We study the decoherence dynamics of Majorana qubit braiding operations in a T-junction topological superconducting chain (TSC) system, in which the braiding is performed by controlling the electron-chemical potentials of the TSCs and the couplings between them. By solving rigorously the Majorana qubit dynamics, we show how the Majorana qubit coherence is generated through Bogoliubon correlations formed by exchanging Majorana zero modes (MZMs) between two TSCs in braiding operations. We also demonstrate with the exact master equation how MZMs and also the Bogoliubon correlations dissipate due to charge fluctuations of the controlling gates at both the zero and finite temperatures. As a result, Majorana qubit coherence and also the fermion parity conservation cannot be immune from local perturbations during braiding operations.

In a magnetic field bilayer graphene supports, at the lowest Landau level, eight characteristic zero-energy levels with an extra twofold degeneracy in Landau orbitals $n=\{0,1\}$. They, under general one-body and many-body interactions, evolve into pseudo-zero-mode (PZM) levels. A close look is made into the detailed structure and characteristics of such PZM levels and cyclotron resonance they host, with full account taken of spin splitting, interlayer bias, weak electron-hole asymmetry, and Coulomb interactions. It is pointed out that the PZM levels generally undergo orbital level mixing (in one valley or both valleys) as they are gradually filled with electrons and that an observation of interband cyclotron resonance over a finite range of filling provides a direct and sensitive probe for exploring many-body effects and orbital mixing in bilayer graphene.

Photoinduced charge separation in transition-metal dichalcogenide heterobilayers is being explored for moiré excitons, spin-valley polarization, and quantum phases of excitons/electrons. While different momentum points can be critically involved in charge separation dynamics, little is known directly from experiments. Here we determine momentum-resolved electron dynamics in the $\mathrm{W}{\mathrm{S}}_2/\mathrm{Mo}{\mathrm{S}}_2$ heterobilayer using time- and angle-resolved photoemission spectroscopy. Upon photoexcitation in the $K$ valleys, we detect electrons in $M$/2, $M$, and $Q$ valleys/points on timescales as short as ∼70 fs, followed by dynamic equilibration in $K$ and $Q$ valleys in ∼400 fs. These findings reveal the essential role of phonon scattering, the coexistence of direct and indirect interlayer excitons, and constraints on spin-valley polarization.

We extend the ab initio molecular dynamics (AIMD) method based on density functional theory to the nonequilibrium situation where an electronic current is present in the electronic system. The dynamics is treated using the semiclassical generalized Langevin equation. We demonstrate how the full anharmonic description of the interatomic forces is important in order to understand the current-induced heating and the energy distribution both in frequency and in real space.

We present magnetoresistivity measurements in high-quality single crystals of the Nowotny chimney ladder compound ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$. We find a linear and nonsaturating magnetoresistance up to 20 T. The magnetoresistance changes with the magnetic field orientation at small magnetic fields, from a positive to a negative curvature. Above 5 T, the magnetoresistance shows no sign of saturation up to 20 T for any measured angle. The shape of the anisotropy in the magnetoresistance remains when increasing temperature and Kohler's rule is obeyed. We associate the linear and nonsaturating magnetoresistance to a small Fermi surface with hot spots, possibly formed as a consequence of the structural transition. We discuss the relevance of electron-electron interactions under magnetic fields and aspects of the topologically nontrivial properties expected in ${\mathrm{Ru}}_2{\mathrm{Sn}}_3$.

A recent work [S. Sachdev et al., Phys. Rev. B 99, 054516 (2019)] proposed a SU(2) gauge theory for optimal doping criticality in the cuprate superconductors. The theory contains $N_h$ Higgs fields transforming under the adjoint representation of SU(2), with $N_h=1$ for the electron-doped cuprates and $N_h=4$ for the hole-doped cuprates. We investigate the strong-coupling dynamics of this gauge theory, while ignoring the coupling to fermionic excitations. We integrate out the SU(2) gauge field in a strong-coupling expansion and obtain a lattice action for the Higgs fields alone. We study such a lattice action, with $O\left(N_h\right)$ global symmetry, in an analytic large-$N_h$ expansion and by Monte Carlo simulations for $N_h=4$ and find consistent results. We find a confining phase with $O\left(N_h\right)$ symmetry preserved (this describes the Fermi-liquid phase in the cuprates), and Higgs phases (describing the pseudogap phase of the cuprates) with different patterns of the broken global $O\left(N_h\right)$ symmetry. One of the Higgs phases is topologically trivial, implying the absence of any excitations with residual gauge charges. The other Higgs phase has ${\mathbb{Z}}_2$ topological order, with “vison” excitations carrying a ${\mathbb{Z}}_2$ gauge charge. We find consistent regimes of stability for the topological Higgs phase in both our numerical and analytical analyses.

