Diluted ferromagnetic semiconductors combining ferromagnetic and semiconducting properties in one material provide numerous new functionalities, attractive for basic studies and potentially useful for novel device applications. The tailoring of the electronic structure in analogy to conventional semiconductors has yet to be explored. Here, we demonstrate the conservation of broken inversion symmetry and band structure tailoring for high-quality molecular-beam-epitaxy-grown (In,Ga,Mn)As films with 3% In plus 2.5% or 5.6% Mn using hard-x-ray photoelectron diffraction (hXPD) and momentum microscopy. Photon energies of 3–5 keV ensure that the results are not corrupted by surface effects, which are known to be strong in semiconductors. The missing inversion center of the GaAs host lattice leads to fingerprint-like hXPD signatures of As and Ga sites. For both concentrations, Mn predominantly occupies Ga substitutional sites. Momentum microscopy reveals a shift of the chemical potential with increasing Mn doping and a highly dispersing band, crossing the Fermi level for high Mn concentration. The Mn doping induces a pronounced modification of the spin-orbit split-off band.

The magnetic properties of black phosphorene nanoribbons are investigated using static and dynamical mean-field theory. Besides confirming the existence of ferromagnetic/antiferromagnetic edge magnetism, our detailed calculations using large unit cells find a phase transition at weak interaction strength to an incommensurate (IC) magnetic phase. A detailed Fourier analysis of the magnetization patterns in the IC phase shows the existence of a second critical interaction strength, where the incommensurate phase changes to an antiferromagnetic (AFM) or ferromagnetic (FM) phase. We demonstrate that the difference of the ground-state energies of the AFM and FM phase is exponentially small, making it possible to switch between both states by a small external field. Finally, we analyze the influence of strain and disorder on the magnetic properties and show that while the IC phase is robust to Anderson type disorder, it is fragile against strain.

We analyze a steady-state thermoelectric engine, whose working substance consists of two capacitively coupled quantum dots. One dot is tunnel-coupled to a hot reservoir serving as a heat source, the other one to two electrically biased reservoirs at a colder temperature, such that work is extracted under the form of a steady-state current against the bias. In single realizations of the dynamics of this steady-state engine autonomous, four-stroke cycles can be identified. The cycles are purely stochastic, in contrast to mechanical autonomous engines which exhibit self-oscillations. In particular, these cycles fluctuate in direction and duration and occur in competition with other spurious cycles. Using a stochastic thermodynamic approach, we quantify the cycle fluctuations and relate them to the entropy produced during individual cycles. We identify the cycle mainly responsible for the engine performance and quantify its statistics with tools from graph theory. We show that such stochastic cycles are made possible because the work extraction mechanism is itself stochastic instead of the periodic time dependence in the working-substance Hamiltonian which can be found in conventional mechanical engines. Our investigation brings new perspectives about the connection between cyclic and steady-state engines.

We investigate the Bi on Si(111) $\sqrt{3}\times \sqrt{3}\beta$-phase surface reconstruction using scanning tunneling microscopy. Details of the bias-dependent images of both the reconstruction and defects are presented. Combining our experimental data with density-functional theory calculations, it is confirmed that the honeycomb pattern at low-bias empty states and hexagonal closest-packed pattern at high-bias empty states originate from the Bi $p_x, p_y$ states and Bi, Si overlapped states outside bulk band gap, respectively. Analysis of the defect images and their associated densities of states provides further insight into the electronic structure of the surface. In particular, we note the presence of a quantum-dotlike localized state associated with an $\alpha$-phase defect structure. These results pave the way for further development of surface structures based on Bi on Si(111) surface with unique electronic properties such as sizable spin-orbit coupling, which might be suitable candidates for spintronic applications.

