Quaternary alloy ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ hosts magnetic and electronic properties that can be tuned by varying the P concentration “$y$”, Mn concentration “$x$” and by annealing. In this work we make use of this tunability to probe the origin of the anomalous Hall effect (AHE) in ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ thin films grown on GaAs that host perpendicular magnetic anisotropy. Specifically, we find that AHE in this class of materials is determined primarily by two contributions: an intrinsic band component arising from the Berry curvature, and a component determined by hopping conduction. As we vary the properties of ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ from the metallic to the semi-insulating regime by changing the value of $y$ and by postgrowth annealing, we observe a clear crossover from a Berry-curvature-induced AHE to one that is caused by hopping. The transition occurs approximately at the point where the numbers of localized and itinerant holes become comparable. In this hopping regime, the conductivity follows the Efros-Shklovskii scaling law versus temperature indicating the presence of a Coulomb gap, but the AHE remains robustly present. These results indicate that ${\mathrm{Ga}}_{1-x}{\mathrm{Mn}}_x{\mathrm{As}}_{1-y}{\mathrm{P}}_y$ can host an interesting interplay between magnetism and Coulomb interactions.

In the anomalous Hall effect (AHE), the magnetization, electric field, and Hall current are presumed to be mutually vertical to each other. In this paper, we propose an unconventional AHE where the magnetization, electric field, and Hall current stay inside the same plane. Such an AHE is odd under time reversal and exists even when the magnetization is parallel to the electric field or Hall current, different from the planar Hall effect which is even under time reversal. Here, we term it parallel anomalous Hall effect (PAHE). We reveal that the PAHE exists when all the point group rotational and reflection symmetries are broken where the Berry curvature field is not necessarily parallel to the magnetization axis. We further demonstrate the PAHE in a ferrimagnetic Weyl semimetal ${\mathrm{FeCr}}_2{\mathrm{Te}}_4$.

Recent experiments on the antiferromagnetic intercalated transition metal dichalcogenide ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ have demonstrated reversible resistivity switching by application of orthogonal current pulses below its magnetic ordering temperature, making ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ promising for spintronics applications. Here, we perform density functional theory calculations with Hubbard $U$ corrections of the magnetic order, electronic structure, and transport properties of crystalline ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$, clarifying the origin of the different resistance states. The two experimentally proposed antiferromagnetic ground states, corresponding to in-plane stripe and zigzag ordering, are computed to be nearly degenerate. In-plane cross sections of the calculated Fermi surfaces are anisotropic for both magnetic orderings, with the degree of anisotropy sensitive to the Hubbard $U$ value. The in-plane resistance, computed within the Kubo linear response formalism using a constant relaxation time approximation, is also anisotropic, supporting a hypothesis that the current-induced resistance changes are due to a repopulating of antiferromagnetic domains. Our calculations indicate that the transport anisotropy of ${\mathrm{Fe}}_{1/3}{\mathrm{NbS}}_2$ in the zigzag phase is reduced relative to stripe, consistent with the relative magnitudes of resistivity changes in experiment. Finally, our calculations reveal the likely directionality of the current-domain response, specifically, which domains are energetically stabilized for a given current direction.

We use large-scale Monte-Carlo simulations to study thermal fluctuations in chiral $p$-wave superconductors in an applied magnetic field in three dimensions. We consider the thermal stability of previously predicted unusual double-quanta flux-line lattice ground states in such superconductors. In previous works it was shown that, neglecting thermal fluctuations, a chiral $p$-wave superconductor forms a hexagonal lattice of doubly-quantized vortices, except extremely close to the vicinity of $H_{c2}$ where double-quanta vortices split apart. We find dissociation of double-quanta vortices driven by thermal fluctuations. However, our calculations also show that the previous predictions of hexagonal doubly-quantized vortices, where thermal fluctuations were ignored, are very robust in the considered model.

