Electron-electron thermalization and electron-phonon relaxation processes in laser-excited solids are often assumed to occur on different timescales. This is true for the majority of the conduction band electrons in a metal. However, electron-phonon interactions can influence the thermalization process of the excited electrons. We study the interplay of the underlying scattering mechanisms for the case of a noble metal with help of a set of complete Boltzmann collision integrals. We trace the transient electron distribution in copper and its deviations from a Fermi-Dirac distribution due to the excitation with an ultrashort laser pulse. We investigate the different stages of electronic nonequilibrium after an excitation with an ultrashort laser pulse of $800\mathrm{nm}$ wavelength and $10\mathrm{fs}$ pulse duration. Our calculations show a strong nonequilibrium during and directly after the end of the laser pulse. Subsequently, we find a fast thermalization of most electrons. Surprisingly, we observe a long-lasting nonequilibrium, which can be attributed to the electron-phonon scattering. This nonequilibrium establishes at energies around peaks in the density of states of the electrons and persists on the timescale of electron-phonon energy relaxation. It influences in turn the electron phonon coupling strength.

Generating efficient and highly spin-polarized currents through nanoscale junctions is essential in the field of nanoelectronics and spintronics. In this paper, using ab initio electron transport calculations, we predict highly conductive and perfect spin filtering of nickel atomic contacts in a nitrogen environment, where a single ${\mathrm{N}}_2$ molecule sits in parallel (energetically most favorable) between two nickel electrodes. Such a particular performance is due to the wave function orthogonality between majority-spin $s$-like states of ferromagnetic electrodes and the lowest unoccupied molecular orbital of the ${\mathrm{N}}_2$ molecule, and thus, majority-spin electrons are completely blocked at the interface. For the minority spin, on the contrary, two almost saturated conducting channels are formed due to the effective coupling between $d_{zx,zy}$ of the Ni atom and $p_{x,y}$ of the N atom, resulting in a large conductance of about $1G_0$ ($=2e^2/h$). As a consequence, a single ${\mathrm{N}}_2$ molecule acts as a highly conductive and half-metallic conductor. On the other hand, the CO and NO incorporated molecular junctions exhibit rather low conductance with a partially spin-polarized current.

The chiral magnet ${\mathrm{CrNb}}_3{\mathrm{S}}_6$ with its solitonic objects has novel magnetic and transport properties, in which the spin-orbit coupling (SOC) plays a central role. Aiming to address the possible existence of orbital moments driven by SOC, we perform soft x-ray magnetic circular dichroism spectroscopy at the Cr $L_{2,3}$ edges with in-plane magnetization. The dichroic signals provide direct experimental evidence that the Cr orbital magnetic moment is not quenched and is coupled antiparallel to the spin counterpart. Application of the orbital sum rule reveals that the magnitude of the Cr orbital moment is about 1% of the total magnetization. These findings are consistent with the first-principles electronic structure calculations that utilize the Cr $2p$ core radial function to define the Cr local $3d$ quantities. The distinct roles of the atomic SOC among the Cr $3d$ and Nb $4d$ states are discussed.

Larkin and Ovchinnikov established that the viscous flow of magnetic flux quanta in current-biased superconductor films placed in a perpendicular magnetic field can lose stability due to a decrease in the vortex viscosity coefficient $\eta$ with increasing velocity of the vortices $v$. The dependence of $\eta$ on $v$ leads to a nonlinear section in the current-voltage ($I-V$) curve, which ends at the flux-flow instability point with a voltage jump to a highly resistive state. At the same time, in contradistinction with the nonlinear conductivity regime, instability jumps often occur in linear $I-V$ sections. Here, for the elucidation of such jumps we develop a theory of local instability of the magnetic flux flow occurring not in the entire film but in a narrow strip across the film width in which vortices move much faster than outside it. The predictions of the developed theory are in agreement with experiments on Nb films for which the heat removal coefficients and the inelastic scattering times of quasiparticles are deduced. The presented model of local instability is also relevant for the characterization of superconducting thin films whose performance is examined for fast single-photon detection.

