A comprehensive picture of polaron and bipolaron physics is essential to understand the optical absorption spectrum in many materials with electron-phonon interactions. In particular, the finite-temperature properties are of interest since they play an important role in many experiments. Here, we combine the parallel two-site time-dependent variational principle algorithm (p2TDVP) with local basis optimization (LBO) and purification to calculate time-dependent current-current correlation functions. From this information, we extract the optical conductivity for the Holstein polaron and bipolaron with dispersive phonons at finite temperatures. For the polaron in the weak and intermediate electron-phonon coupling regimes, we analyze the influence of phonon dispersion relations on the spectra. For strong electron-phonon coupling, the known result of an asymmetric Gaussian is reproduced for a flat phonon band. For a finite phonon bandwidth, the center of the Gaussian is either shifted to larger or smaller frequencies, depending on the sign of the phonon hopping. We illustrate that this can be well understood by considering the Born-Oppenheimer surfaces. Similar behavior is seen for the bipolaron for strong coupling. For the bipolaron with weak and intermediate coupling strengths and a flat phonon band, we obtain two very different spectra. The latter also has a temperature-dependent resonance at a frequency below the phonon frequency.

We investigate the interplay between the Kondo effect and the ferromagnetism by a one dimensional Anderson impurity model with a spin partially polarized bath, using the projective truncation approximation under a Lacroix basis. The equal-time spatial spin-spin correlation function (SSCF) is calculated. For the case of spin-unpolarized conduction electrons, it agrees qualitatively with the results from the density matrix renormalization group. For a system with partially spin-polarized conduction electrons, an oscillation in the envelope of SSCF emerges due to the beating of two Friedel oscillations associated to two spin-split Fermi surfaces of conduction electrons. The period is proportional to the inverse of magnetic field $h$. A fitting formula is proposed to perfectly fit the numerical results of SSCF in both the short- and long-range regions. For large enough bath spin polarization, a bump appears in the curve of the integrated SSCF. It marks the boundary between the suppressed Kondo cloud and the polarized bath sites.

Electrically driven spin resonance (EDSR) is an established tool for controlling semiconductor spin qubits. Here, we theoretically study a frequency-mixing variant of EDSR, where two driving tones with different drive frequencies are applied, and the resonance condition connects the spin Larmor frequency with the sum of the two drive frequencies. Focusing on flopping-mode operation of a single electron in a double quantum dot with spin-orbit interaction, we calculate the parameter dependence of the Rabi frequency and the Bloch-Siegert shift. A shared-control spin qubit architecture could benefit from this bichromatic EDSR scheme, as it enables simultaneous single-qubit gates.

Characterizing and understanding the environment affecting quantum systems is critical to elucidate its physical properties and engineer better quantum devices. We develop an approach to reduce the quantum environment causing single-qubit dephasing to a simple yet predictive noise model. Our approach, inspired by quantum noise spectroscopy, is to define a “self-consistent” classical noise spectrum, that is, compatible with all observed decoherence under various qubit dynamics. We demonstrate the power and limits of our approach by characterizing, with nanoscale spatial resolution, the noise experienced by two electronic spins in diamond that, despite their proximity, surprisingly reveal the presence of a complex quantum spin environment, both classically reducible and not. Our results overcome the limitations of existing noise spectroscopy methods and highlight the importance of finding predictive models to accurately characterize the underlying environment. Extending our work to multiqubit systems would enable spatially resolved quantum sensing of complex environments and quantum device characterization, notably to identify correlated noise between qubits, which is crucial for practical realization of quantum error correction.

We study the Landau level spectrum of bulk graphene monolayers beyond the Dirac Hamiltonian with linear dispersion. We consider an effective Wannier-like tight-binding model obtained from ab initio calculations that includes long-range electronic hopping integral terms. We employ the Haydock-Heine-Kelly recursive method to numerically compute the Landau level spectrum of bulk graphene in the quantum Hall regime and demonstrate that this method is both accurate and computationally much faster than the standard numerical approaches used for this kind of study. The Landau level energies are also obtained analytically for an effective Hamiltonian that accounts for up to third-nearest-neighbor hopping processes. We find an excellent agreement between both approaches. We also study the effect of disorder on the electronic spectrum. Our analysis helps to elucidate the discrepancy between theory and experiment for the high-energy Landau level energies.