We consider a noncompact lattice formulation of the three-dimensional electrodynamics with $N$-component complex scalar fields, i.e., the lattice Abelian-Higgs model with noncompact gauge fields. For any $N\ge 2$, the phase diagram shows three phases differing for the behavior of the scalar-field and gauge-field correlations: The Coulomb phase (short-ranged scalar and long-ranged gauge correlations), the Higgs phase (condensed scalar-field and gapped gauge correlations), and the molecular phase (condensed scalar-field and long-ranged gauge correlations). They are separated by three transition lines meeting at a multicritical point. Their nature depends on the phases they separate, and on the number $N$ of components of the scalar field. In particular, the Coulomb-to-molecular transition line (where gauge correlations are irrelevant) is associated with the Landau-Ginzburg-Wilson ${\mathrm{\Phi }}^4$ theory sharing the same SU($N$) global symmetry but without explicit gauge fields. On the other hand, the Coulomb-to-Higgs transition line (where gauge correlations are relevant) turns out to be described by the continuum Abelian-Higgs field theory with explicit gauge fields. Our numerical study is based on finite-size scaling analyses of Monte Carlo simulations with $C^{*}$ boundary conditions (appropriate for lattice systems with noncompact gauge variables, unlike periodic boundary conditions), for several values of $N$, i.e., $N=2$, 4, 10, 15, and 25. The numerical results agree with the renormalization-group predictions of the continuum field theories. In particular, the Coulomb-to-Higgs transitions are continuous for $N\gtrsim 10$, in agreement with the predictions of the Abelian-Higgs field theory.

Many-body localization is a fascinating theoretical concept describing the intricate interplay of quantum interference, i.e., localization, with many-body interaction-induced dephasing. Numerous computational tests and also several experiments have been put forward to support the basic concept. Typically, averages of time-dependent global observables have been considered, such as the charge imbalance. We here investigate within the disordered spinless Hubbard ($t\text{−}V$) model how dephasing manifests in time-dependent variances of observables. We find that after quenching a Néel state the local charge density exhibits strong temporal fluctuations with a damping that is sensitive to disorder $W$: variances decay in a power-law manner, $t^{-\zeta }$, with an exponent $\zeta \left(W\right)$ strongly varying with $W$. A heuristic argument suggests the form $\zeta \approx \alpha \left(W\right){\xi }_{\text{sp}}$, where ${\xi }_{\text{sp}}\left(W\right)$ denotes the noninteracting localization length and $\alpha \left(W\right)$ characterizes the multifractal structure of the dynamically active volume fraction of the many-body Hilbert space. In order to elucidate correlations underlying the damping mechanism, exact computations are compared with results from the time-dependent Hartree-Fock approximation. Implications for experimentally relevant observables, such as the imbalance, will be discussed.

Manipulating the electronic properties of graphene has been a subject of great interest since it can aid material design to extend the applications of graphene to many different areas. In this paper, we systematically investigate the effect of lead (Pb) intercalation on the structural and electronic properties of epitaxial graphene on the SiC(0001) substrate. We show that the band structure of Pb-intercalated few-layer graphene can be effectively tuned through changing intercalation conditions, such as coverage, location of Pb, and the initial number of graphene layers. Lead intercalation at the interface between the buffer layer (BL) and the SiC substrate decouples the BL from the substrate and transforms the BL into a $p$-doped graphene layer. We also show that Pb atoms tend to donate electrons to neighboring layers, leading to an $n$-doping graphene layer and a small gap in the Dirac cone under a sufficiently high Pb coverage. This paper provides useful guidance for manipulating the electronic properties of graphene layers on the SiC substrate.

Transition-metal dichalcogenides (TMDs) are promising for two-dimensional (2D) semiconducting devices and novel phenomena. For 2D applications, their work function, ionization energy, and electron affinity are required as a function of thickness, but research on this is yet to cover the full family of compounds. Here, we present the work function, ionization energy, and electron affinity of few-layer and bulk $M{X}_2$ ($M=\text{Mo},$ W and $X=\text{S}$, Se, Te) in 2H phase obtained accurately by the density functional theory and GW calculations. For each compound, we consider one-, two-, three-, four-layer, and bulk geometry. In GW calculations, accurate results are obtained by nonuniform $q$ sampling for two-dimensional geometry. From band energies including the GW self-energy correction, we estimate the work function, band gap, ionization energy, and electron affinity as functions of the number of layers. We compare our results with available theoretical and experimental reports, and we discuss types of band alignments in in-plane and out-of-plane junctions of these few-layer and bulk TMDs.

