We systemically investigate the quantized local Andreev reflection (LAR) and crossed Andreev reflection (CAR) in four-terminal topological insulator–superconductor hybrid systems based on the Kane-Mele model and Haldane model corresponding to the quantum spin Hall (QSH) and quantum anomalous Hall (QAH) systems. We find that although both are driven by edge states, the LAR and CAR behave in different ways. The essential difference between the LAR and CAR is the coherent phases of incoming electrons and outgoing holes in the transport processes. For the LAR, the total phase remains zero, and quantum interference is always constructive. However, for the CAR, the coherent phase varies with system parameters. Fortunately, we find a weak magnetic field can smoothly modify the edge states and then the interference phase; finally, a perfect quantized CAR is realized in the QAH-superconductor hybrid systems. In addition, the quantized LAR is more robust against random disorder because the coherent phases carried by electrons are completely canceled out by holes.

We demonstrate that the prototypical two-dimensional Chern insulator hosts exotic quantum multicriticality in the presence of an appropriate periodic driving: a linear Dirac-like transition coexists with a quadratic nodal looplike transition. This nodal loop gap closure is characterized by an enhanced chiral-mirror symmetry that is induced by the driving procedure. The existence of multiple universality classes can be unambiguously captured by extracting critical exponents and scaling laws with a single renormalization group approach based on the curvature function of the stroboscopic Floquet Hamiltonian. This procedure is effective regardless of whether the topological phase transitions are associated with anomalous edge modes or not. We comment on possible experimental realizations of the model and detection schemes for the curvature function.

The effects of a turnstile operation on the current-induced vibron dynamics in nanoelectromechanical systems (NEMS) are analyzed in the framework of the generalized master equation. In our simulations each turnstile cycle allows the pumping of up to two interacting electrons across a biased mesoscopic subsystem which is electrostatically coupled to the vibrational mode of a nanoresonator. The time-dependent mean vibron number is very sensitive to the turnstile driving, rapidly increasing/decreasing along the charging/discharging sequences. This sequence of heating and cooling cycles experienced by the nanoresonator is due to specific vibron-assisted sequential tunneling processes along a turnstile period. At the end of each charging/discharging cycle the nanoresonator is described by a linear combination of vibron-dressed states $s_{\nu }$ associated to an electronic configuration $\nu$. If the turnstile operation leads to complete electronic depletion the nanoresonator returns to its equilibrium position, i.e., its displacement vanishes. It turns out that a suitable bias applied on the NEMS leads to a slow but complete cooling at the end of the turnstile cycle. Our calculations show that the quantum turnstile regime switches the dynamics of the NEMS between vibron-dressed subspaces with different electronic occupation numbers. We predict that the turnstile control of the electron-vibron interaction induces measurable changes on the input and output transient currents.

The correlation kinetic equation approach is developed that allows describing spin correlations in a material with hopping transport. The quantum nature of spin is taken into account. The approach is applied to the problem of the bipolaron mechanism of organic magnetoresistance in the limit of large Hubbard energy and small applied electric field. The spin relaxation that is important to magnetoresistance is considered to be due to hyperfine interaction with atomic nuclei. It is shown that the line shape of magnetoresistance depends on short-range transport properties. Different model systems with identical hyperfine interaction but different statistics of electron hops lead to different line shapes of magnetoresistance including the two empirical laws $H^2/(H^2+H_0^2)$ and $H^2/\left(\right|H|+H_0{)}^2$ that are commonly used to fit experimental results.

