Electron transport in ultraclean two-dimensional materials has received much attention. However, the sign of the magnetoresistance effect in various electron flow regimes remains controversial. In this work, the complete electron Boltzmann transport equation is numerically solved with the discrete ordinate method in the real space to clarify the condition of the negative magnetoresistance effect under a weak magnetic field. It turns out from the numerical results that this effect occurs only within the ballistic regime under a low electric field rather than the hydrodynamic regime. It is noteworthy that the existence of momentum-conserving scattering dramatically reverses the sign of magnetoresistance in the ballistic regime. When the electric field becomes strong enough compared to the magnetic field, its effect on the deflection of the electrons is not negligible and will lead to positive magnetoresistance in the whole parameter domain. The possible influence of boundary conditions and internal electric field models on the sign of magnetoresistance is also discussed. Our work provides insight into electron fluid transport under electromagnetic fields.

Machine learning techniques have been shown to be effective to recognize different phases of matter and produce phase diagrams in the parameter space of interest, while they usually require prior labeled data to perform well. Here, we propose a machine learning procedure, mainly in an unsupervised manner, which can first identify topological/nontopological phases and then refine the locations of phase boundaries. By following this proposed procedure, we expand our previous work on the one-dimensional $p$-wave superconductor [Tsai et al., Phys. Rev. B 102, 054512 (2020)] and further on the Su-Schrieffer-Heeger model, with an emphasis on using the quantum-entanglement-based quantities as the input features. We find that our method not only reproduces similar results to the previous work with sharp phase boundaries but importantly it also does not rely on prior knowledge of the phase space, e.g., the number of phases present. We conclude with a few remarks about its potential, limitations, and explanatory power.

Modulating charge density wave (CDW) states in layered materials has both fundamental scientific value and application potential in future electronic devices. Based on first-principles electronic structure calculations, we have studied the modulation of the CDW states in ${\mathrm{TaSe}}_2$ by using a typical electride ${\mathrm{Ca}}_2\mathrm{N}$ as the substrate. We find that the ${\mathrm{Ca}}_2\mathrm{N}$ monolayer can donate 0.49 electrons/f.u. to the ${\mathrm{TaSe}}_2$ monolayer and meanwhile avoids the disorder effect in conventional chemical substitution approach. With the uniform electron doping from ${\mathrm{Ca}}_2\mathrm{N}$, the CDW order in $1H-{\mathrm{TaSe}}_2$ is completely suppressed; in comparison, the CDW period in $1T-{\mathrm{TaSe}}_2$ transforms from the Star of David pattern to a $\sqrt{3}\times \sqrt{3}$ triangular pattern. Our studies enrich the phase diagram of ${\mathrm{TaSe}}_2$ and highlight the effective manipulation of the CDW states via an electride substrate, which calls for future experimental verification.

Recently, the kagome lattice metal $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K}$, Rb, Cs) family has received wide attention due to its coexistence of superconductivity, charge density wave (CDW), and peculiar properties arising from a topological nontrivial electronic structure. With time-resolved pump-probe spectroscopy, we show that the excited quasiparticle relaxation dynamics can be explained by the formation of a temperature-dependent energy gap below the phase transition that is similar to a usual second-order CDW condensate. By contrast, the structural change shows a predominantly first-order phase transition. Furthermore, no CDW amplitude mode is identified in the ordered phase. The results suggest that the CDW order is very different from a conventional CDW condensate. We also find that a weak pump pulse can nonthermally melt the CDW order and drive the sample into its high-temperature phase, revealing a small difference in lattice potential between those phases.

We study gated field-effect transistors (FETs) with an eccentric Corbino-disk geometry, such that the drain spans its circumference while the off-center inner ring acts as a source. An alternating current terahertz potential difference is applied between source and gate while a static source-drain voltage, rectified by the nonlinearities of FET electrons, is measured. When a magnetic field is applied perpendicular to the device, a strong resonance appears at the cyclotron frequency. The strength of the resonance can be tuned by changing the eccentricity of the disk. We show that there is an optimum value of the eccentricity that maximizes the responsivity of the FET.

We theoretically study the current-induced dynamics of a domain wall in a ferromagnetic nanotube by developing a theory for the orbital angular momentum of a domain wall and the current-induced torque on it. Specifically, a domain wall with nontrivial magnetization winding along the circumference is shown to possess finite orbital angular momentum, which is proportional to the product of its topological charge and position, and the current is shown to exert a torque changing the orbital angular momentum of the domain wall and thereby driving it. The current-induced torque is interpreted as the transfer of orbital angular momentum from electrons to the domain wall, which occurs due to the emergent magnetic field associated with the topological charge. Our results reveal a hitherto unrecognized utility of the orbital degree of freedom of magnetic solitons.