Topological insulators have emerged as an important material class for efficient spin-charge interconversion. Most topological insulators considered to date are binary or ternary compounds, with the exception of $\alpha -\mathrm{Sn}$. Here we report a comprehensive characterization of the growth, magnetotransport properties, and current-induced spin-orbit torques of $\alpha -\mathrm{Sn}$ and $\beta -\mathrm{Sn}$-based ferromagnetic heterostructures. We show that $\alpha -\mathrm{Sn}$ grown with a Bi surfactant on CdTe(001) promotes large spin-orbit torques in a ferromagnetic FeCo layer at room temperature, comparable to Pt, whereas $\alpha -\mathrm{Sn}$ grown without Bi surfactant and the nontopological phase, $\beta -\mathrm{Sn}$, induce lower torques. The dampinglike and fieldlike spin-orbit torque efficiency in $\alpha -\mathrm{Sn}$ with Bi are 0.12 and 0.18, respectively. Further, we show that $\alpha -\mathrm{Sn}$ grown with and without Bi presents a spin-Hall-like magnetoresistance comparable to that found in heavy metal/ferromagnet bilayers. Our work demonstrates direct and efficient charge-to-spin conversion in $\alpha -\mathrm{Sn}$ ferromagnetic heterostructures, showing that $\alpha -\mathrm{Sn}$ is a promising material for current-induced magnetization control in spintronic devices.

We present a detailed low-energy muon spin rotation and x-ray magnetic circular dichroism (XMCD) investigation of the magnetic structure in ultrathin tetragonal (T)-CuO films. The measured muon-spin polarization decay indicates an antiferromagnetic (AFM) order with a transition temperature higher than 200 K. The XMCD signal obtained around the $\mathrm{Cu}{L}_{2,3}$ edges indicates the presence of pinned ${\mathrm{Cu}}^{2+}$ moments that are parallel to the sample surface, and additionally, isotropic paramagnetic moments. The pinning of some of the Cu moments is caused by an AFM ordering consisting of moments that lie most likely in plane of the film. Moreover, pinned moments show a larger orbital magnetic moment contribution with an approximate ratio of $m_{\mathrm{orb}}/m_{\mathrm{spin}}=1.5$, indicating that these spins are located at sites with reduced symmetry. Some fractions of the pinned moments remain pinned from an AFM background even at 360 K, indicating that $T_{\mathrm{N}}>360\mathrm{K}$. We propose a simple model that explains these experimental findings, showing that the magnetic order of the $T$-CuO ultrathin films differs from the theoretical predictions.

We report the magnetic susceptibility, specific heat, and muon spin relaxation results of the $5d^1$ double perovskite ${\mathrm{Ba}}_2{\mathrm{YWO}}_6$. The dc magnetic susceptibility shows two distinct Curie-Weiss regimes and a sub-Curie-Weiss increase $T^{-n}$ with $n=0.75\left(1\right)$ for $T=3\text{–}100$ K, alluding to the presence of random magnetism. The ac magnetic susceptibility reveals a spin freezing at $T_f\sim 0.3$ K with the activation energy of $\mathrm{\Delta }/k_B=27.6$ K. The specific heat data exhibit the pseudogap behavior at 25 K and the subsequent power-law dependence with decreasing temperature, indicating the gradual spin freezing with the quenching of orbital fluctuations. However, the muon spin relaxation data display only a weak muon spin depolarization with lacking long-range magnetic ordering down to 26 mK. Taken together, our results suggest that ${\mathrm{Ba}}_2{\mathrm{YWO}}_6$ is the proximate realization of a random spin-orbit dimer state.

