Charge noise remains the primary obstacle to the development of quantum information technologies with semiconductor spin qubits. We use an exact analytical calculation to determine the effects of quasistatic charge noise on a ring of three equally spaced exchange-coupled quantum dots. We calculate the disorder-averaged return probability from a specific initial state and use it to determine the coherence time $T_2^{*}$ and show that it depends on only the disorder strength and not the mean interaction strength. We also use a perturbative approach to investigate other arrangements of three or four qubits, finding that the return probability contains multiple oscillation frequencies. These oscillations decay in a Gaussian manner, determined by differences in energy levels of the Hamiltonian. We give quantitative values for gate times resulting in several target fidelities. We find that the decoherence time decreases with an increasing number of qubits. Our work provides useful analytical insight into the charge noise dynamics of coupled spin qubits.

Twisted bilayer graphene is highly sensitive to external perturbations. Strains, and the presence of the substrate, break the symmetries of the central bands. The resulting changes in the Berry curvature lead to valley currents and to a nonlinear Hall effect. We show that these effects, described by a Berry dipole, can be very significant, such that the nonlinear effects surpass the linear response for moderate applied fields, $\sim 0.1\mathrm{mV}/\mu \mathrm{m}$. The dependence of these effects on applied strain, coupling to the substrate, density of carriers, and temperature, makes them highly tunable.

We have grown and characterized three samples of Co-doped MnSi and studied their physical properties (magnetization and magnetic susceptibility, heat capacity, and electrical resistance). All three samples show non-Fermi liquid physical properties. From literature data and current results it follows that impurities (Co and Fe) eliminate the first-order phase transition peaks and spread the fluctuation maxima in such a way that the low-temperature part effectively reaches the zero temperature, where the fluctuations inevitably become quantum. The behavior of low-temperature parts of the heat capacity of the samples suggests that a gradual transition from classical to quantum fluctuations can be described by a simple power function of temperature with the exponent less than one. The $d\rho /dT$ data generally support this suggestion. The values of the heat capacity exponents immediately lead to the diverging ratio $C_p/T$ and hence to the diverging effective electron mass. We found that at a large concentration of the dopant there are no distinct phase transition points. What we observe is probably a region of the helical fluctuations spreading over a significant range of concentrations and temperatures, which become quantum close to 0 K.

Topological semimetals have been a focal point of research in recent years. One of the salient characters of topological semimetals is the anomalous Hall effect arising from the nontrivial momentum-space Berry phase. In contrast, the anomalous transverse thermoelectric coefficient and thermal Hall effect have been much less explored to date. In this paper, we report thermoelectric and thermal transport properties of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$, a prototypical Dirac semimetal. We find that ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ exhibits a large topological Nernst effect which is associated with the nonzero spin chirality of the skyrmion bubble phase. Furthermore, ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$ shows a prominent anomalous thermal Hall effect and the anomalous Nernst effect with features similar to the anomalous Hall effect. The anomalous Nernst signal is about $2.1\mu \mathrm{V}/\mathrm{K}$ at room temperature, which is comparable to the largest value reported thus far. These results highlight the synergic effects of the Berry phase in both real space and momentum space of ${\mathrm{Fe}}_3{\mathrm{Sn}}_2$, and this study demonstrates an effective strategy of investigating topological materials by measuring their transverse thermal and thermoelectric responses.

The magnetization-induced gap at the surface state is widely believed to be the kernel of magnetic topological insulators (MTIs) because of its relevance to various topological phenomena, such as the quantum anomalous Hall effect and the axion insulator phase. However, whether the magnetic gap exists in an intrinsic MTI, such as ${\mathrm{MnBi}}_2{\mathrm{Te}}_4$, still remains elusive, with significant discrepancies between theoretical predictions and various experimental observations. Here, including the previously overlooked self-doping in real MTIs, we find that in general a doped MTI prefers a ground state with a gapless surface state. We use a simple model based on Koopmans' theorem to elucidate the mechanism and further demonstrate it in the self-doped ${\mathrm{MnBi}}_2{\mathrm{Te}}_4/{\left({\mathrm{Bi}}_2{\mathrm{Te}}_3\right)}_n$ family through first-principles calculations. Our work sheds light on the design principles of MTIs with magnetic gaps by revealing the critical role of doping effects in understanding the delicate interplay between magnetism and topology.