Hitherto, the discrete identification of quantum spin liquid phase, holy grail of condensed-matter physics, remains a challenging task experimentally. However, the precursor of quantum spin liquid state may reflect in the spin dynamics even in the paramagnetic phase over a wide temperature range as conjectured theoretically. Here we report comprehensive inelastic light- (Raman) scattering measurements on the Ir-based double perovskite, ${\mathrm{Gd}}_2{\mathrm{ZnIrO}}_6$, as a function of different incident photon energies and polarization in a broad temperature range. Our results evidence the spin fractionalization within the paramagnetic phase reflected in the emergence of a polarization-independent quasielastic peak at low energies with lowering temperature. Also, the fluctuating scattering amplitude measured via dynamic Raman susceptibility increases with lowering temperature and decreases mildly upon entering into long-range magnetic ordering phase, below 23 K, suggesting the magnetic origin of these quantum fluctuations. This anomalous scattering response is thus indicative of fluctuating fractional spin evincing the proximate quantum spin liquid phase in a three-dimensional double-perovskite system.

The study of superconductivity in compressed hydrides is of great interest due to measurements of high critical temperatures $\left(T_c\right)$ in the vicinity of room temperature, beginning with the observations of ${\mathrm{LaH}}_{10}$ at 170–190 GPa. However, the pressures required for synthesis of these high-$T_c$ superconducting hydrides currently remain extremely high. Here we show the investigation of crystal structures and superconductivity in the La-B-H system under pressure with particle-swarm intelligence structure-searches methods in combination with first-principles calculations. Structures with seven stoichiometries, LaBH, ${\mathrm{LaBH}}_4, {\mathrm{LaBH}}_6, {\mathrm{LaBH}}_7, {\mathrm{LaBH}}_8$, La(${\mathrm{BH})}_3$, and $\mathrm{La}{\left({\mathrm{BH}}_4\right)}_3$ were predicted to become stable under pressure. Remarkably, the hydrogen atoms in ${\mathrm{LaBH}}_8$ were found to bond with B atoms in a manner that is similar to that in ${\mathrm{H}}_3\mathrm{S}$. Lattice dynamics calculations indicate that ${\mathrm{LaBH}}_7$ and ${\mathrm{LaBH}}_8$ become dynamically stable at pressures as low as 109 and 48 GPa, respectively. Moreover, the two phases were predicted to be superconducting with a critical temperature $T_c$ of 93 K and 156 K at 110 GPa and 55 GPa, respectively (${\mu }^{*}$ = 0.1). The present results provide guidance for future experiments targeting hydride superconductors with both low synthesis pressures and high $T_c$.

We show that Josephson $\psi$ junctions with a half-metallic (HM) weak link coupled to superconducting (S) electrodes through ferromagnetic (F) layers host collective excitations of the magnetic moment and the Josephson phase. This results in a shift of the ferromagnetic resonance frequency, anomalies in the current-voltage characteristics, and the appearance of additional magnetic anisotropy in the F layers. In contrast to previously studied S/F/S junctions, coupling between magnetic and plasma modes emerges even in the long wavelength limit. Such coupling is shown to enable controllable magnetization reversal in the F layer governed by a dc current pulse, which provides an effective mechanism for the magnetic moment manipulation in the devices of superconducting spintronics.