Interfacial perpendicular magnetic anisotropy in the $\mathrm{MgO}/{\mathrm{Co}}_2\mathrm{FeAl}$ heterostructure is desired for technological applications, while the origin of the large interfacial anisotropy constant $K_i$ remains controversial. Here we show that, by modeling four types of interface models for the $\mathrm{MgO}/{\mathrm{Co}}_2\mathrm{FeAl}$ system using first-principles calculations, the $\mathrm{MgO}/{\mathrm{Co}}_2$ interface is energetically more favorable than the MgO/FeAl interface and the interfacial Co atoms at the former interface produce out-of-plane $K_i$, while the interfacial Fe atoms at the latter interface produce in-plane $K_i$. The origin of these different behaviors can be explained by the atomic-resolved and orbital-resolved $K_i$ along with the perturbation theory energy analysis. In addition, we also studied the influence of 26 capping layers on the interfacial magnetic anisotropy of $\mathrm{MgO}/{\mathrm{Co}}_2\mathrm{FeAl}$ and found that Fe and W capping can significantly enhance $K_i$ in the $\mathrm{MgO}/{\mathrm{Co}}_2\mathrm{FeAl}$ with a particularly large $K_i$ of $4.90\mathrm{mJ}/{\mathrm{m}}^2$ in the W-capped model. This work clarifies the atomistic origin of the interfacial perpendicular magnetic anisotropy and provides guidance to further enhance interfacial $K_i$ by adding capping layers in the $\mathrm{MgO}/{\mathrm{Co}}_2\mathrm{FeAl}$.

$\mathrm{C}{\mathrm{o}}_2\mathrm{MnAl}$ has been predicted to have Weyl points near the Fermi level in $L{2}_1$-ordered structure, which is expected to give rise to exotic transverse transport properties such as large anomalous Hall (AHE) and Nernst effects (ANE) due to large Berry curvature. In this study, the effects of Fermi level position and atomic ordering on AHE and ANE in $\mathrm{C}{\mathrm{o}}_2\mathrm{MnA}{\mathrm{l}}_{1-x}\mathrm{S}{\mathrm{i}}_x$ were studied systematically. The $\mathrm{C}{\mathrm{o}}_2\mathrm{MnAl}$ film keeps $B2$-disordred structure regardless of annealing temperature, which results in much smaller anomalous Hall conductivity ${\sigma }_{xy}$ and transverse Peltier coefficient ${\alpha }_{xy}$ than those calculated for $L{2}_1$-ordered $\mathrm{C}{\mathrm{o}}_2\mathrm{MnAl}$. Our newly performed calculation of ${\sigma }_{xy}$ with taking $B2$ disordering into account well reproduces the experimental result; thus it was concluded that Berry curvature originating from the Weyl points is largely reduced by $B2$ disordering. It was also revealed Al substitution with Si shifts the position of the Fermi level and greatly improves the $L{2}_1$ atomic ordering, leading to strong enhancement of ${\alpha }_{xy}$, which also agreed with our theoretical calculation. The highest thermopower of ANE of 5.7 μV/K, which is comparable to the recent reports for $\mathrm{C}{\mathrm{o}}_2\mathrm{MnGa}$, was observed for $\mathrm{C}{\mathrm{o}}_2\mathrm{MnA}{\mathrm{l}}_{0.63}\mathrm{S}{\mathrm{i}}_{0.37}$ because of dominant contribution of ${\alpha }_{xy}$. This study clearly shows the importance of both Fermi level tuning and high atomic ordering for obtaining the effect of topological features in Co-based Heusler alloys on transverse transport properties.

The experimental parameter ranges needed to generate superfluidity in optical and drag experiments in GaAs double quantum wells are determined using a formalism that includes self-consistent screening of the Coulomb pairing interaction in the presence of the superfluid. The very different electron and hole masses in GaAs make this a particularly interesting system for superfluidity with exotic superfluid phases predicted in the BCS-Bose-Einstein condensation crossover regime. We find that the density and temperature ranges for superfluidity cover the range for which optical experiments have observed indications of superfluidity but that existing drag experiments lie outside the superfluid range. We also show that, for samples with low mobility with no macroscopically connected superfluidity, if the superfluidity survives in randomly distributed localized pockets, standard quantum capacitance measurements could detect these pockets.

