arXiv subjects

Q. Niu

Publications and source records attributed to Q. Niu.

At least 19 recordsLinked to original sources

arXiv2020

Stabilization of antiferromagnetism in 1T-Fe$_{0.05}$TaS$_2$

1T-TaS$_2$ is a prototypical charge-density-wave (CDW) system with a Mott insulating ground state. Usually, a Mott insulator is accompanied by an antiferromagnetic state. However, the antiferromagnetic order had never been observed in 1T-TaS$_2$. Here, we report the stabilization of the antiferromagnetic order by the intercalation of a small amount of Fe into the van der Waals gap of 1T-TaS$_2$, i.e. forming 1T-Fe$_{0.05}$TaS$_2$. Upon cooling from 300~K, the electrical resistivity increases with a decreasing temperature before reaching a maximum value at around 15~K, which is close to the Neel temperature determined from our magnetic susceptibility measurement. The antiferromagnetic state can be fully suppressed when the sample thickness is reduced, indicating that the antiferromagnetic order in Fe$_{0.05}$TaS$_2$ has a non-negligible three-dimensional character. For the bulk Fe$_{0.05}$TaS$_2$, a comparison of our high pressure electrical transport data with that of 1T-TaS$_2$ indicates that, at ambient pressure, Fe$_{0.05}$TaS$_2$ is in the nearly commensurate charge-density-wave (NCCDW) phase near the border of the Mott insulating state. The temperature-pressure phase diagram thus reveals an interesting decoupling of the antiferromagnetism from the Mott insulating state.

cond-mat.str-el↗

arXiv2019

Nonsaturating large magnetoresistance in the high carrier density nonsymmorphic metal CrP

The band structure of high carrier density metal CrP features an interesting crossing at the Y point of the Brillouin zone. The crossing, which is protected by the nonsymmorphic symmetry of the space group, results in a hybrid, semi-Dirac-like energy-momentum dispersion relation near Y. The linear energy-momentum dispersion relation along Y-$Γ$ is reminiscent of the observed band structure in several semimetallic extremely large magnetoresistance (XMR) materials. We have measured the transverse magnetoresistance of CrP up to 14 T at temperatures as low as $\sim$ 16 mK. Our data reveal a nonsaturating, quadratic magnetoresistance as well as the behaviour of the so-called `turn-on' temperature in the temperature dependence of resistivity. Despite the difference in the magnitude of the magnetoresistance and the fact that CrP is not a semimetal, these features are qualitatively similar to the observations reported for XMR materials. Thus, the high-field electrical transport studies of CrP offer the prospect of identifying the possible origin of the nonsaturating, quadratic magnetoresistance observed in a wide range of metals.

cond-mat.str-el↗

arXiv2018

Anisotropic two-gap superconductivity and the absence of a Pauli paramagnetic limit in single-crystalline LaO$_{0.5}$F$_{0.5}$BiS$_2$

Ambient-pressure-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$ with a superconducting transition temperature $T_{c}\sim$3K possesses a highly anisotropic normal state. By a series of electrical resistivity measurements with a magnetic field direction varying between the crystalline $c$-axis and the $ab$-plane, we present the first datasets displaying the temperature dependence of the out-of-plane upper critical field $H_{c2}^{\perp}(T)$, the in-plane upper critical field $H_{c2}^{\parallel}(T)$, as well as the angular dependence of $H_{c2}$ at fixed temperatures for ambient-pressure-grown LaO$_{0.5}$F$_{0.5}$BiS$_2$ single crystals. The anisotropy of the superconductivity, $H_{c2}^{\parallel}/H_{c2}^{\perp}$, reaches $\sim$16 on approaching 0 K, but it decreases significantly near $T_{c}$. A pronounced upward curvature of $H_{c2}^{\parallel}(T)$ is observed near $T_{c}$, which we analyze using a two-gap model. Moreover, $H_{c2}^{\parallel}(0)$ is found to exceed the Pauli paramagnetic limit, which can be understood by considering the strong spin-orbit coupling associated with Bi as well as the breaking of the local inversion symmetry at the electronically active BiS$_2$ bilayers. Hence, LaO$_{0.5}$F$_{0.5}$BiS$_2$ with a centrosymmetric lattice structure is a unique platform to explore the physics associated with local parity violation in the bulk crystal.