The capacity to electrically detect phonons, ultimately at the single-phonon limit, is a key requirement for many schemes for phonon-based quantum computing; so-called quantum phononics. Here, we predict that by exploiting the strong coupling of their electrons to surface-polar phonons, van der Waals heterostructures can offer a suitable platform for phonon sensing, capable of resolving energy transfer at the single-phonon level. The geometry we consider is one in which a drag momentum is exerted on electrons in a graphene layer, by a single out-of-equilibrium phonon in a dielectric layer of hexagonal boron nitride, giving rise to a measurable induced voltage $\left(V_{\mathrm{drag}}\right)$. Our numerical solution of the Boltzmann transport equation shows that this drag voltage can reach a level of a few hundred microvolts per phonon, well above experimental detection limits. Furthermore, we predict that $V_{\mathrm{drag}}$ should be highly insensitive to the mobility of carriers in the graphene layer and to increasing the temperature to at least 300 K, offering the potential of a versatile material platform for single-phonon sensing.

The Thouless pump is a phenomenon in which $U\left(1\right)$ charges are pumped from an edge of a fermionic system to another edge. The Thouless pump has been generalized in various dimensions and for various charges. In this paper we investigate the generalized Thouless pumps of fermion parity in both trivial and nontrivial phases of $(1+1)$-dimensional interacting fermionic short-range entangled (SRE) states. For this purpose, we use matrix product states (MPSs). MPSs describe many-body systems in $(1+1)$ dimensions and can characterize SRE states algebraically. We prove fundamental theorems for fermionic MPSs (fMPSs) and use them to investigate the generalized Thouless pumps. We construct nontrivial pumps in both the trivial and nontrivial phases, and we show the stability of the pumps against interactions. Furthermore, we define topological invariants for the generalized Thouless pumps in terms of fMPSs and establish consistency with existing results. These are invariants of the family of SRE states that are not captured by the higher dimensional Berry curvature. We also argue a relation between the topological invariants of the generalized Thouless pump and the twist of the $K$ theory in the Donovan-Karoubi formulation.

${\mathrm{InSb}}_{1-x}{\mathrm{As}}_x$ is a promising material system for exploration of topological superconductivity in hybrid superconductor/semiconductor devices due to its large effective $g$-factor and enhanced spin-orbit coupling when compared to binary InSb and InAs. Much remains to be understood concerning the fundamental properties of the two-dimensional electron gas (2DEG) in InSbAs quantum wells. We report on the electrical properties of a series of 30 nm ${\mathrm{InSb}}_{1-x}{\mathrm{As}}_x$ quantum wells grown 40 nm below the surface with three different arsenic mole fractions, $x=$ 0.05, 0.13, and 0.19. The dependencies of mobility on 2DEG density and arsenic mole fraction are analyzed. For the $x=0.05$ sample, the 2DEG displays a peak mobility $\mu =2.4\times {10}^5{\mathrm{cm}}^2$/Vs at a density of $n=2.5\times {10}^{11}{\mathrm{cm}}^{-2}$. High mobility, small effective mass, and strong spin-orbit coupling result in beating in the Shubnikov de Haas oscillations at low magnetic field. Fourier analysis of the Shubnikov de Haas oscillations facilitates extraction of the Rashba spin-orbit parameter $\alpha$ as a function of 2DEG density and quantum well mole fraction. For $x=0.19$ at $n=3.1\times {10}^{11}$ cm ${}^{-2}$, $\alpha \approx 300$ meVÅ, among the highest reported values in III-V materials.