We investigate the temperature-dependent electronic structure of the van der Waals ferromagnet, ${\mathrm{CrGeTe}}_3$. Using angle-resolved photoemission spectroscopy, we identify atomic- and orbital-specific band shifts upon cooling through $T_{\mathrm{C}}$. From these, together with x-ray absorption spectroscopy and x-ray magnetic circular dichroism measurements, we identify the states created by a covalent bond between the Te $5p$ and the Cr $e_g$ orbitals as the primary driver of the ferromagnetic ordering in this system, while it is the Cr $t_{2g}$ states that carry the majority of the spin moment. The $t_{2g}$ states furthermore exhibit a marked bandwidth increase and a remarkable lifetime enhancement upon entering the ordered phase, pointing to a delicate interplay between localized and itinerant states in this family of layered ferromagnets.

Topological insulator with antiferromagnetic order can serve as an ideal platform to realize axion electrodynamics. In this paper, we report a systematic study of the axion topological insulator candidate ${\mathrm{EuIn}}_2{\mathrm{As}}_2$. A linear energy dispersion across the Fermi level reveals a hole-type Fermi pocket. The orientation of the magnetic moment for ground state is determined within the $ab$-plane by anisotropic magnetic behavior. Besides long-range antiferromagnetic order, magnetization and magnetotransport measurements indicate existence of the ferromagnetic orders and ferromagnetic correlation, suggesting the formation of the magnetic polarons. These ferromagnetic clusters can persist above the antiferromagnetic transition leading to unconventional transport properties. Our results suggest multiple magnetic orders and states in ${\mathrm{EuIn}}_2{\mathrm{As}}_2$, which is vital to understanding its topological nature.

Pressurizing strategically selected compositions of the $\mathrm{EuC}{\mathrm{u}}_2{\left(\mathrm{G}{\mathrm{e}}_{1-x}\mathrm{S}{\mathrm{i}}_x\right)}_2$ series affords an opportunity for gaining microscopic insight into the ground-state properties and interplay between magnetism and valence fluctuations across a quantum critical point. This is investigated by way of systematic ${}^{151}\mathrm{Eu}$ Mössbauer spectroscopy measurements on $x=0$ and $x=0.5$ compositions in the series, pressurized up to 7 GPa including variable temperature scans in the range 300–4.2 K. In $\mathrm{EuC}{\mathrm{u}}_2\mathrm{G}{\mathrm{e}}_2$ the temperature and pressure dependences of the hyperfine interaction parameters indicate that both the magnetic and divalent state, $\mathrm{E}{\mathrm{u}}^{\nu +}$ where $\nu =2$, are stable up to 6–7 GPa, thus serving as a useful reference. Whereas in the $x=0.5$ composition which initially involves $\mathrm{E}{\mathrm{u}}^{2+}$, collapse of the magnetically ordered state is onset at ∼1.3 GPa and there is emergence of a nonmagnetic intermediate valence state coexisting with the magnetically ordered state. This regime of mixed states is a precursor of a quantum phase transition to a nonmagnetic homogeneous intermediate valence state $\nu \sim 2.45$, across a quantum critical point at 3.6 GPa, suggesting a first-order phase transition. X-ray-diffraction pressure studies at 300 K up to 6 GPa of the $x=0.5$ composition indicate there is no change in lattice symmetry from the tetragonal $\mathrm{ThC}{\mathrm{r}}_2\mathrm{S}{\mathrm{i}}_2$-type structure. There are also no obvious discontinuities in pressure dependences of the lattice parameters upon evolving through the quantum critical point at 3.6 GPa. Increasing pressure changes the starting $\mathrm{E}{\mathrm{u}}^{2+}$ valence monotonically, until the mean valence attains its largest value $\nu \sim 2.45$ indicative of enhanced charge fluctuations at the quantum critical point and plateaus thereafter. High-pressure resistance measurements at low temperatures down to 40 mK near the quantum critical point reveal no evidence for superconductivity.

We investigate the stability of $xy$ orbital ordering and the nature of the magnetic longitudinal excitations in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. To this end, we employ the local-density approximation $+$ dynamical mean-field theory approach, in combination with many-body perturbation theory. We show that the crossover to a nonperturbative spin-orbit regime—-in which $xy$-like orbital ordering is to a large extent quenched—only takes place when the crystal field is sizably reduced with respect to its actual value in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$. In the small crystal-field splitting limit, the spin-orbit interaction favors the metal-to-insulator transition. We find that the effects of the spin-orbit interaction remain perturbative even for the less distorted ${\mathrm{Ca}}_{2-x}{\mathrm{Sr}}_x{\mathrm{RuO}}_4$ with $0<x<0.2$ ($S$-Pbca phase) and for the $S^{*}$ phase. We show that, nevertheless, a Higgs mode at $\omega \sim 50\mathrm{meV}$ is compatible with $xy$-like orbital ordering in ${\mathrm{Ca}}_2{\mathrm{RuO}}_4$.