Similarly to their purely electric counterparts, spintronic circuits may be presented as networks of lumped elements. Due to the interplay between spin and charge currents, each element is described by a matrix conductance. We establish reciprocity relations between the entries of the conductance matrix of a multiterminal linear device, comprising normal metallic and strong-ferromagnetic elements with vanishing spin-orbit interactions and spin-inactive interfaces. In particular, reciprocity equates the spin transmissions through a two-terminal element in opposite directions. When applied to “geometric spin ratchets,” reciprocity shows that certain effects, announced for such devices, are, in fact, impossible. We describe the relation between our work and the spintronic circuit theory formalism and contrast our results with the requirements following from the Onsager symmetry of kinetic coefficients. Consequences of finite spin-orbit interactions are also discussed.

We report a description and comprehensive study on four successive magnetic transitions in ferromagnetic $\mathrm{T}{\mathrm{b}}_2\mathrm{MnNi}{\mathrm{O}}_6$ double perovskite. In the ground state ($P{2}_1^{\prime }$), the moments of magnetic $A$ and $B$ sites order according to different nonpolar magnetic modes, but the coupling between them generates an overall polar symmetry which makes this oxide potentially multiferroic due to magnetic trilinear coupling, and therefore ferromagnetic and ferroelectric in its ground state. Its macroscopic magnetization is large ($5{\mu }_{\mathrm{B}}/\mathrm{f}.\mathrm{u}$) and not related to a weak ferromagnetic component induced by Dzyaloshinskii-Moriya interaction. However, a sharp and severe spin reorientation of the ferromagnetic transition-metal moments has been observed which opens the door to the magnetic switching of the ferroelectric state in this perovskite, and conversely to the control of the magnetization direction by electrical fields applied parallel to $b$. We also anticipate that in this material the direction of the magnetization (towards $c/a$) could be used as the key to switch the polar/nonpolar (ferroelectric/antiferroelectric) transformation.

In muon spin rotation experiments, the diamagnetic fraction observed at low temperatures is a consequence of the muon interaction with the lattice during the incorporation process and depends on the properties of the material. An increase in the diamagnetic fraction with decreasing temperature is observed in some cases, which we attribute to a thermal spike at the end of the muon trajectory. The thermal spike is due to energy liberated not only during the muon stopping process but also as a consequence of the stress release when the electron bound to a muon in an unrelaxed configuration jumps to a neighboring lattice site. The magnitude and time extension of this effect are more significant for highly disturbed lattices which exhibit very low thermal conductivities at low temperatures. A phenomenological description of some selected examples is presented, together with a discussion of heat transport, using existing models for the thermal spike.

When light is confined in all three directions and in dimensions of the order of the light wavelength, discretization of the photon spectra and distinctive phenomena occur, the Purcell effect and the inhibition of emission of atoms being two paradigmatic examples. Diverse solid-state devices that confine light in all three dimensions have been developed and applied. Typically the confinement volume, operating wavelength, and quality factor of these resonators are set by construction, and small variations of these characteristics with external perturbations are targeted for applications including light modulation and control. Here we describe full three-dimensional light trapping, that is set and tuned by laser excitation in an all-optical scheme. The proposed device is based on a planar distributed Bragg reflector GaAs semiconductor microcavity operated at room temperature. Lateral confinement is generated by an in-plane gradient in the refractive index of the structure's materials due to localized heating, which is in turn induced by carriers photoexcited by a focused laser. Strong three-dimensional trapping of light is evidenced by the laser-induced changes on the spectral, spatial, and $k$-space distribution of the emission. The dynamics of the laser-induced photonic potential is studied using modulated optical excitation, highlighting the central role of thermal effects at the origin of the observed phenomena.