Tremendous efforts have been devoted to studying ordinary and novel properties in molecular-scale. Especially the reversible and controlled electrical and thermoelectric switches, which have been extensively studied experimentally and theoretically. They possess two distinct electronic and thermoelectric transport states actuated by external triggers, such as light, temperature, and electric field. Here, a photochromic diarylethene, linked to a nontriazatriangulene platform covalently, is studied by first-principles calculation. By means of density functional theory, we calculate the density of states and scanning tunneling microscope (STM) images of the designed diarylethene adsorbed on a Au(111) surface. Moreover, the thermoelectric transport properties of diarylethene-based molecular junction with a Au(111) electrode are studied systematically by using nonequilibrium Green's function transport formalism. We illustrate that due to the structure chang e of the designed diarylethene induced by light, the electronic and thermoelectric transport properties of the junction are completely different. Those differences can be visualized by the calculated electronic transmission eigenchannel and transport pathway. Furthermore, we demonstrate that the structure change of the designed diarylethene can be detected by STM experiments. The present study shows that the designed diarylethene could be a desirable candidate for electronic and thermoelectric molecular switch driven by light.

The quantum Hall (QH) effect, the quantum spin Hall (QSH) effect, and the quantum valley Hall (QVH) effect are three peculiar topological insulating phases in graphene. They are characterized by three different types of edge states. These three effects are caused by the external magnetic field, the intrinsic spin-orbit coupling (SOC) and the strain-induced pseudomagnetic field, respectively. Here we theoretically study phase diagrams when these effects coexist and analyze how the edge states evolve between the three. We find the real magnetic field and the pseudomagnetic field will compete above the SOC energy gap while the QSH effect is almost unaffected within the SOC energy gap. The edge-state transitions from the QH effect or the QVH effect to the QSH effect directly relies on the arrangement of the zeroth Landau levels. Using edge-state transitions, we raise a device similar to a spin field effect transistor (spin-FET) and also design a spintronics multiple-way switch.

The interface of manganese(II) phtalocyanine (MnPc) with an $\alpha \text{−}{\mathrm{RuCl}}_3$ single crystal is studied by means of photoelectron spectroscopy. The organic molecule was evaporated on top of an in situ cleaved $\alpha \text{−}{\mathrm{RuCl}}_3$ bulk crystal in steps. The work function and electron affinity of $\alpha \text{−}{\mathrm{RuCl}}_3$ is determined to be 6.3 and 5.4 eV, respectively, in line with common fluorinated acceptor molecules, but with the potential to absorb more electrons per area at interfaces with other materials. These favorable electronic properties lead to a charge transfer from MnPc to $\alpha \text{−}{\mathrm{RuCl}}_3$ which manifests itself mainly in peak shape changes of core level photoemission spectra and further filling of Ru $4d$ states as revealed by valence band photoemission. This work demonstrates the promising potential of $\alpha \text{−}{\mathrm{RuCl}}_3$ as a strong acceptor material for interfaces in general, and for organic molecules in particular.

We demonstrate the tunability of the ultrafast energy flow in magnetic/nonmagnetic bilayer structures by changing the wavelength of the optical excitation. This is achieved by an advanced description of the temperature-based $\mu T$ model that explicitly considers the wavelength- and layer-dependent absorption profile within multilayer structures. For the exemplary case of a Ni/Au bilayer, our simulations predict that the energy flow from Ni to Au is reversed when changing the wavelength of the excitation from the infrared to the ultraviolet spectral range. These predictions are fully supported by characteristic signatures in the time-dependent magneto-optical Kerr traces of the Ni/Au model system. Our results will open up avenues to steer and control the energy transport in designed magnetic multilayers for ultrafast spintronic applications.