We use time-resolved measurement and modeling to study the spin-torque induced motion of a domain wall in perpendicular anisotropy magnets. We show that the most important factor governing domain wall dynamics is the energy difference between a wall at the center of the disk with either a Bloch-type configuration or the Néel-type configuration; this energy difference strongly depends on the disk diameter. When between 70 and 100 nm, the wall drifts across the disk with pronounced back-and-forth oscillations that arise because the wall moves in the Walker regime. Several switching paths occur stochastically and lead to distinct switching durations. The wall can cross the disk center either in a ballistic manner or with variably marked oscillations before and after the crossing. The crossing of the center can even occur multiple times if a vertical Bloch line nucleates within the wall. The wall motion is analyzed using a collective coordinate model parametrized by the wall position $q$ and the tilt $\varphi$ of its in-plane magnetization projection. The dynamics results from the stretch field, which describes the affinity of the wall to reduce its length and the wall stiffness field describing the wall tendency to reduce dipolar energy by rotating its tilt. The wall oscillations result from the continuous exchange of energy between to the two degrees of freedom $q$ and $\varphi$. The stochasticity of the wall dynamics can be understood from the concept of the retention pond: a region in the $q\text{−}\varphi$ space in which walls are transiently bound to the disk center. Walls having trajectories close to the pond must circumvent it and therefore have longer propagation times. The retention pond disappears for a disk diameter of typically 40 nm: the wall then moves in a ballistic manner irrespective of the dynamics of its tilt. The propagation time is then robust against fluctuations hence reproducible.

We develop a theory of the spin battery effect in superconductor/ferromagnetic insulator (SC/FI) systems taking into account the magnetic proximity effect. We demonstrate that the spin-energy mixing enabled by the superconductivity leads to the enhancement of spin accumulation by several orders of magnitude relative to the normal state. This finding can explain the recently observed giant inverse spin Hall effect generated by thermal magnons in the SC/FI system. We suggest a nonlocal electrical detection scheme which can directly probe the spin accumulation driven by the magnetization dynamics. We predict a giant Seebeck effect converting the magnon temperature bias into the nonlocal voltage signal. We also show how this can be used to enhance the sensitivity of magnon detection even up to the single-magnon level.

In this work, we propose how to load and manipulate chiral states in a Josephson junction ring in the so-called transmon regime. We characterize these states by their symmetry properties under time-reversal and parity transformations. We describe an explicit protocol to load and detect the states within a realistic set of circuit parameters and show simulations that reveal the chiral nature. Finally, we explore the utility of these states in quantum technological nonreciprocal devices.

Graphene-based moiré superlattices are now established as an interesting platform for strongly correlated many-electron physics, and they have so far been characterized mainly by transport and scanning tunneling microscopy (STM) measurements. Motivated by recent experimental progress, we present a theoretical model study whose aim is to assess the potential of angle-resolved photoemission spectroscopy (ARPES) to resolve some of the many open issues in these systems. The theory is developed specifically for graphene on hexagonal boron nitride (G/hBN) and twisted bilayer graphene (TBG) moiré superlattices, but it is readily generalized to any system with active degrees of freedom in graphene sheets.

Quantum computation requires high-fidelity single-qubit and two-qubit gates on a scalable platform. Silicon spin qubits are a promising platform toward realization of this goal. In this paper we show how to perform single-qubit and controlled-Z (cz) gates in a linear chain of three spin qubits with always-on exchange coupling, which is relevant for certain dot- and donor-based silicon devices. We also show how to make the cz gate robust against both charge noise and pulse length error using a two-tone pulse shaping method. The robust pulse maintains a fidelity of 99.99% at 3.5% fluctuations in exchange or pulse amplitude, which is an improvement over the uncorrected pulses where this fidelity can only be maintained for fluctuations in exchange up to 2% or up to 0.2% in amplitude.

Solutions of the Peierls-Boltzmann transport equation using inputs from density functional theory calculations have been successful in predicting the thermal conductivity in a wide range of materials. In the case of two-dimensional (2D) materials, the accuracy of this method can depend highly on the shape of the dispersion curve for flexural phonon (ZA). As a universal feature, very recent theoretical studies have shown that the ZA branch of 2D materials is quadratic. However, many prior thermal conductivity studies and conclusions are based on a ZA branch with linear components. In this paper, we systematically study the impact of the long-wavelength dispersion of the ZA branch in graphene, silicene, and $\alpha$-nitrophosphorene to highlight its role on thermal conductivity predictions. Our results show that the predicted $\kappa$ value, its convergence and anisotropy, as well as phonon lifetimes and mean free path can change substantially even with small linear to pure quadratic corrections to the shape of the long-wavelength ZA branch. Also, having a pure quadratic ZA dispersion can improve the convergence speed and reduce uncertainty in this computational framework when different exchange-correlation functionals are used in the density functional theory calculations. Our findings may provide a helpful guideline for more accurate and efficient thermal conductivity estimation in mono- and few-layer 2D materials.