Low-density two-dimensional electron gases (2DEGs) with spin-orbit coupling are highly sensitive to an in-plane magnetic field, which impacts their Fermi surfaces and transport properties. Such 2DEGs, formed at transition metal oxide surfaces or interfaces, can also undergo surface phase transitions leading to polar metals that exhibit electronic nematicity. Motivated by experiments on such systems, we theoretically study magnetotransport in $t_{2g}$ orbital systems, using Hamiltonians that include atomic spin-orbit coupling and broken inversion symmetry, for both square symmetry (001) and hexagonal symmetry (111) 2DEGs. Using a numerical solution to the full multiband matrix-Boltzmann equation, together with insights gleaned from the impurity scattering overlap matrix, we explore the anisotropic magnetoresistance (AMR) in the presence of impurities that favor small momentum scattering. We find that transport in the (001) 2DEG is dominated by a single pair of bands, weakly coupled by impurity scattering, one of which has a larger Fermi velocity while the other provides an efficient current-relaxation mechanism. This leads to strong angle-dependent current damping and a large AMR with many angular harmonics. In contrast, AMR in the (111) 2DEG typically features a single $cos\left(2\vartheta \right)$ harmonic, with the angle-averaged magnetoresistance being highly tunable by a symmetry-allowed trigonal distortion. We also explore how the (111) 2DEG Fermi surfaces are impacted by electronic nematicity via a surface phase transition into a 2D polar metal for which we discuss a Landau theory, and we show that this leads to distinct symmetry components and higher angular harmonics in the AMR. Our results are in qualitative agreement with experiments from various groups for 2DEGs at the ${\mathrm{SrTiO}}_3$ surface and the ${\mathrm{LaAlO}}_3-{\mathrm{SrTiO}}_3$ interface.

The recent emergence of atomically thin two-dimensional (2D) materials has opened up possibilities for the design of ultrathin and flexible nanoelectronic devices. As truly 2D materials, they exhibit an optimal surface-to-volume ratio, which results in an extremely high sensitivity to external changes which can not be achieved by conventional semiconductors. This makes these materials optimal candidates for sensing applications. Here, we propose a dark exciton based concept for ultrasensitive strain sensors. By investigating both dark and bright excitons in tungsten-based monolayer transition metal dichalcogenides, we demonstrate that the dark-bright-exciton separation can be controlled by strain, which has a crucial impact on the activation of dark excitonic states. The predicted opposite strain-induced shifts for dark and bright excitons result in a pronounced change in photoluminescence stemming from dark excitons even at very small strain values. The predicted high optical gauge factors of up to 8000 are promising for the design of optical strain sensors.

The phase diagram of the quantum spin-$\frac{1}{2}$ antiferromagnetic $J_1^{}\text{−}{J}_2^{}$ XXZ chain was obtained by Haldane using bosonization techniques [Haldane, Phys. Rev. B 25, 4925 (1982); Phys. Rev. B 26, 5257 (1982)]. It supports three distinct phases for $0\le J_2^{}/J_1^{}<\frac{1}{2}$, i.e., a gapless algebraic spin-liquid phase, a gapped long-range ordered Neel phase, and a gapped long-range ordered dimer phase. Even though the Neel and dimer phases are not related hierarchically by a pattern of symmetry breaking, it was shown that they meet along a line of quantum critical points with a U(1) symmetry and central charge $c=1$. Here, we extend the analysis made by Haldane on the quantum spin-$\frac{1}{2}$ antiferromagnetic $J_1^{}\text{−}{J}_2^{}$ XYZ chain using both bosonization and numerical techniques. We show that there are three Neel phases and the dimer phase that are separated from each other by six planes of phase boundaries realizing Gaussian criticality when $0\le J_2^{}/J_1^{}<\frac{1}{2}$. We also show that each long-range ordered phase harbors topological point defects (domain walls) that are dual to those across the phase boundary in that a defect in one ordered phase locally binds the other type of order around its core. By using the bosonization approach, we identify the critical theory that describes simultaneous proliferation of these dual point defects, and show that it supports an emergent U(1) symmetry that originates from the discrete symmetries of the XYZ model. To confirm this numerically, we perform exact diagonalization and density-matrix renormalization-group calculations and show that the critical theory is characterized by the central charge $c=1$ with critical exponents that are consistent with those obtained from the bosonization approach. Furthermore, we generalize the field-theoretic description of direct continuous phase transition to higher dimensions, especially in $d=3$, by using a nonlinear sigma model (NLSM) with a topological term. In particular, we propose the $\pi$-flux phase on the cubic lattice with local quartic interactions as a platform for direct continuous phase transition and deconfined criticality. We discuss possible phase diagrams for the $\pi$-flux phase on the cubic lattice with these quartic interactions from the renormalization flow of NLSMs.