We revisit a superconducting phase diagram and electronic structures across the charge density wave (CDW) phase transition of Cu-underdoped $1T\text{−}{\mathrm{Cu}}_{\delta }\mathrm{Ti}{\mathrm{Se}}_2$ ($\delta \sim 0.03$) under hydrostatic pressure. Superconductivity appears right after the complete collapse of the long-range CDW at a critical pressure of $P_{\mathrm{c}}\sim 2.48\mathrm{GPa}$, apart from the reported superconducting phase diagrams; it is found that the superconducting transition temperature shows a domelike pressure dependence covering a narrow pressure range with a maximum of ${T_{\mathrm{c}}}^{\max }\sim 2.80\mathrm{K}$ at 4.80 GPa. Accordingly, the residual resistivity ${\rho }_0$ and temperature exponent $n$ of normal-state resistivity (from ∼3.30 at ambient pressure to ∼2.38 at $P_{\mathrm{c}}$ and ∼4.0 at 6.50 GPa) reduce sizably while the quadratic temperature coefficient $A$ of normal-state resistivity is enhanced by one order in magnitude; these results indicate the importance of CDW quantum fluctuation in superconducting pairing; low-$T$ resistivity upwarps with a −ln$T$ dependence below a characteristic temperature $T^{*}$ which has a domelike shape in the pressure range of 2.82–4.80 GPa. Based on two-band analysis of Hall conductivity and Kohler-fitting of magnetotransport (MR), energy bands are dominated by electron-type carriers across the CDW phase transition for $P<P_{\mathrm{c}}$, and they reverse into hole-type for $P>P_{\mathrm{c}}$; interestingly, the mobility of carriers increases up to five times at $P_{\mathrm{c}}$, but carrier concentration shows a weak pressure dependence. The MR value increases with the pressure for $P<P_{\mathrm{c}}$ and then jump up to a saturated value after the collapse of the CDW. Our results show that the collapse of the CDW is accompanied by the reconstruction of the Fermi surface, and the enhancement in MR can be mainly attributed to the change of mobility. Possible mechanisms are discussed.

X-ray fluorescence holography (XFH) measurements were carried out for multiferroic $\mathrm{Pb}\left({\mathrm{Fe}}_{1/2}{\mathrm{Nb}}_{1/2}\right){\mathrm{O}}_3$ (PFN) single crystal to investigate variations in the local structures around Nb ions with decreasing temperature from 300 to 100 K, in combination with single-crystal x-ray diffraction (XRD) experiments. We find that the Nb–Pb correlation is more significantly changed by cooling than the average structure derived from the single-crystal XRD experiments, indicating a mobile feature of Nb ions. Also, we analyze previously obtained XFH results around Fe ions in detail for comparison with those around Nb ions. In contrast to the case of Nb ions, the variation in the Fe–Pb correlation with decreasing temperature is well explained by the average structure from 300 K down to the Néel temperature of about 150 K. On the other hand, a remarkable positional fluctuation between Fe and Pb ions is observed at 100 K, which is evaluated to be 90% larger than that between Nb and Pb ions. This observation strongly indicates the presence of spin-lattice coupling in PFN. The present study clearly elucidates the differences of local environment around the Nb ions from that around the Fe ions located at the same $B$-site.

The magnetic Hamiltonian of the Heisenberg quantum antiferromagnet $\mathrm{Sr}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ is studied by inelastic neutron scattering technique on powder and single-crystalline samples above and below the magnetic transition temperatures at 8 and 2 K. The high-temperature spectra reveal a characteristic diffuse scattering corresponding to a multispinon continuum, confirming the dominant quantum spin chain behavior due to the third neighbor interaction $J_{\text{intra}}=4.22$ meV (49 K). The low-temperature spectra exhibit sharper excitations at energies <1.25 meV, which can be explained by considering a combination of weak antiferromagnetic first nearest neighbor interchain coupling $J_1=0.17$ meV ($1.9$ K) and even weaker ferromagnetic second nearest neighbor $J_2=-0.037$ meV ($-0.4$ K) or a weak ferromagnetic $J_2=-0.11$ meV ($-1.3$ K) and antiferromagnetic $J_6=0.16$ meV ($1.85$ K), giving rise to the long-range magnetic order and spin-wave excitations at low energies. These results suggest that $\mathrm{Sr}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ is a highly one-dimensional Heisenberg system with three mutually perpendicular spin chains coupled by a weak ferromagnetic $J_2$ in addition to the antiferromagnetic $J_1$ or $J_6$, presenting a contrasting scenario from the highly frustrated hyper-hyperkagome lattice (equally strong antiferromagnetic $J_1$ and $J_2$) found in the isostructural quantum spin liquid candidate $\mathrm{Pb}\mathrm{Cu}{\mathrm{Te}}_2{\mathrm{O}}_6$ .

How to efficiently manipulate the Néel vector of antiferromagnets by electric methods is one of the major focuses in current antiferromagnetic spintronics. In this work, we investigated the ferroelectric control of magnetism in antiferromagnetic $L10\text{−}\mathrm{Mn}\mathrm{Pt}/\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$ bilayer structures by using first-principles calculation. We studied the effect of ferroelectric polarization reversal on magnetic crystalline anisotropy (MCA) of $L10$-MnPt films with different interface structures. Our results predict a large perpendicular MCA in $L10$-MnPt films with Pt-O interface, and an in-plane MCA with Mn-O interface when they are interfaced with ferroelectric $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$. In addition, the magnitude of MCA for both interfaces can be modulated efficiently by the polarization reversal of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_3$. The ferroelectric control of MCA has been analyzed based on second order perturbation theory, and it can be mainly attributed to the ferroelectric polarization driven redistribution of Pt-$5d$ orbital occupation around Fermi energy. Especially, for the Mn-O interface, the Néel vector can be switched between in-plane [100] and [110] directions, or even from in plane to out of plane at certain film thickness by reversing ferroelectric polarization. Our results may provide a nonvolatile concept for ferroelectric control of Néel vector in $L10$ antiferromagnets, which could stimulate experimental investigations on magnetoelectric effect of antiferromagnets and promote its applications in low-power consumption spintronic memory devices.