cond-mat.supr-con↗

arXiv2017

Nearly isotropic superconductivity in layered Weyl semimetal WTe$_2$ at 98.5 kbar

Layered transition metal dichalcogenide WTe$_2$ has recently attracted significant attention due to the discovery of an extremely large magnetoresistance, a predicted type-II Weyl semimetallic state, and the pressure-induced superconducting state. By a careful measurement of the superconducting upper critical fields as a function of the magnetic field angle at a pressure as high as 98.5 kbar, we provide the first detailed examination of the dimensionality of the superconducting condensate in WTe$_2$. Despite the layered crystal structure, the upper critical field exhibits a negligible field anisotropy. The angular dependence of the upper critical field can be satisfactorily described by the anisotropic mass model from 2.2 K ($T/T_c\sim0.67$) to 0.03 K ($T/T_c\sim0.01$), with a practically identical anisotropy factor $γ\sim1.7$. The temperature dependence of the upper critical field, determined for both $H\perp ab$ and $H\parallel ab$, can be understood by a conventional orbital depairing mechanism. Comparison of the upper critical fields along the two orthogonal field directions results in the same value of $γ\sim1.7$, leading to a temperature independent anisotropy factor from near $T_c$ to $<0.01T_c$. Our findings thus identify WTe$_2$ as a nearly isotropic superconductor, with an anisotropy factor among one of the lowest known in superconducting transition metal dichalcogenides.

cond-mat.supr-con↗

arXiv2017

Maximizing Tc by tuning nematicity and magnetism in FeSe1-xSx superconductors

A fundamental issue concerning iron-based superconductivity is the roles of electronic nematicity and magnetism in realising high transition temperature ($T_{\rm c}$). To address this issue, FeSe is a key material, as it exhibits a unique pressure phase diagram involving nonmagnetic nematic and pressure-induced antiferromagnetic ordered phases. However, as these two phases in FeSe overlap with each other, the effects of two orders on superconductivity remain perplexing. Here we construct the three-dimensional electronic phase diagram, temperature ($T$) against pressure ($P$) and isovalent S-substitution ($x$), for FeSe$_{1-x}$S$_{x}$, in which we achieve a complete separation of nematic and antiferromagnetic phases. In between, an extended nonmagnetic tetragonal phase emerges, where we find a striking enhancement of $T_{\rm c}$. The completed phase diagram uncovers two superconducting domes with similarly high $T_{\rm c}$ on both ends of the dome-shaped antiferromagnetic phase. The $T_{\rm c}(P,x)$ variation implies that nematic fluctuations unless accompanying magnetism are not relevant for high-$T_{\rm c}$ superconductivity in this system.

cond-mat.supr-con↗

arXiv2017

Weakening of the diamagnetic shielding in FeSe$_{1-x}$S$_x$ at high pressures

The superconducting transition of FeSe$_{1-x}$S$_x$ with three distinct sulphur concentrations $x$ was studied under hydrostatic pressure up to $\sim$70 kbar via bulk AC susceptibility. The pressure dependence of the superconducting transition temperature ($T_c$) features a small dome-shaped variation at low pressures for $x=0.04$ and $x=0.12$, followed by a more substantial $T_c$ enhancement to a value of around 30 K at moderate pressures. In $x=0.21$, a similar overall pressure dependence of $T_c$ is observed, except that the small dome at low pressures is flattened. For all three concentrations, a significant weakening of the diamagnetic shielding is observed beyond the pressure around which the maximum $T_c$ of 30 K is reached near the verge of pressure-induced magnetic phase. This observation points to a strong competition between the magnetic and high-$T_c$ superconducting states at high pressure in this system.