Transition metal dichalcogenides are expected to be used in transparent, flexible, and highly efficient light-emitting devices. Exciton diffusion is a key factor in device applications. In this study, we measured the photoluminescence (PL) decay of excitons in a ${\mathrm{MoS}}_2$ monolayer synthesized by chemical vapor deposition on a sapphire substrate. The PL decay of A and B excitons in the femtosecond regime was observed. The PL decay curves were analyzed based on a model of exciton trapping at the deactivation center via diffusion, and the diffusion time until trapping was obtained. The diffusion coefficient and the corresponding exciton mobility were also determined. This study demonstrates the two-dimensional exciton diffusion in the femtosecond regime in the ${\mathrm{MoS}}_2$ monolayer.

In classical mechanics, solutions can be classified according to their stability. Each of them is part of the possible trajectories of the system. However, the signatures of unstable solutions are hard to observe in an experiment, and most of the times if the experimental realization is adiabatic, they are considered just a nuisance. Here we use a small number of XY magnetic dipoles subject to an external magnetic field for studying the origin of their collective magnetic response. Using bifurcation theory we have found all the possible solutions being stable or unstable, and explored how those solutions are naturally connected by points where the symmetries of the system are lost or restored. Unstable solutions that reveal the symmetries of the system are found to be the culprit that shape hysteresis loops in this system. The complexity of the solutions for the nonlinear dynamics is analyzed using the concept of boundary basin entropy, finding that the damping timescale is critical for the emergence of fractal structures in the basins of attraction. Furthermore, we numerically found domain wall solutions that are the smallest possible realizations of transverse walls and vortex walls in magnetism. We experimentally confirmed their existence and stability showing that our system is a suitable platform to study domain wall dynamics at the macroscale.

We study the effects of static magnetic and electric fields on Kitaev's honeycomb model. Using the electric polarization operator appropriate for Kitaev materials, we derive the effective Hamiltonian for the emergent Majorana fermions to second order in both the electric and magnetic fields. We find that while individually each perturbation does not qualitatively alter Kitaev spin liquid, the cross term induces a finite chemical potential at each Dirac node, giving rise to a Majorana-Fermi surface. We argue this gapless phase is stable and exhibits typical metallic phenomenology, such as linear in temperature heat capacity and finite, but nonquantized, thermal Hall response. Finally, we speculate on the potential for realization of this physics in Kitaev materials.

We study forward scattering of two-dimensional massless Dirac electrons at Fermi energy $\varepsilon >0$ in single-layer graphene (SLG) through a one-dimensional rectangular barrier of height $u_0$ in the presence of uniform Rashba spin-orbit coupling (of strength $\lambda$). The role of the Klein paradox in graphene spintronics is thereby exposed. It is shown that (1) for $\varepsilon -2\lambda <u_0<\varepsilon +2\lambda$, there is partial Klein tunneling, the transmission coefficient $T\left(\lambda \right)<1$, and quite remarkably, $T(\lambda \ge 0.1\text{meV})\approx 0$ when the scattering energy equals the barrier height $\varepsilon =u_0$ [whereas $T(\lambda =0)=2$]. (2) Spin density and spin-current density are remarkably different than in bulk SLG. They are sensitive to $\lambda$ and $u_0$. (3) Spin current densities are space dependent, implying the occurrence of nonzero spin torque density. Such a system may serve as a graphene-based spintronic device without the use of an external magnetic field or magnetic materials.