We report that close to a Mott transition there is an emergence of large thermal noise ($S_{\mathrm{th}}$) which occurs concomitantly with large correlated flicker noise ($\frac{1}{f}$ noise) with significant non-Gaussian content. This was observed in films of ${\text{NdNiO}}_3$ (thickness 15 nm) grown on crystalline ${\text{SrTiO}}_3$ substrates with different crystallographic orientations that show a hysteretic transition from a high temperature metallic phase to a low temperature insulating phase in the temperature range 160 to 211 K depending on the substrate orientation and the heating and cooling cycle. The thermal noise, which is distinct from the flicker noise, deviates from the canonical Johnson-Nyquist value of $4k_{B}TR$ as measured through the ratio $\zeta \left(T\right)(\equiv \frac{S_{\mathrm{th}}\left(T\right)}{4k_{B}TR})$. The ratio $\zeta$ reaches a maximum value of ${\zeta }_M$ at a temperature $T^{*}$ that is close to but distinct from the metal-insulator transition (MIT) temperature $T_{\mathrm{MI}}$. In all the films near $T^{*}$, the scaled thermal noise maxima ${\zeta }_M\gg 1$. The films were found to be largely strain relaxed with residual in-plane and out-of-plane strain as measured by x-ray reciprocal space mapping. It has been observed that the ratio $\frac{T^{*}}{T_{\mathrm{MI}}}$ as well as ${\zeta }_M$ have a close dependence on the in-plane-strain in the film. The enhanced thermal noise that occurs along with large correlated flicker noise both arise from slow kinetics of relaxation as established from temperature dependence of the correlation time ($\tau$) that gets significantly larger in the temperature range around $T^{*}$, reaching a maxima at $T=T^{*}$. It has been proposed that the existence of large noise (both thermal and flicker noise) owes its origin to electronic phase separation (EPS) that exists near the MIT. A physical model has been suggested that EPS near MIT temperature can give rise to a sparse phase of nanometric small pockets of metallic phases (nanopuddles) that are surrounded by and embedded within the minority insulating phase. The nanopuddles act as a source of charge fluctuations and are coupled weakly to the majority metallic phase by tunneling through the layer of the minority insulating phase. Such isolated metallic nanopuddles can be Coulomb charged if the charging energy $E_C\ge k_{B}T$ and can have slow relaxation of fluctuations acting as a source of large noise. It has been argued that the size distribution of the nanopuddles, their average size $\langle d\rangle$, as well as the temperature dependence of their number density $N_d$ can determine the temperature $T^{*}$.

We employ the periodic Anderson model with superconducting correlations in the conduction band at half filling to study the behavior of the in-gap bands in a heterostructure consisting of a molecular layer deposited on the surface of a conventional superconductor. We use the dynamical mean-field theory to map the lattice model on the superconducting single impurity model with self-consistently determined bath and use the continuous-time hybridization expansion (CT-HYB) quantum Monte Carlo and the iterative perturbation theory (IPT) as solvers for the impurity problem. We present phase diagrams for square and triangular lattice that both show two superconducting phases that differ by the sign of the induced pairing, in analogy to the 0 and $\pi$ phases of the superconducting single impurity Anderson model and discuss the evolution of the spectral function in the vicinity of the transition. We also discuss the failure of the IPT for superconducting models with spinful ground state and the behavior of the average expansion order of the CT-HYB simulation.

Smooth interfaces of topological systems are known to host massive surface states in addition to the topologically protected chiral one. We show that in Weyl semimetals these massive states, along with the chiral Fermi arc, strongly alter the form of the Fermi-arc plasmon. Most saliently, they yield further collective plasmonic modes that are absent in a conventional interface. The plasmon modes are completely anisotropic as a consequence of the underlying anisotropy in the surface model and expected to have a clear-cut experimental signature, e.g., in electron-energy loss spectroscopy.