Using density functional theory and a thermodynamic model [V. Sokolovskiy et al. Phys. Rev. B 86, 134418 (2012)] in this paper we provide an approach to systematically screen compounds of a given Heusler family to predict ones that can yield giant magnetocaloric effect driven by a first-order magnetostructural transition. We apply this approach to two Heusler series ${\mathrm{Ni}}_{2-x}{\mathrm{Fe}}_x{\mathrm{Mn}}_{1+z-y}{\mathrm{Cu}}_y{\mathrm{Sb}}_{1-z}$ and ${\mathrm{Ni}}_{2-x}{\mathrm{Co}}_x{\mathrm{Mn}}_{1+z-y}{\mathrm{Cu}}_y{\mathrm{Sb}}_{1-z}$ obtained by cosubstitution at Ni and Mn sites. We predict four new compounds with potentials to achieve the target properties. Our computations of the thermodynamic parameters relevant for magnetocaloric applications show that the improvement in the parameters in the predicted cosubstituted compounds can be as large as four times in comparison to the off-stoichiometric Ni-Mn-Sb and a compound derived by single substitution at the Ni site where magnetocaloric effects have been observed experimentally. This work establishes a protocol to select new compounds that can exhibit large magnetocaloric effects and demonstrate cosubstitution as a route for more flexible tunability to achieve outcomes better than the existing ones.

Ferromagnesite $\left[\left({\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x\right){\mathrm{CO}}_3\right]$, also referred to as magnesiosiderite at high iron concentration ($x>0.5$), is a solid solution of magnesite $(\mathrm{Mg}{\mathrm{CO}}_3$) and siderite ($\mathrm{Fe}{\mathrm{CO}}_3$). Ferromagnesite is believed to enter the Earth's lower mantle via subduction and is considered a major carbon carrier in the Earth's lower mantle, playing a key role in the Earth's deep carbon cycle. Experiments have shown that ferromagnesite undergoes a pressure-induced spin crossover, accompanied by volume and elastic anomalies, in the lower-mantle pressure range. In this work, we investigate thermal properties of $\left({\mathrm{Mg}}_{1-x}{\mathrm{Fe}}_x\right){\mathrm{CO}}_3$ ($0<x\le 1$) using first-principles calculations. We show that nearly all thermal properties of ferromagnesite are drastically altered by iron spin crossover, including anomalous reduction of volume, anomalous softening of bulk modulus, and anomalous increases of thermal expansion, heat capacity, and the Grüneisen parameter. Remarkably, the anomaly of heat capacity remains prominent (up to $\sim 40\%$) at high temperature without smearing out, which suggests that iron spin crossover may significantly affect the thermal properties of subducting slabs and the Earth's deep carbon cycle.

We study spin-torque and electrical transport properties of Dirac electrons on the surface of magnetic topological insulators. Usually, surface Dirac electrons are described by a linear-dispersion model, which exhibits various peculiar features originating from the spin-velocity equivalence or spin-momentum locking. In this paper, a focus is placed on the effects of hexagonal warping (parametrized by $\lambda$) of the otherwise linear dispersion, which removes such peculiar features. Focusing on perpendicular magnetization, we calculate Gilbert damping, spin-orbit torque (SOT), and electrical conductivity, which are “degenerate” in the absence of hexagonal warping, $\lambda =0$, but this degeneracy is lifted in its presence, $\lambda \ne 0$. The electrical conductivity was found to be greatly enhanced by $\lambda$ at large doping, whereas the other two remain moderate. Their reactive counterparts (spin renormalization, reactive SOT, and Hall conductivity), which are nonzero even in the undoped (insulating) state and also mutually degenerate at $\lambda =0$, become differentiated by $\lambda$, with the Hall conductivity remaining unchanged at the universal value. We next study current-induced torques with magnetization gradient. We first classify all possible forms allowed by symmetry, and find there are three ways of representation (SOT, spin-transfer torque, and gradient torque), which we call “viewpoints,” each of which forms a complete set of torques. Explicit calculation is then performed in the SOT viewpoint. While such torques are exhausted by two SOT-type ones at $\lambda =0$, examination of magnon Doppler shift and effects on domain-wall motion indicates that the spin-transfer torques (in a narrow sense) do exist even at $\lambda =0$. Finally, the current-induced Dzyaloshinskii-Moriya interaction is shown to arise starting linearly with $\lambda$, in contrast to the other effects mentioned above that start quadratically $(\sim {\lambda }^2)$.