We report a comprehensive study of the spin ladder compound ${\mathrm{BaFe}}_2{\mathrm{S}}_{2.5}{\mathrm{Se}}_{0.5}$ using neutron diffraction, inelastic neutron scattering, high pressure synchrotron diffraction, and high pressure transport techniques. We find that ${\mathrm{BaFe}}_2{\mathrm{S}}_{2.5}{\mathrm{Se}}_{0.5}$ possesses the same $Cmcm$ structure and stripe antiferromagnetic order as does ${\mathrm{BaFe}}_2{\mathrm{S}}_3$, but with a reduced Néel temperature of $T_N=98$ K compared to 120 K for the undoped system, and a slightly increased ordered moment of $1.40{\mu }_B$ per iron. The low-energy spin excitations in ${\mathrm{BaFe}}_2{\mathrm{S}}_{2.5}{\mathrm{Se}}_{0.5}$ are likewise similar to those observed in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$. However, unlike the reports of superconductivity in ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ below $T_c\sim 14$ K under pressures of 10 GPa or more, we observe no superconductivity in ${\mathrm{BaFe}}_2{\mathrm{S}}_{2.5}{\mathrm{Se}}_{0.5}$ at any pressure up to 19.7 GPa. In contrast, the resistivity exhibits an upturn at low temperature under pressure. Furthermore, we show that additional high-quality samples of ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ synthesized for this study likewise fail to become superconducting under pressure, instead displaying a similar upturn in resistivity at low temperature. These results demonstrate that microscopic, sample-specific details play an important role in determining the ultimate electronic ground state in this spin ladder system. We suggest that the upturn in resistivity at low temperature in both ${\mathrm{BaFe}}_2{\mathrm{S}}_3$ and ${\mathrm{BaFe}}_2{\mathrm{S}}_{2.5}{\mathrm{Se}}_{0.5}$ may result from Anderson localization induced by S vacancies and random Se substitutions, enhanced by the quasi-one-dimensional ladder structure.

Three-dimensional topological insulators or axion insulators exhibit the topological magnetoelectric effect, which is isotropic with a universal coefficient of proportionality quantized in units of $e^2/2h$. Here we study the finite-size effect of topological magnetoelectric effect, and find the magnetoelectric coefficients are anisotropic, namely ${\alpha }_{xx}\ne {\alpha }_{zz}$. Both of them are shown to converge to a quantized value when the thickness of topological insulator film $d$ increases reaching the three-dimensional bulk limit. The nonzero value of $({\alpha }_{xx}-{\alpha }_{zz})\propto 1/d$ could be measured by using the gyrotropic or nonreciprocal birefringence of terahertz light. The unique $1/d$ dependence on film thickness of the rotation angle of optical principle axes is the manifestation of topological magnetoelectric effect, which may also serve as a smoking gun signature for axion insulators.

Electrical breakdown at hydrogen/oxygen interfaces is limiting the use of nanoscaled diamond devices for the control of optically active defect centers. In this work electron transport across an oxygen-terminated potential barrier in a hydrogen-terminated surface conductive diamond is investigated. We analyze temperature-dependent current-voltage characteristics for different barrier widths and report on a reduced effective barrier height compared to theoretical expectations. This is ascribed to an inhomogeneous potential landscape, as observed by Kelvin probe and conductive force microscopy. Furthermore, we use photocurrent spectroscopy to discuss possible transport processes and identify a field dependent absorption feature. A defect state involved in transport across the barrier is proposed at the hydrogen-oxygen barrier approximately $1\mathrm{eV}$ above the valence band maximum. The new understanding enabled by our work may help to overcome the current limitations of diamond surface electronics.

A high degree of controllability and long coherence time make the nuclear spin of a phosphorus donor in isotopically purified silicon a promising candidate for a quantum bit. However, long-distance two-qubit coupling and fast, robust gates remain outstanding challenges for these systems. Here, following recent proposals for long-distance coupling via dipole-dipole interactions, we present a simple method to implement fast, high-fidelity arbitrary single- and two-qubit gates in the absence of charge noise. Moreover, we provide a method to make the single-qubit gates robust to moderate levels of charge noise to well within an error bound of ${10}^{-3}$.