Copper monotelluride CuTe has a layered orthorhombic structure (space group Pmmn, No. 59) but exhibits a quasi-one-dimensional charge density wave (CDW) order. Here, we present a high-pressure study of CuTe in a diamond anvil cell at pressures up to 41.8–49.8 GPa, through a combination of electrical transport, synchrotron x-ray diffraction (XRD), and Raman spectroscopy measurements as well as theoretical calculations. Our transport experiments show that the CDW transition is manifested as a hump in resistivity; with the application of external pressure, its transition temperature $T_{\mathrm{CDW}}$ decreases gradually and the amplitude of the resistivity hump decays rapidly. The CDW gets suppressed completely above ∼10 GPa as estimated by a linear extrapolation of the $T_{\mathrm{CDW}}$ versus $P$ trend. Upon further compression, a sudden drop in resistivity is initially observed at ∼2.4 K and ∼20 GPa, and zero resistance is established above ∼37 GPa, suggesting the occurrence of superconductivity (SC). Combined XRD and Raman data as well as theoretical calculations together evidence that the emergence of SC at ∼20 GPa is accompanied by an orthorhombic to monoclinic (space group $\mathit{Cm}$, No. 8) structural transition.

Strain and defects in crystalline materials are responsible for the distinct mechanical, electric, and magnetic properties of a desired material, making their study an essential task in material characterization, fabrication, and design. Existing techniques for the visualization of strain fields, such as transmission electron microscopy and diffraction, are destructive and limited to thin slices of the materials. On the other hand, nondestructive x-ray imaging methods either have a reduced resolution or are not robust enough for a broad range of applications. Here we present x-ray ptychographic topography, a method for strain imaging, and demonstrate its use on an InSb micropillar after microcompression, where the strained region is visualized with a spatial resolution of 30 nm. Thereby, x-ray ptychographic topography proves itself as a robust nondestructive approach for the imaging of strain fields within bulk crystalline specimens with a spatial resolution of a few tens of nanometers.

Instanton processes are present in a variety of quantum field theories relevant to high energy as well as condensed matter physics. While they have led to important theoretical insights and physical applications, their underlying features often remain elusive due to the complicated computational treatment. Here, we address this problem by studying topological as well as nontopological instantons using Monte Carlo methods on lattices of interacting spins. As a proof of principle, we systematically construct instanton solutions in $O\left(3\right)$ nonlinear sigma models with a Dzyaloshinskii-Moriya interaction in $(1+1)$ and $(1+2)$ dimensions, thereby resembling an example of a chiral magnet. We demonstrate that, due to their close correspondence, Monte Carlo techniques in spin-lattice systems are well suited to describe topologically nontrivial field configurations in these theories. In particular, by means of simulated annealing, we demonstrate how to obtain domain walls, merons, and critical instanton solutions.

Using quantum Monte Carlo simulations, we map out the phase diagram of Hamiltonians interpolating between trivial and nontrivial bosonic symmetry-protected topological phases, protected by ${\mathbb{Z}}_2$ and ${\mathbb{Z}}_2^3$ symmetries, in two dimensions. In all cases, we find that the trivial and the topological phases are separated by an intermediate phase in which the protecting symmetry is spontaneously broken. Depending on the model, we identify a variety of magnetic orders on the triangular lattice, including ferromagnetism, $\sqrt{3}\times \sqrt{3}$ order, and stripe orders (both commensurate and incommensurate). Critical properties are determined through a finite-size scaling analysis. Possible scenarios regarding the nature of the phase transitions are discussed.

We study the key features of the Josephson transport through a curved semiconducting nanowire. Based on numerical simulations and analytical estimates within the framework of the Bogoliubov–de Gennes equations we find the ground-state phase difference ${\phi }_0$ between the superconducting leads tuned by the spin splitting field $h$ driving the system from the topologically trivial to the nontrivial superconducting state. The phase ${\phi }_0$ vanishes for rather small $h$, grows in a certain field range around the topological transition, and then saturates at large $h$ in the Kitaev regime. Both the subgap and the continuum quasiparticle levels are responsible for the above behavior of the anomalous Josephson phase. It is demonstrated that the crossover region on ${\phi }_0\left(h\right)$ dependencies reveals itself in the superconducting diode effect. The resulting tunable phase battery can be used as a probe of topological transitions in Majorana networks and can become a useful element of various quantum computation devices.