Correlated electron systems, particularly iron-based superconductors, are extremely sensitive to strain, which inevitably occurs in the crystal growth process. Built-in strain of this type has been proposed as a possible explanation for experiments where nematic order has been observed at high temperatures corresponding to the nominally tetragonal phase of iron-based superconductors. Strain is assumed to produce linear defect structures, e.g., dislocations, which are quite similar to O vacancy chainlets in the underdoped cuprate superconductor YBCO. Here we investigate a simple microscopic model of dislocations in the presence of electronic correlations which create defect states that can drive magnetic anisotropy of this kind, if spin-orbit interaction is present. We estimate the contribution of these dislocations to magnetic anisotropy as detected by current torque magnetometry experiments in both cuprates and Fe-based systems.

We present a polarization-resolved, high-resolution Raman scattering study of the three consecutive charge density wave (CDW) regimes in $1T-{\mathrm{TaS}}_2$ single crystals, supported by ab initio calculations. Our analysis of the spectra within the low-temperature commensurate (C-CDW) regime shows $P\overline{3}$ symmetry of the system, thus excluding the previously proposed triclinic stacking of the “star-of-David” structure, and promoting trigonal or hexagonal stacking instead. The spectra of the high-temperature incommensurate (IC-CDW) phase directly project the phonon density of states due to the breaking of the translational invariance, supplemented by sizable electron-phonon coupling. Between 200 and 352 K, our Raman spectra show contributions from both the IC-CDW and the C-CDW phases, indicating their coexistence in the so-called nearly commensurate (NC-CDW) phase. The temperature dependence of the symmetry-resolved Raman conductivity indicates the stepwise reduction of the density of states in the CDW phases, followed by a Mott transition within the C-CDW phase. We determine the size of the Mott gap to be ${\mathrm{\Omega }}_{\mathrm{gap}}\approx 170\text{–}190$ meV, and track its temperature dependence.

It is well known that Lorentzian voltage pulses with integer quantum flux can inject integer-charged wave packets without electron-hole pairs. The wave packets are composed of solitonlike quasiparticles on top of the Fermi sea, which have been named “levitons.” However, it is not clear what kind of charged quasiparticles can be injected by Lorentzian pulses with fractional quantum flux. To answer this question, we study the wave packets injected by a train of Lorentzian pulses with repetition period $T$. We introduce a set of one-body wave functions, within which the quantum state of the charged quasiparticles can be described for pulses with arbitrary quantum flux. We find that in the general case, the injection of the charged quasiparticles is characterized by two different timescales: one is decided by the repetition period $T$ of the pulse train, while the other one is $T$ rescaled by a factor related to the flux of the pulses. For pulses with integer quantum flux, the two timescales match each other. In this case, the charged quasiparticles are levitons, which are injected with a single period $T$. For pulses with fractional quantum flux, the two timescales mismatch. The charged quasiparticles can then be injected in a multiperiodic way. This makes each quasiparticle carry only a fractional electric charge into the quantum conductor within a single period $T$. These quasiparticles can have pronounced impact on the charge injection. In particular, they can lead to the cycle-missing event, in which the voltage pulse fails to inject an electron within a single period $T$. The cycle-missing event can be seen intuitively from the waiting time distribution between electrons above the Fermi sea, which exhibits a series of peaks at multiples of the period $T$. By using the wave functions of the charged quasiparticles, we elucidate in detail how a leviton evolves as the flux of the pulse changes. In the meantime, we also clarify how additional $eh$ pairs can be excited.