cond-mat.supr-con↗

arXiv2016

Quasilinear quantum magnetoresistance in pressure-induced nonsymmorphic superconductor CrAs

In conventional metals, modification of electron trajectories under magnetic field gives rise to a magnetoresistance that varies quadratically at low field, followed by a saturation at high field for closed orbits on the Fermi surface. Deviations from the conventional behaviour, e.g. the observation of a linear magnetoresistance, or a non-saturating magnetoresistance, have been attributed to exotic electron scattering mechanisms. Recently, linear magnetoresistance has been observed in many Dirac materials, in which the electron-electron correlation is relatively weak. The strongly correlated helimagnet CrAs undergoes a quantum phase transition to a nonmagnetic superconductor under pressure. Near the magnetic instability, we observe a large and non-saturating quasilinear magnetoresistance from the upper critical field to 14 T at low temperatures. We show that the quasilinear magnetoresistance arises from an intricate interplay between a nontrivial band crossing protected by nonsymmorphic crystal symmetry and strong magnetic fluctuations

cond-mat.supr-con↗

arXiv2014

Anomalous Nernst and Hall effects in magnetized platinum and palladium

We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in proximity-induced ferromagnetic palladium and platinum which is widely used in spintronics, within the Berry phase formalism based on the relativistic band structure calculations. We find that both the anomalous Hall ($σ_{xy}^A$) and Nernst ($α_{xy}^A$) conductivities can be related to the spin Hall conductivity ($σ_{xy}^S$) and band exchange-splitting ($Δ_{ex}$) by relations $σ_{xy}^A =Δ_{ex}\frac{e}{\hbar}σ_{xy}^S(E_F)'$ and $α_{xy}^A = -\frac{π^2}{3}\frac{k_B^2TΔ_{ex}}{\hbar}σ_{xy}^s(μ)"$, respectively. In particular, these relations would predict that the $σ_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since the $σ_{xy}^S(E_F)'$ is positive (negative). Furthermore, both $σ_{xy}^A$ and $α_{xy}^A$ are approximately proportional to the induced spin magnetic moment ($m_s$) because the $Δ_{ex}$ is a linear function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic platinum and palladium would be gigantic, being up to ten times larger than, e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be significant.

cond-mat.mtrl-sci↗

arXiv2006

Coordinate shift in the semiclassical Boltzmann equation and the anomalous Hall effect

We propose a gauge invariant expression for the side jump associated with scattering between particular Bloch states. Our expression for the side jump follows from the Born series expansion for the scattering T-matrix in powers of the strength of the scattering potential. Given our gauge invariant side jump expression, it is possible to construct a semiclassical Boltzmann theory of the anomalous Hall effect which expresses all previously identified contributions in terms of gauge invariant quantities and does not refer explicitly to off-diagonal terms in the density-matrix response.

cond-mat.mes-hall↗

arXiv2004

Non-vanishing spin Hall currents in disordered spin-orbit coupling systems

Spin currents that flow perpendicular to the electric field direction are generic in metals and doped semiconductors with spin-orbit coupling. It has recently been argued that the spin Hall conductivity can be dominated by an intrinsic contribution which follows from Bloch state distortion in the presence of an electric field. Here we report on an numerical demonstration of the robustness of this effect in the presence of disorder scattering for the case of a two-dimensional electron-gas with Rashba spin-orbit interactions (R2DES).

cond-mat.mes-hall↗

arXiv2004

Semiclassical theory of spin transport in spin-orbit coupled systems

Motivated by recent interest in novel spintronics effects, we develop a semiclassical theory of spin transport that is valid for spin-orbit coupled bands. Aside from the obvious convective term in which the average spin is transported at the wavepacket group velocity, the spin current has additional contributions from the wavepacket's spin and torque dipole moments. Electric field corrections to the group velocity and carrier spin contribute to the convective term. Summing all terms we obtain an expression for the intrinsic spin-Hall conductivity of a hole-doped semiconductor, which agrees with the Kubo formula prediction for the same quantity. We discuss the calculation of spin accumulation, which illustrates the importance of the torque dipole near the boundary of the system.