Low-frequency perturbations at the boundary of critical quantum chains can be understood in terms of the sequence of boundary conditions imposed by them, as has been previously demonstrated in the Ising and related fermion models. Using extensive numerical simulations, we explore the scaling behavior of the Loschmidt echo under longitudinal field perturbations at the boundary of a critical ${\mathbb{Z}}_3$ Potts model. We show that at times much larger than the relaxation time after a boundary quench, the Loschmidt echo has a power-law scaling with time as expected from interpreting the quench as insertion of boundary condition changing operators. Similar scaling is observed as a function of time period under a low-frequency square-wave pulse. We present numerical evidence for one cycle that indicates that under a sinusoidal or triangular pulse, scaling with time period is modified by the Kibble-Zurek mechanism, again similar to the case of the Ising model. Results confirm the validity, beyond the Ising model, of the treatment of the boundary perturbations in terms of the effect on boundary conditions.

We investigate the dynamical properties of an interacting many-body system with a nontrivial energy potential landscape that may induce a singular continuous single-particle energy spectrum. Focusing on the Aubry-André model, whose anomalous transport properties in the presence of interaction was recently demonstrated experimentally in an ultracold-gas setup, we discuss the anomalous slowing down of the dynamics it exhibits and show that it emerges from the singular-continuous nature of the single-particle excitation spectrum. Our study demonstrates that singular-continuous spectra can be found in interacting systems, unlike previously conjectured by treating the interactions in the mean-field approximation. This, in turns, also highlights the importance of the many-body correlations in giving rise to anomalous dynamics, which, in many-body systems, can result from a nontrivial interplay between geometry and interactions.

We theoretically study the tunnel magnetoresistance (TMR) effect in (111)-oriented junctions Co/MgO/Co(111) and Ni/MgO/Ni(111). The Co-based junction is shown to have a TMR ratio over $2000\%$, which is one order higher than that of the Ni-based one. The high TMR ratio is attributed to the interfacial resonance effect: The interfacial $d\text{−}p$ antibonding states are formed close to the Fermi level in the majority-spin channel and these states in both interfaces resonate with each other. This differs essentially from the conventional coherent tunneling mechanism of high TMR ratios in Fe(Co)/MgO/Fe(Co)(001).

We show that transverse resistivity anomalies in $\mathrm{SrRu}{\mathrm{O}}_3$ epitaxial thin films, which have been claimed to be a manifestation of the topological Hall effect, can be modulated by applying gate voltages ($V_{\mathrm{G}}$). The electric field induced effects on the anomalies are found to be the same as those of the saturated anomalous Hall resistivity (${\rho }_{\mathrm{AHE}}$), revealing that the anomalies arise as a result of the coexistence of the intrinsic and extrinsic ${\rho }_{\mathrm{AHE}}$. Furthermore, the $V_{\mathrm{G}}$-induced effects on the anomalies are dominated by the $V_{\mathrm{G}}$-induced changes in the intrinsic ${\rho }_{\mathrm{AHE}}$ while the extrinsic ${\rho }_{\mathrm{AHE}}$ remains unchanged under the $V_{\mathrm{G}}$ applications. Our results reveal that electric field induced modulations in the anomalous Hall effect depend on its mechanism.

Magnetization plateaus emerging in quantum spin systems due to spontaneously breaking of translational symmetry have been reported both theoretically and experimentally. The broken symmetry can induce reconstruction of elementary excitations such as Goldstone and Higgs modes, whereas its microscopic mechanism and reconstructed quasiparticle in magnetization-plateau phases have remained unclear so far. Here we theoretically study magnetic excitations in the magnetization-plateau phases of a frustrated spin ladder by using the dynamical density-matrix renormalization-group method. Additionally, analytical approaches with the perturbation theory are performed to obtain an intuitive view of magnetic excitations. A comparison between numerical and analytical results indicates the presence of a reconstructed quasiparticle originating from spontaneously broken translational symmetry, which is realized as a collective mode of the spin trimer called a trimeron.