We show that exotic spontaneous symmetry breaking appears in thermal topological phases by perturbing the exact solutions of quantum rotor models coupled to the three-dimensional toric code. The exotic Ising and XY transitions are shown to be in the same universality class, in drastic contrast to the conventional Wilson-Fisher classes without topological orders. Our results indicate that topological orders must be included to pin down the universality classes of thermal transitions in addition to order parameter symmetry and spatial dimension. We evaluate all the critical exponents and find that the exotic universality class is more stable under the couplings to acoustic phonons and disorder. Applying our results to experiments, we provide a plausible scenario in puzzlings of strongly correlated systems, including the absence of a specific heat anomaly in doped ${\mathrm{RbFe}}_2{\mathrm{As}}_2$.

Taking the clue from the modern theory of polarization [Rev. Mod. Phys. 66, 899 (1994)], we identify an operator to distinguish between ${\mathbb{Z}}_2$-even (trivial) and ${\mathbb{Z}}_2$-odd (topological) insulators in two spatial dimensions. Its definition extends the position operator [Phys. Rev. Lett. 82, 370 (1999)], which was introduced in one-dimensional systems. We first show a few examples of noninteracting models where single-particle wave functions are defined and allow for a direct comparison with standard techniques on large system sizes. Then, we illustrate its applicability for an interacting model on a small cluster where exact diagonalizations are available. Its formulation in the Fock space allows a direct computation of expectation values over the ground-state wave function (or any approximation of it), thus, allowing us to investigate generic interacting systems, such as strongly correlated topological insulators.

X-ray diffraction measurements under laser-driven dynamic compression now allow us to investigate the atomic structure of matter at TPa pressures and thousands of degree temperatures, with broad implications for condensed matter physics, planetary science, and astronomy. Pressure determination in these experiments often relies on velocimetry measurements coupled with modeling that requires accurate knowledge of the optical and thermomechanical properties of a window material, resulting in significant systematic uncertainty. Here we report on a series of x-ray diffraction experiments on five metals dynamically compressed to 600 GPa. In addition to simultaneously collecting atomic structure information for multiple compressed samples, namely Pt, Ta, Au, W, and Fe, we demonstrate a different approach for pressure determination applicable to x-ray diffraction experiments under quasi-isentropic ramp compression. The method, based on the use of in situ pressure calibrants, is similar to the techniques often adopted in static compression with diamond anvil cells. Focusing on experiments using a diamond window, we discuss challenges and mitigation strategies for the novel approach. Our study provides lattice-level information on five different metals compressed to hundreds of GPa and validation to the currently used methods for pressure determination based on time-resolved measurement of the diamond free-surface velocity, revealing that the use of in situ calibrants enables a factor of four reduction in the pressure uncertainty in these experiments.

We bridge an essential knowledge gap on the understanding of all-optical ultrafast switching in ferrimagnets, namely, the connection between atomistic spin dynamics methods and macroscopic phenomenological models. All-optical switching of the magnetization occurs after the application of a single femtosecond laser pulse to specific ferrimagnetic compounds. This strong excitation puts the involved degrees of freedom, electrons, lattice, and spins out-of-equilibrium between each other. Atomistic spin models have quantitatively described all-optical switching in a wide range of experimental conditions, while having failed to provide a simple picture of the switching process. Phenomenological models are able to qualitatively describe the dynamics of the switching process. However, a unified theoretical framework is missing that describes the element-specific spin dynamics as atomistic spin models with the simplicity of phenomenology. Here, we bridge this gap and present an element-specific macrospin dynamical model which fully agrees with atomistic spin dynamics simulations and symmetry considerations of the phenomenological models.

We present a ferromagnetic resonance (FMR) method that we term the “Ferris” FMR. It is wideband, has at least an order of magnitude higher sensitivity as compared to conventional FMR systems, and measures the absorption line rather than its derivative. It is based on large-amplitude modulation of the externally applied magnetic field that effectively magnifies signatures of the spin-transfer torque making its measurement possible even at the wafer level. Using the Ferris FMR, we report the generation of spin currents from the orbital Hall effect taking place in pure Cu and Al. To this end, we use the spin-orbit coupling of a thin Pt layer introduced at the interface that converts the orbital current to a measurable spin current. While Cu reveals a large effective spin Hall angle exceeding that of Pt, Al possesses an orbital Hall effect of opposite polarity in agreement with the theoretical predictions. Our results demonstrate additional spin- and orbit functionality for two important metals in the semiconductor industry beyond their primary use as interconnects with all the advantages in power, scaling, and cost.