We apply the Green's function coupled cluster singles and doubles (GFCCSD) impurity solver to realistic impurity problems arising for strongly correlated solids within the self-energy embedding theory (SEET) framework. We describe the details of our GFCC solver implementation, investigate its performance, and highlight potential advantages and problems on examples of impurities created during the self-consistent SEET for antiferromagnetic MnO and paramagnetic $\mathrm{Sr}\mathrm{Mn}{\mathrm{O}}_3$. GFCCSD provides satisfactory descriptions for weakly and moderately correlated impurities with sizes that are intractable by existing accurate impurity solvers such as exact diagonalization. However, our data also shows that when correlations become strong, the singles and doubles approximation used in GFCC could lead to instabilities in searching for the particle number present in impurity problems. These instabilities appear especially severe when the impurity size gets larger and multiple degenerate orbitals with strong correlations are present. We conclude that, to fully check the reliability of GFCCSD results and use them in fully ab initio calculations in the absence of experiments, a verification from a GFCC solver with higher order excitations is necessary.

Recent experimental success in the realization of two-dimensional (2D) magnetism has stimulated the search for new magnetic 2D materials with strong magnetic anisotropy and high Curie temperature. One promising subgroup of 2D magnetic systems are Dirac half-metals (DHMs) which have gained a lot of interest recently, as they host a high-temperature quantum anomalous Hall effect. This paper discusses predictions for intrinsic DHMs and identifies them as realizations of the Kane-Mele Hubbard model at quarter filling. This proposed unification contributes to a firmer understanding of these materials and suggests pathways for the discovery of new DHM systems.

Two-dimensional spin-orbital magnets with strong exchange frustration have recently been predicted to facilitate the realization of a quantum critical point in the Gross-Neveu-SO(3) universality class. In contrast to previously known Gross-Neveu-type universality classes, this quantum critical point separates a Dirac semimetal and a long-range-ordered phase, in which the fermion spectrum is only partially gapped out. Here, we characterize the quantum critical behavior of the Gross-Neveu-SO(3) universality class by employing three complementary field-theoretical techniques beyond their leading orders. We compute the correlation-length exponent $\nu$, the order-parameter anomalous dimension ${\eta }_{\varphi }$, and the fermion anomalous dimension ${\eta }_{\psi }$ using a three-loop $\epsilon$ expansion around the upper critical space-time dimension of four, a second-order large-$N$ expansion (with the fermion anomalous dimension obtained even at the third order), as well as a functional renormalization group approach in the improved local potential approximation. For the physically relevant case of $N=3$ flavors of two-component Dirac fermions in $2+1$ space-time dimensions, we obtain the estimates $1/\nu =1.03\left(15\right), {\eta }_{\varphi }=0.42\left(7\right)$, and ${\eta }_{\psi }=0.180\left(10\right)$ from averaging over the results of the different techniques, with the displayed uncertainty representing the degree of consistency among the three methods.

We revisit the interplay between superconductivity and quantum criticality when thermal effects from virtual static bosons are included. These contributions, which arise from an effective theory compactified on the thermal circle, strongly affect field-theoretic predictions even at small temperatures. We argue that they are ubiquitous in a wide variety of models of non-Fermi liquid behavior, and generically produce a parametric suppression of superconducting instabilities. We apply these ideas to non-Fermi liquids in $d=2$ space dimensions, obtained by coupling a Fermi surface to a Landau-damped soft boson. Extending previous methods developed for $d=3-\epsilon$ dimensions, we determine the dynamics and phase diagram. It features a naked quantum critical point, separated by a continuous infinite order transition from a superconducting phase with strong non-Fermi liquid corrections. We also highlight the relevance of these effects for (numerical) experiments on non-Fermi liquids.

The axial magnetic effect (AME) is one of the anomalous transport phenomena in which the energy current is induced by an axial magnetic field. Here, we numerically study the AME for the relativistic Wilson fermion in the axial magnetic field and a twisted Dirac semimetal. The AME current density inside the bulk is nonzero, and particularly in the low-energy regime for the former model, it is explained by the field-theoretical results without any fitting parameter. However, for both models, the average AME current density vanishes owing to the surface contribution. The axial gauge field is regarded as the spatially modulated (effective) Zeeman field and induces the spatially modulated energy magnetization. The AME is attributed to the magnetization energy current and hence cannot be observed in transport experiments.