Using ultrafast x-ray diffraction, we directly monitor the lattice dynamics induced by femtosecond laser pulses in nanoscale thin films of bismuth iron garnet in external magnetic fields $H_{\mathrm{ext}}$. We control the ultrafast laser-induced lattice strain amplitude by changing the laser pulse helicity. The strength of $H_{\mathrm{ext}}$ is used as an external parameter to switch the helicity dependence on and off, respectively. Based on magneto-optical spectroscopic measurements, we explain these phenomena by magnetic circular dichroism. Our findings highlight an important approach for ultrafast manipulation of lattice strain in magnetic materials, in particular insulators, and open exciting perspectives towards ultrafast control of lattice strain and heat-induced magnetization switching and spin waves in bismuth substituted iron garnets using the polarization of light.

We have successfully grown single crystals of $\mathrm{Pb}{\mathrm{Mn}}_2{\mathrm{Ni}}_6{\mathrm{Te}}_3{\mathrm{O}}_{18}$ and present a comprehensive study of their magnetic, thermodynamic, and Raman spectroscopic properties. $\mathrm{Pb}{\mathrm{Mn}}_2{\mathrm{Ni}}_6{\mathrm{Te}}_3{\mathrm{O}}_{18}$ consists of a planar network of pairwise rotated $\mathrm{Ni}{\mathrm{O}}_6$ dimers coupled by corners. Similarities to ${\mathrm{Ni}}_3\mathrm{Te}{\mathrm{O}}_6$ exist, which forms honeycomb layers. The magnetic susceptibility χ and heat capacity $C_p$ data reveal an antiferromagnetic phase transition around 84 K, which is evidently hysteretic on warming and cooling between 94 and 40 K with a loop width of about 1.83± 0.41 K; thus the transition appears to be of first order. χ is anisotropic, with larger values for in-plane fields over the entire measured temperature range. Raman spectroscopy has been employed to investigate the lattice and magnetic excitations of the $\mathrm{Pb}{\mathrm{Mn}}_2{\mathrm{Ni}}_6{\mathrm{Te}}_3{\mathrm{O}}_{18}$ from 5 to 300 K. Besides an anharmonic phonon behavior, i.e., a decay into acoustic phonons, we find a coupling to the spin system at $T_N=84\mathrm{K}$, as well as weak anomalies at $T^{*}\approx 200\mathrm{K}$. This second characteristic temperature gives evidence for an instability of the coupled spin/lattice system, as here the phonon linewidths and intensities evidence similar behavior as at $T_N=84\mathrm{K}$. Furthermore, magnetic Raman scattering at $240\mathrm{c}{\mathrm{m}}^{-1}$ is used to estimate an exchange coupling of $J=–86\mathrm{K}$ in general agreement with the Curie-Weiss temperature.

Huang's experiment [Phys. Rev. Lett. 109, 156802 (2012)] found, in the quantum Hall bilayer of the Corbino geometry, the interlayer tunneling currents at two edges are coupled to each other and one of two tunneling currents is referred to as the compensating current of the other. The recent theoretical work [Hsu et al., arXiv:2006.15329] has explained this exotic coupling phenomenon as a result of excitonic Josephson effect induced by interlayer tunneling current. In this paper, we study the same setup—excitonic Josephson junction—but in the weak Josephson coupling regime, which occurs for large junction length. Interestingly, we find the compensating current drives the other edge to undergo a nonequilibrium phase transition from a superfluid to resistive state, which is signaled by an abrupt jump of the critical tunneling current. We also identify the critical exponent and furthermore offer more experimental prediction.