Can we find a physical intuitive language to represent the complex mathematical properties of anyon models? Is there a hidden organization in the space of anyon models? Can we systematically construct complex anyon models from other, more simple, anyon models? This work is an attempt to understand and explore the anatomy of the space of anyon models. I develop a construction to systematically generate and interconnect anyon models. The construction is formulated in a very physical, visual, and intuitive manner. The complex mathematical properties of anyon models, usually expressed in the complex language of braided modular tensor categories, are described here using familiar simple physical concepts like bosons in lattices, Fock states, and tunneling Hamiltonians. The construction formalism is conceptually simple and appealing. As in a Lego construction, a set of building block anyon models are assembled to construct towers of other, more complex, anyon models. A principle of assembly is established that dictates how to articulate the building blocks, setting out the global blueprint for the constructed anyon model. By assembling sets of the simple Abelian ${\mathbb{Z}}_n$ anyon models, the construction succeeds in generating towers of well known nontrivial anyon models. Moreover, novel anyon models (nontabulated, to my knowledge) arise. Furthermore, by applying the construction principle to other building blocks, such as Ising or Fibonacci, further series of anyon models are generated. In an intriguing connection, the boson-lattice construction unveils an alluring duality between anyon models and curved space geometries. Bosons induce curvature in the lattice geometry. I disclose a geography in the space of anyon models, where one is engendered from another by deforming the geometry of space. Moreover, the potential of the boson-lattice construction can be further explored via the quantum simulation of the corresponding boson-lattice systems in the versatile accessible playground of atomic systems.

The radiative rate of GaInN/GaN quantum well structures on nonpolar substrates is investigated for different quantum well widths, showing a significant decrease of the radiative emission towards larger well widths. This effect can be explained by the strict selection rules that apply for radiative transitions in nonpolar structures without any polarization fields in the direction of quantization. The selection rule $\mathrm{\Delta }n=0$ reduces the number of possible radiative transitions that involve higher quantized hole states. These states will get occupied towards room temperature for wider quantum wells due to the decrease in quantization energies. Since the effective masses are strongly different in the conduction and valence bands, the thermal population of higher states is imbalanced between electrons and holes. Applying a simple model in a nondegenerate limit, we can well describe the width dependence of the experimentally determined radiative rates. At room temperature, the decrease amounts to a factor of 2–4 for nonpolar quantum wells of $8\mathrm{nm}$ thickness.

We study theoretically how electron-phonon interaction affects the energies and level broadening (inverse lifetime) of Majorana bound states (MBSs) in a clean topological nanowire at low temperatures. At zero temperature, the energy splitting between the right and left MBSs remains exponentially small with increasing nanowire length $L$. At finite temperatures, however, the absorption of thermal phonons leads to the broadening of energy levels of the MBSs that does not decay with system length, and the coherent absorption/emission of phonons at opposite ends of the nanowire results in MBSs energy splitting that decays only as an inverse power law in $L$. Both effects remain exponential in temperature. In the case of quantized transverse motion of phonons, the presence of Van Hove singularities in the phonon density of states causes additional resonant enhancement of both the energy splitting and the level broadening of the MBSs. This is the most favorable case to observe the phonon-induced energy splitting of MBSs as it becomes much larger than the broadening even if the topological nanowire is much longer than the coherence length. We also calculate the charge and spin associated with the energy splitting of the MBSs induced by phonons. We consider both a spinless low-energy continuum model, which we evaluate analytically, as well as a spinful lattice model for a Rashba nanowire, which we evaluate numerically.

The transmission across a graphene bilayer region is calculated for two different types of connections to monolayer leads. A transfer-matrix algorithm based on a tight-binding model is developed to obtain the ballistic transmission beyond linear response. The two configurations are found to behave similarly when no gate voltage is applied. For a finite gate voltage, both develop a conductance gap characteristic of a biased bilayer, but only one shows a pronounced conductance step at the gap edge. A gate voltage domain wall applied to the bilayer region renders the conductance of the two configurations similar. For a microstructure consisting of equally spaced domain walls, we find a high sensitivity to the domain size. This is attributed to the presence of topologically protected in-gap states localized at domain walls, which hybridize as the domain size becomes of the order of their confining scale. Our results show that transmission through a bilayer region can be manipulated by a gate voltage in ways not previously anticipated.

