of Bi 2Sr 2CaCu 2O 8

The out-of-plane resistance of BSCCO-2212 single crystals with T c0=91–93 K has been measured in magnetic fields up to 50 T over a wide temperature range. The results are characterized by a positive linear magnetoresistance in the superconducting state and a negative linear magnetoresistance in the normal state. We show that the zero-field normal state c-axis resistance, the negative linear normal state magnetoresistance and the divergent upper critical field, H c2, may be understood within the framework of the bipolaron theory of superconductivity.

Melting of the vortex lattice inYBa2Cu4O8in parallel fields

The mixed state c-axis resistivity of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ has been measured with the magnetic field applied parallel to the a, b, and c axes. For all orientations of the magnetic field, a ``kink'' is observed in the resistive transition, associated with the first-order melting of an anisotropic three-dimensional vortex lattice. While the melting lines for $H\ensuremath{\parallel}b$ and $H\ensuremath{\parallel}c$ obey the expected relation ${H}_{m}{=H}_{0}(1\ensuremath{-}{T/T}_{c}{)}^{n}$ with $n=1.5, {H}_{m}(T)$ for $H\ensuremath{\parallel}a$ follows a different T dependence with a lower exponent. This result is consistent with a reduction in the dimensionality of ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{4}{\mathrm{O}}_{8}$ in this field geometry, as observed in normal-state magnetoresistance measurements.