We present a comprehensive structural study of the charge-orbital ordering and magnetic phase transitions observed in the $A$-site ordered $\mathrm{SmBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$ perovskite combining synchrotron radiation x-ray powder diffraction and symmetry-adapted modes analysis. In $\mathrm{SmBa}{\mathrm{Mn}}_2{\mathrm{O}}_6$, successive phase transitions in charge, spin, and lattice degrees of freedom take place with decreasing temperature at $T_{\mathrm{CO}1}\approx 380\mathrm{K}, T_{\mathrm{CO}2}\approx 190\mathrm{K}$, and $T_{\mathrm{N}}\approx 250\mathrm{K}$. The main difference between the two charge-ordered phases concerns the stacking sequence along the $c$ axis, which is double for the high temperature charge-ordered phase and has led to controversy in the literature. We show that both charge-ordered phases are pseudosymmetric with respect to the ideal undistorted tetragonal structure of $A$-site ordered $R\mathrm{Ba}{\mathrm{Mn}}_2{\mathrm{O}}_6$ perovskites and lead to two nonequivalent Mn sites. However, the charge segregation stabilizes at about $0.35e^{-}$ in the low temperature charge-ordered phase, clearly below the nominal separation of one charge unit between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ and undergoes a prominent increase in the high temperature charge-ordered phase when warming above $\approx 250\mathrm{K}$. The two Mn sites are anisotropic in both charge-ordered phases but the analysis of the active modes discloses that only the low temperature charge-ordered phase displays a Jahn-Teller-like distortion for one of the Mn sites. In addition, this low temperature charge-ordered phase has polar symmetry compatible with ferroelectricity along the $a$ axis.

We use density functional theory (DFT) and structure searching methods to show that the ground state of elemental sulfur is a barium-IVa type incommensurate host-guest (HG) phase between 386 and 679 GPa, becoming the first group-VI element predicted to possess such a structure. Within the HG phase, sulfur undergoes a series of transitions in which adjacent guest chains are not aligned but rather offset by different amounts, which can be described as a rearrangement of the stacking order of the chains. We show that these chain rearrangements are intimately coupled to modulations of the host and guest atoms, which prove crucial to stabilizing the HG structure. Unlike the high-pressure HG phases of other elements, sulfur does not exhibit interstitial charge localization, and instead features strongly localized “voids” that are depleted of electronic charge. Prior to adopting the HG structure, we predict that sulfur possesses an orthorhombic structure of $Fdd2$ symmetry. We calculate the superconducting critical temperatures of these newly discovered phases, and we show that $T_c$ is expected to peak between 24.8 and 28.2 K at 271 GPa.

We study non-Hermitian Aubry-André-Harper models with $p$-wave pairing, where the non-Hermiticity is introduced by on-site complex quasiperiodic potentials. By analyzing the $\mathcal{PT}$ symmetry breaking, winding numbers of energy spectra, localization and fractal dimensions of states, and fate of Majorana fermions, a complete phase diagram on localization and topological phase transitions is obtained. In particular, the non-Hermitian topological nature of localization phase transitions from extended to critical and then to localized phases is identified, using both analytical and numerical methods. In the critical phase, the complex spectrum is topologically nontrivial with a fractional winding number. In the localized phase the analytical localization length of states can apply to the Hermitian case, which is absent so far. Both the non-Hermiticity and quasiperiodicity are detrimental to Majorana fermions.

We investigate using local structural disorder to induce a topologically nontrivial phase in a solid state system. Using first-principles calculations, we introduce structural disorder in the trivial insulator BiTeI and observe the emergence of a topological insulating phase. By modifying the bonding environments, the crystal-field splitting is enhanced, with spin-orbit interactions producing a band inversion in the bulk electronic structure. Analysis of the Wannier charge centers and the surface electronic structure reveals a strong topological insulator with Dirac surface states. Finally, we propose a prescription for inducing topological states from disorder in crystalline materials. Understanding how local environments produce topological phases is a key step for predicting disordered and amorphous topological materials.