We report measurements of the electrical resistivity and ac magnetic susceptibility of single-crystalline ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ under pressure, in order to investigate the interplay of superconductivity and charge density wave (CDW) order. ${\mathrm{LaPt}}_2{\mathrm{Si}}_2$ exhibits a first-order phase transition from a tetragonal to orthorhombic structure, accompanied by the onset of CDW order below $T_{\mathrm{CDW}}$ = 76 K, while superconductivity occurs at a lower temperature of $T_{\mathrm{c}}$ = 1.87 K. We find that the application of pressure initially suppresses the CDW transition, but enhances $T_{\mathrm{c}}$. At pressures above 2.4 GPa, CDW order vanishes, while both $T_{\mathrm{c}}$ and the resistivity $A$ coefficient reach a maximum value around this pressure. Our results suggest that the occurrence of a superconducting dome can be accounted for within the framework of BCS theory, where there is a maximum in the density of states upon the closure of the CDW gap.

Disorder such as impurities and dislocations in Weyl semimetals drives a quantum critical point (QCP) where the density of states at the Weyl point gains a nonzero value. Near the QCP, the asymptotic low-energy singularities of physical quantities are controlled by the critical exponents $\nu$ and $z$. The nuclear spin-lattice relaxation rate, which originates from the hyperfine coupling between a nuclear spin and long-range orbital currents in Weyl fermion systems, shows intriguing critical behavior. Based on the self-consistent Born approximation for impurities, we study the nuclear spin-lattice relaxation rate $1/T_1$ due to the orbital currents in disordered Weyl SMs. We find that ${\left(T_{1}T\right)}^{-1}\sim E^{2/z}$ at the QCP where $E$ is the maximum of temperature $T$ and chemical potential $\mu \left(T\right)$ relative to the Weyl point. This scaling behavior of ${\left(T_{1}T\right)}^{-1}$ is also confirmed by the self-consistent $T$-matrix approximation, where a remarkable temperature dependence of $\mu \left(T\right)$ could play an important role. We hope these results of ${\left(T_{1}T\right)}^{-1}$ will serve as an impetus for exploration of the disorder-driven quantum criticality in Weyl materials.

We present an implementation of a truncated unity parquet solver (TUPS) which solves the parquet equations using a truncated form-factor basis for the fermionic momenta. This way fluctuations from different scattering channels are treated on an equal footing. The essentially linear scaling of computational costs in the number of untruncated bosonic momenta allows us to treat system sizes of up to $76\times 76$ discrete lattice momenta, unprecedented by previous unbiased methods that include the frequency dependence of the vertex. With TUPS, we provide the first numerical evidence that the parquet approximation might indeed respect the Mermin-Wagner theorem and further systematically analyze the convergence with respect to the number of form factors. Using a single form factor seems to qualitatively describe the physics of the half-filled Hubbard model correctly, including the pseudogap behavior. Quantitatively, using a single or a few form factors only is not sufficient at lower temperatures or stronger coupling.

Addressability of spin qubits in a silicon double quantum dot setup in the (1,1) charge configuration relies on having a large difference between the Zeeman splittings of the electrons. When the difference is not sufficiently large, the rotating wave approximation becomes inaccurate. We consider a device working in this regime, with always-on exchange coupling, and describe how a controlled-$Z$ (cz) gate and arbitrary one-qubit gates which are robust against charge noise can be implemented by smoothly pulsing the microwave source while eliminating the cross talk. We find that the most significant deviations from the rotating wave approximation, which are analogous to the Bloch-Siegert shift in a two-level system, can be compensated using local virtual gates.

We present a magnetotransport study in an InAs/GaSb double quantum well structure in the weak localization regime. As the charge carriers are depleted using a top-gate electrode, we observe a crossover from weak antilocalization (WAL) to weak localization (WL), when the inelastic phase-breaking time decreases below spin-orbit characteristic time as a result of enhanced electron-electron interactions at lower carrier concentrations. The same crossover is observed with increasing temperature. The linear temperature behavior of inelastic scattering rate indicates that the dominant phase-breaking mechanism in our 2D system is due to electron-electron interaction.