We present a method to accurately and efficiently calculate many-body states of interacting carriers in quantum nanostructures based on a combination of iterative selection of configurations and perturbation theory. This method enables investigations of large excitonic complexes and multielectron systems with near full configuration interaction accuracy, even though only a small subspace of the full many-body Hilbert space is sampled, thus saving orders of magnitudes in computational resources. Important advantages of this method are that the convergence is controlled by a single parameter, the threshold, and that ground and excited states can be treated on an equal footing. On the example of InAsP nanowire quantum dots described using a million-atom tight-binding approach, we demonstrate the extreme efficiency of the method by numerical studies of large excitonic complexes filling up to the fourth electronic shell. We find that the method generally converges fast as a function of the threshold, profiting from a significant acceleration due to the perturbative corrections. The role of the choice of single-particle basis states is discussed. It is found that the algorithm converges faster in the Hartree-Fock basis only for highly charged systems, where Coulomb repulsion dominates. Finally, based on the observation that second-order perturbative energy corrections only depend on off-diagonal elements of the many-body Hamiltonian, we present a way to accurately calculate many-body states that requires only a relatively small number of Coulomb matrix elements.

For topological characterization of non-Hermitian multiband systems, Majorana's stellar representation (MSR) is applied to one-dimensional multiband models consisting of asymmetric nearest-neighbor hopping and imaginary on-site potentials. The number of edge states isolated from the continuous bulk bands in the complex energy plane is successfully linked with a topological invariant constructed from MSR. Specifically, the number of isolated edge states can be obtained from a winding number defined for the Majorana stars, which also allows for a geometric visualization of the topology related to the isolated edge modes. A remarkable success of our approach is that our winding number characterization remains valid even in the presence of exceptional points of the continuous bulk bands, where the Hamiltonian becomes nondiagonalizable and hence conventional topological invariants such as the Zak phase and the Chern number cannot be properly defined. Furthermore, cases with the so-called non-Hermitian skin effect are also studied, showing that the bulk-boundary correspondence between our defined winding numbers and isolated edge states can be restored. Of particular interest is a four-band example with an odd number of isolated edge states, where the Zak phase approach necessarily fails upon removing the skin effect, but our MSR-based characterization works equally well. For these reasons, our study is expected to be widely useful in topological studies of non-Hermitian multiband systems, regardless of the skin effect or the presence of the exceptional points in non-Hermitian systems.

Near-surface indium arsenide quantum wells have recently attracted a great deal of interest since they can be interfaced epitaxially with superconducting films and have proven to be a robust platform for exploring mesoscopic and topological superconductivity. In this paper, we present magnetotransport properties of two-dimensional electron gases confined to an indium arsenide quantum well near the surface. The electron mass extracted from the envelope of the Shubnikov–de Haas oscillations shows an average effective mass $m^{*}=0.04$ at a low magnetic field. Complementary to our magnetotransport study, we employed cyclotron resonance measurements and extracted the electron effective mass in the ultrahigh magnetic-field regime. Both regimes can be understood by considering a model that includes nonparabolicity of the indium arsenide conduction bands.

Short-range ordered polar nanoregions are key to the giant electromechanical properties exhibited by relaxor ferroelectrics. Stabilization of the long-range ferroelectric order in relaxor systems has typically been achieved by applying external fields. In this work, spontaneous (zero-field) ferroelectric order is demonstrated in the composites constituting of nonergodic relaxor matrix phase $0.91\mathrm{N}{\mathrm{a}}_{1/2}\mathrm{B}{\mathrm{i}}_{1/2}\mathrm{Ti}{\mathrm{O}}_3\text{−}0.09\mathrm{BaTi}{\mathrm{O}}_3$ with ZnO inclusions. Direct structural evidence is provided for the long-range ferroelectric order in the composites using in situ electric-field-dependent synchrotron investigations and ${}^{23}\mathrm{Na}$ nuclear magnetic resonance spectroscopy. Thermodynamic analysis incorporating microelasticity reveals the role of spatial residual stress in stabilizing the ferroelectric order. The work provides a direct correlation between the stabilized ferroelectric order and enhanced thermal stability, which can be utilized to guide the design of spontaneous long-range order in other relaxor systems.

Angle-dispersive x-ray powder diffraction experiments have been performed on samarium metal up to 222 GPa. Up to 50 GPa we observe the Sm type ($hR9$) $\to$ dhcp ($hP4$) $\to$ fcc ($cF4$) $\to$ distorted fcc ($hR24$) $\to hP3$ transition sequence reported previously. The structure of the high-pressure phase above 93 GPa, previously reported as having a monoclinic structure with space group C2/m, is found to be orthorhombic, space group Fddd, with eight atoms per unit cell ($oF8$ in Pearson notation). This structure is the same as that found in Am, Cm, and Cf at high pressures. Analysis of samarium's equation of state reveals marked changes in compressibility in the $hP3$ and $oF8$ phases, with the compressibility of the $oF8$ phase being that of a “regular” metal.