Impurity spins randomly distributed at the surfaces and interfaces of superconducting wires are known to cause flux noise in superconducting quantum interference devices (SQUIDs), providing a dominant mechanism for decoherence in flux-tunable superconducting qubits. While flux noise is well characterized experimentally, the microscopic model underlying spin dynamics remains a great puzzle. The main problem is that first-principles theories based on an integration of the quantum Heisenberg equations of motion for interacting spins are too computationally expensive to capture spin diffusion over large length scales, hindering comparisons between microscopic models and experimental data. In contrast, third-principles approaches lump spin dynamics into a single phenomenological spin-diffusion operator $D{\nabla }^2$ that is not able to describe the quantum noise regime and connect to microscopic models and different disorder scenarios such as spin clusters. Here we propose an intermediate “second-principles” method to describe general spin dissipation and flux noise in the quantum regime. It leads to the interpretation that flux noise arises from the density of paramagnon excitations at the edge of the superconducting wire, with paramagnon-paramagnon interactions leading to spin diffusion, and interactions between paramagnons and other degrees of freedom such as phonons, electrons, and two-level systems leading to spin energy relaxation. At high frequency $\omega$, we obtain an upper bound for flux noise, showing that the (super)Ohmic noise observed in experiments is not originating from interacting spin impurities. We apply the method to Heisenberg models in two-dimensional square lattices with a random distribution of vacancies, with nearest-neighbor spins coupled by a constant exchange. Explicit numerical calculations of flux noise show that it follows the observed power law $A/{\omega }^{\alpha }$, with amplitude $A$ and exponent $\alpha$ depending on temperature and inhomogeneities such as spatial confinement and disorder. These results are compared to experiments in niobium and aluminum devices. The method establishes a connection between flux noise experiments and microscopic Hamiltonians with the goal of identifying relevant microscopic mechanisms and guiding strategies for reducing flux noise.

Vanadium-based topological kagome metals ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) have drawn great attention recently due to the discoveries of charge order, nematic phase, time-reversal symmetry breaking, and superconductivity. In this work, we study the Fermi-surface instabilities of topological kagome metals ${\mathrm{AV}}_3{\mathrm{Sb}}_5$ ($A=\mathrm{K}$, Rb, Cs) in all the charge-orbital-spin channels based on first principles calculations, and provide a first principles theory for the current-loop state in this class of materials. After comprehensive analysis of the interaction-renormalized generalized susceptibility tensor combined with unrestricted Hartree-Fock calculations, we find that the leading instability modes at the Fermi surface are three spin magnetic modes at $\mathrm{\Gamma }$ point, giving rise to ferromagnetism within the kagome plane with small magnetization on the V atoms. We further show that counter-propagating current loops within the kagome plane can be generated due to the interplay between spin magnetism and spin-orbit coupling, and the current pattern is consistent with recent muon spin spectroscopy measurements. Moreover, some other puzzling experiments in this system such as the giant anomalous Hall effect and nematicity can also be explained from our results.

Long-range interactions exhibit surprising features which have been less explored so far. Here, studying a one-dimensional fermionic chain with long-range hopping and pairing, we discuss some general features associated to the presence of long-range entanglement. In particular, after determining the algebraic decays of the correlation functions, we prove that a long-range quantum mutual information exists if the exponent of the decay is not larger than one. Moreover, we show that the time evolution triggered by a quantum quench between short-range and long-range regions, can be characterized by dynamical quantum phase transitions without crossing any phase boundary. We show, also, that the adiabatic dynamics is dictated by the divergence of a topological length scale at the quantum critical point, clarifying the violation of the Kibble-Zurek mechanism for long-range systems.