In a previous work, it was shown that the inclusion of exact exchange is essential for a first-principles description of both the electronic and the vibrational properties of $\mathrm{Ti}{\mathrm{Se}}_2$, M. Hellgren et al. [Phys. Rev. Lett. 119, 176401 (2017)]. The $GW$ approximation provides a parameter-free description of screened exchange but is usually employed perturbatively ($G_0{W}_0$), making results more or less dependent on the starting point. In this work, we develop a quasi-self-consistent extension of $G_0{W}_0$ based on the random phase approximation (RPA) and the optimized effective potential of hybrid density functional theory. This approach generates an optimal $G_0{W}_0$ starting point and a hybrid exchange parameter consistent with the RPA. While self-consistency plays a minor role for systems such as Ar, BN, and ScN, it is shown to be crucial for $\mathrm{Ti}{\mathrm{S}}_2$ and $\mathrm{Ti}{\mathrm{Se}}_2$. We find the high-temperature phase of $\mathrm{Ti}{\mathrm{Se}}_2$ to be a semimetal with a band structure in good agreement with experiment. Furthermore, the optimized hybrid functional agrees well with our previous estimate and therefore accurately reproduces the low-temperature charge-density-wave phase.

The quantized Hall conductivity of integer and fractional quantum Hall (IQH and FQH) states is directly related to a topological invariant, the many-body Chern number. The conventional calculation of this invariant in interacting systems requires a family of many-body wave functions parameterized by twist angles to calculate the Berry curvature. In this paper, we demonstrate how to extract the Chern number given a single many-body wave function, without knowledge of the Hamiltonian. For FQH states, our method requires one additional integer invariant as input: the number of $2\pi$ flux quanta, $s$, that must be inserted to obtain a topologically trivial excitation. As we discuss, $s$ can be obtained in principle from the degenerate set of ground state wave functions on the torus, without knowledge of the Hamiltonian. We perform extensive numerical simulations involving IQH and FQH states to validate these methods.

We discuss a proof-of-principle experiment in which we detect triple electron emission from a surface due to primary electron impact. The new aspect is the ability to record the energies and emission directions of the ejected electrons. We selected NiO films as a target, which have shown in previous electron pair emission studies to give an enhanced intensity compared to other materials. The triple sum energy spectrum displays a shape consistent with a self-convolution of the electronic density of states. We define two different emission geometries. While the energy distributions are essentially identical, the intensity levels differ by a factor of 2. Imposing a geometrical constraint on one of the emitted triples shows that the available energy is equally shared among the other two electrons. We discuss our findings within a simplified scattering model. We also present angular distributions. Prominent intensity minima for electron emission in the same direction are not observed in contrast to our previous electron pair emission studies.

Fermions in a Fermi gas obey the Pauli exclusion principle restricting any two fermions from occupying the same quantum state. Strong interactions between fermions can completely change the properties of the Fermi gas. In our theoretical study we find an exotic quantum phase in strongly interacting Fermi gases subject to a certain condition imposed on the Fermi surfaces that we call the Fermi surface resonance. The phase is quantum critical in time and space and can be identified by the power-law dependence of the spectral density in frequency and momentum. The linear-response functions are singular in the static limit and at the Kohn anomalies. We analyze the quantum critical state at finite temperatures $T$ and finite size $L$ of the Fermi gas and provide a qualitative $L-T$ phase diagram. The quantum critical phase can be experimentally found in typical semiconductor heterostructures.

Expansive open Fermi arcs covering most of the surface Brillouin zone (SBZ) are desirable for detection and control of many topological phenomena, but they have generally been reported for Kramers-Weyl points, or unconventional chiral fermions, pinned at time-reversal invariant momentum in chiral materials. Here using first-principles band structure calculations, we show that for conventional Weyl points in $\mathrm{Zr}{\mathrm{Te}}_5$ with the chirality of +1/−1 near the BZ center at general momentum induced by one of the infrared phonons—the second lowest $B_{1u}$ mode for breaking inversion symmetry—they can also form expansive open Fermi arcs across the SBZ boundary to occupy most of the SBZ when projected on the (001) surface. We reveal that such expansive open Fermi arcs are evolved from the topological surface states that connect multiple surface Dirac points on the (001) surface of the topological insulator phases without lattice distortion in $\mathrm{Zr}{\mathrm{Te}}_5$. Furthermore, we find that the connectivity of the induced open Fermi arcs can be changed by the magnitude of the lattice distortion of this infrared phonon mode. Thus, we propose that using coherent optical phonons to modulate lattice parameters can offer ways to induce unique topological features including expansive open Fermi arcs and to dynamically control Fermi arc connectivity in $\mathrm{Zr}{\mathrm{Te}}_5$.