cond-mat.mes-hall↗

arXiv2004

Universal Intrinsic Spin-Hall Effect

We describe a new effect in semiconductor spintronics that leads to dissipationless spin-currents in paramagnetic spin-orbit coupled systems. We argue that in a high mobility two-dimensional electron system with substantial Rashba spin-orbit coupling, a spin-current that flows perpendicular to the charge current is intrinsic. In the usual case where both spin-orbit split bands are occupied, the spin-Hall conductivity has a universal value.

cond-mat.mes-hall↗

arXiv2003

Pumping in an interacting quantum wire

We study charge and spin pumping in an interacting one-dimensional wire. We show that a spatially periodic potential modulated in space and time acts as a quantum pump inducing a dc-current component at zero bias. The current generated by the pump is strongly affected by the interactions. It has a power law dependence on the frequency or temperature with the exponent determined by the interaction in the wire, while the coupling to the pump affects the amplitudes only. We also show that pure spin-pumping can be achieved, without the presence of a magnetic field.

cond-mat.mes-hall↗

arXiv2000

The Edge-State Theory of Integer-Quantum-Hall-Effect to Insulator Transition

Direct transitions, driven by disorder, from several integral quantum Hall states to an insulator have been observed in experiment. This finding is enigmatic in light of a theoretical phase diagram, based on rather general considerations, that predicts a sequence of transitions in which the integer $n$ characterizing the Hall conductivity is reduced successively by unity, eventually going from $n=1$ into an insulator. In this work, we suggest that the direct transition occurs because, in certain parameter regime, the edge states of different Landau levels are strongly coupled and behave as a single edge state. It is indicated under what conditions successive transitions may be seen.

cond-mat.mes-hall↗

arXiv1999

Screening, nonadiabaticity, and quantized acoustoelectric current

Quantized single-electron transport driven by surface acoustic waves (SAW) through a pinched-off narrow constriction is studied theoretically. Long-range Coulomb interaction causes the tunneling coupling between the two-dimensional electron gas (2DEG) and the moving minimum of the SAW-induced potential to decay rapidly with time. The energy scale, associated with the characteristic time of this decay, controls both the width of the transition regions between the plateaus and the slope of the plateaus. This sets a limit for the accuracy of the quantization of acoustoelectric current at low temperature.

cond-mat.mes-hall↗

arXiv1998

Aharonov-Bohm and Aharonov-Casher Effects: Connections to Dynamics of Topological Singularities

We analyze the physical processes involved in the Aharonov-Bohm (A-B) and the Aharonov-Casher (A-C) effects, showing that an incomplete A-B effect knowledge can lead a totally wrong conclusion on the A-C effect. Based on this we demonstrate that the Magnus force, the net force, is the only transverse force on a moving vortex, in analogous to the net charge in A-C effect. This conclusion has been arrived both theoretically and experimentally.

cond-mat↗

arXiv1996

Derivation of the transverse force on a moving vortex in a superfluid

We describe an exact derivation of the total nondissipative transverse force acting on a quantized vortex moving in a uniform background. The derivation is valid for neutral boson or fermion superfluids, provided the order parameter is a complex scalar quantity. The force is determined by the one-particle density matrix far away from the vortex core, and is found to be the Magnus force proportional to the superfluid density.

cond-mat↗

arXiv1996

Vortex Dynamics in Superfluid Systems: Cyclotron Type Motion

Vortex dynamics in superfluids is investigated in the framework of the nonlinear Schrödinger equation. The natural motion of the vortex is of cyclotron type, whose frequency is found to be on the order of phonon velocity divided by the coherence length, and may be heavily damped due to phonon radiation. Trapping foreign particles into the vortex core can reduce the cyclotron frequency and make the cyclotron motion underdamped. The density fluctuations can follow the vortex motion adiabatically within the phonon wave length at the cyclotron frequency, which results in a further downward renormalization of the cyclotron frequency. We have also discussed applications on the dynamics of vortices in superconducting films.

cond-mat↗