A major open question in studies of nonequilibrium quantum dynamics is the identification of the time scales involved in the relaxation process of isolated quantum systems that have many interacting particles. We demonstrate that long time scales can be analytically found by analyzing dynamical manifestations of spectral correlations. Using this approach, we show that the Thouless time $t_{\text{Th}}$ and the relaxation time $t_{\text{R}}$ increase exponentially with system size. We define $t_{\text{Th}}$ as the time at which the spread of the initial state in the many-body Hilbert space is complete and verify that it agrees with the inverse of the Thouless energy. $t_{\text{Th}}$ marks the point beyond which the dynamics acquire universal features, while relaxation happens later when the evolution reaches a stationary state. In chaotic systems, $t_{\text{Th}}\ll t_{\text{R}}$, while for systems approaching a many-body localized phase, $t_{\text{Th}}\to t_{\text{R}}$. Our analytical results for $t_{\text{Th}}$ and $t_{\text{R}}$ are obtained for the survival probability, which is a global quantity. We show numerically that the same time scales appear also in the evolution of the spin autocorrelation function, which is an experimental local observable. Our studies are carried out for realistic many-body quantum models. The results are compared with those for random matrices.

Utilizing the Pauli equation based multislice method, introduced in a previous paper [A. Edström, A. Lubk, and J. Rusz, Phys. Rev. Lett. 116, 127203 (2016)], we study the atomic-resolution differential phase contrast (DPC) imaging on an example of a hard magnet FePt with in-plane magnetization. Simulated center-of-mass pattern in a scanning transmission electron microscopy experiment carries information about both electric and magnetic fields. The momentum transfer remains curl free, which has consequences for interpretation of the integrated DPC technique. The extracted magnetic component of the pattern is compared to the expected projected microscopic magnetic field as obtained by density functional theory calculation. Qualitative agreement is obtained for low sample thicknesses and a suitable range of collection angles.

We develop a theory for the thermal Hall coefficient in a spin-$\frac{1}{2}$ system on a strip of kagome lattice, where a chiral spin-interaction term is present. To this end, we model the kagome strip as a three-leg $XXZ$ spin-ladder, and use bosonization to derive a low-energy theory for the spinons in this system. Introducing further a Dzyaloshinskii-Moriya interaction ($D$) and a tunable magnetic field ($B$), we identify three distinct $B$-dependent quantum phases: a valence-bond crystal (VBC), a “metallic” spin liquid (MSL), and a chiral spin liquid (CSL). In the presence of a temperature difference $\mathrm{\Delta }T$ between the top and the bottom edges of the strip, we evaluate the net heat current $J_h$ along the strip, and consequently the thermal Hall conductivity ${\kappa }_{xy}$. We find that the VBC-MSL-CSL transitions are accompanied by a pronounced qualitative change in the behavior of ${\kappa }_{xy}$ as a function of $B$. In particular, analogously to the quantum Hall effect, ${\kappa }_{xy}$ in the CSL phase exhibits a quantized plateau centered around a commensurate value of the spinon filling factor ${\nu }_s\propto B/D$.

The model strongly correlated electron system ${\mathrm{Ho}}_{0.8}{\mathrm{Lu}}_{0.2}{\mathrm{B}}_{12}$, which demonstrates a cooperative Jahn-Teller instability of the boron sublattice in combination with rattling modes of Ho(Lu) ions, dynamic charge stripes, and an unusual antiferromagnetic (AF) ground state, has been studied in detail at low temperatures by magnetoresistance (Δρ/ρ), magnetization, and heat capacity measurements. Based on received results it turns out that the angular H-φ-T magnetic phase diagrams of this nonequilibrium AF metal can be reconstructed in the form of a “Maltese cross”. The dramatic AF ground state symmetry lowering of this dodecaboride with fcc crystal structure can be attributed to the redistribution of conduction electrons. These leave the Ruderman-Kittel-Kasuya-Yosida oscillations of the electron spin density to participate in the dynamic charge stripes providing extraordinary changes in the indirect exchange interaction between magnetic moments of $\mathrm{H}{\mathrm{o}}^{3+}$ ions and resulting in the emergence of a number of various magnetic phases. It is also shown that the two main contributions to magnetoresistance in the complex AF phase, the (i) positive linear on magnetic field and the (ii) negative quadratic $-\mathrm{\Delta }\rho /\rho \sim H^2$ component can be separated and analyzed quantitatively, correspondingly, in terms of charge carrier scattering on the spin density wave ($5d$) component of the magnetic structure and on local $4f-5d$ spin fluctuations of holmium sites.