In ultrahigh-quality two-dimensional (2D) materials the mean free paths of phonons and electrons relative to all mechanisms of scattering can be much larger than a size of a sample. In this case the most frequent type of scattering of particles is their collisions with sample edges and the ballistic regime of heat and charge transport is realized. We study the ballistic transport of classical interacting 2D particles in a long narrow sample. We show that the interparticle scattering conserving momentum leads to a positive hydrodynamic correction to the ballistic conductance, which is a precursor of the viscous Poiseuille flow. We examine the effect of weak magnetic field on the electron ballistic conductance and predict a classical kinematic mechanism for negative magnetoresistance. Our analysis demonstrates that, apparently, such a mechanism explains the temperature-independent part of the giant negative magnetoresistance recently observed in the ultrahigh-mobility GaAs quantum wells.

The probe of antiferromagnetic (AFM) spin dynamics in thin films has traditionally relied on the short x-ray pulses based on the magnetic dichroism or diffraction effect. Here, we demonstrate the direct optical probe of ultrafast laser-induced AFM spin dynamics of thin AFM CoO films using time-resolved magnetic linear dichroism effect in reflection geometry. The ultrafast laser excitation of the CoO film leads to the quench of AFM order, manifested as large polarization rotations of the reflected probe light. Far below the Néel temperature $\left(T_{\mathrm{N}}\right)$, the quench of the AFM order occurs within 300 fs, which is faster than the lattice thermalization $(\sim 1\mathrm{ps})$ via electron-lattice scattering. This AFM quench process, however, slows down near $T_{\mathrm{N}}$, where an additional slower quench process with the time constant longer than 20 ps is emergent. Such an AFM spin dynamics is dramatically different than the transient reflectivity dynamics which is nearly invariant in a wide temperature range across $T_{\mathrm{N}}$. We attribute the quench of AFM order in CoO to the charge-transfer excitation and thermal effect, but the latter mechanism only plays a significant role near $T_{\mathrm{N}}$.

We report a quantitative study of two-magnon scattering in ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/NiO bilayers with various NiO thicknesses $d_{\mathrm{NiO}}$. We found that the magnetic damping of the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/NiO bilayer strongly depends on $d_{\mathrm{NiO}}$, which evidences that the amplitude of the two-magnon scattering increases with increasing the thickness of the antiferromagnetic layer. The enhancement of the two-magnon scattering in the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/NiO bilayer is attributed to the increase of randomly distributed antiferromagnetic exchange fields. We calculated the spin-mixing conductance by eliminating the effect of the two-magnon scattering, and found that the value is at $8.1{\mathrm{nm}}^{-2}$ for the ${\mathrm{Ni}}_{81}{\mathrm{Fe}}_{19}$/NiO interface. Our result gives further insight on the role of the two-magnon scattering in manipulating magnetic damping, which is crucial for generation and transmission of spin currents in ferromagnet/antiferromagnet systems.

Spin-valve structures are usually associated with the ability to modify the resistance of electrical currents. We here demonstrate a profoundly different effect of a spin-valve. In combination with a topological insulator and superconducting materials, we show that a spin-valve can be used to toggle quantum vortices in and out of existence. In the antiparallel configuration, the spin valve causes superconducting vortex nucleation. In the parallel configuration, however, no vortices appear. This switching effect suggests a new way to control quantum vortices.

Kohn's theorem and the $f$-sum rule are powerful theorems, the first applying to translationally invariant single-band electronic systems with parabolic electronic dispersion relations and the second applying to materials in general, that impose restrictions on the effects of electron-electron interactions on electrical conductivity and on dielectric response, respectively. We show rigorously that Kohn's theorem does not hold for intrinsic Dirac-Weyl materials with filled Fermi seas where the chemical potential is pinned at the band touching points. We also demonstrate that the low-energy effective “relativistic” theories used in many-body studies of these materials violate the $f$-sum rule due to the neglect of the full band structure of the materials in the effective low-energy relativistic approximations.

Pancharatnam's experimental findings on amplitude interferometry of polarized light during 1950's was an early example of the Berry phase. But a similar experimental realization of the geometric phase in the context of solid-state electronic systems where the polarization state of the photon is replaced by spin-polarized states of the electron remains unexplored. This is primarily due to the fact that the generation of Pancharatnam's geometric phase involves a discrete number of cyclic projective measurements on the polarized states of light, and an equivalent cyclic operation on electron spin is much harder to implement in a solid-state setting. In the present paper, we show that the edge states of the quantum spin Hall effect in conjunction with tunnel coupled spin-polarized electrodes (SPEs) provide us with a unique opportunity to generate Pancharatnam's type of geometric phase locally in space, which can be detected via electronic current measurements. We show that the controlled manipulation of the polarization directions of the SPEs results in coherent oscillations in the cross-correlated current noise, which can be attributed to a multiparticle version of Pancharatnam's geometric phase, and is directly related to the phenomenon of intensity interferometry. We demonstrate that the interference patterns produced due to the manipulation of the geometric phase in our proposed setup show a remarkable immunity to orbital dephasing owing to their spatially local origin.

We study interaction-induced broken symmetry phases that can arise in metallic or semimetallic band structures with two nested Weyl or Dirac loops. The ordered phases can be of the charge or (pseudo)spin density wave type, or superconductivity from interloop pairing. A general analysis for two types of Weyl loops is given, according to whether a local reflection symmetry in momentum space exists or not, for Hamiltonians having a global PT symmetry. The resulting density wave phases always have lower total energy, and can be metallic, insulating, or semimetallic (with nodal loops), depending on both the reflection symmetry of the loops and the symmetry transformation that maps one loop onto the other. We extend this study to nested ${\mathbb{Z}}_2$ nodal lines, for which the ordered phases include also nodal point and nodal chain semimetals, and to spinful Dirac nodal lines. Superconductivity from interloop pairing can be fully gapped only if the initial double-loop system is semimetallic.

We consider pure dephasing of Bell states of electron spin qubits interacting with a sparse bath of nuclear spins. Using the newly developed two-qubit generalization of cluster-correlation expansion method, we calculate the spin-echo decay of $|\mathrm{\Psi }\rangle$ and $|\mathrm{\Phi }\rangle$ states for various interqubit distances. Comparing the results with calculations in which dephasing of each qubit is treated independently, we identify signatures of influence of common part of the bath on the two qubits. At large interqubit distances, this common part consists of many nuclei weakly coupled to both qubits, so that decoherence caused by it can be modeled by considering multiple uncorrelated sources of noise (clusters of nuclei), each of them weakly affecting the qubits. Consequently, the resulting genuinely two-qubit contribution to decoherence can be described as being caused by classical Gaussian noise. On the other hand, for small interqubit distances the common part of the environment contains clusters of spins that are strongly coupled to both qubits, and their contribution to two-qubit dephasing has visibly non-Gaussian character. We show that one can easily obtain information about non-Gaussianity of environmental noise affecting the qubits from the comparison of dephasing of $|\mathrm{\Psi }\rangle$ and $|\mathrm{\Phi }\rangle$ Bell states. Numerical results are obtained for two nitrogen vacancy centers interacting with a bath of ${}^{13}\mathrm{C}$ nuclei of natural concentration, for which we obtain that Gaussian description of a correlated part of environmental noise starts to hold for centers separated by about 3 nm.

Although interesting per se, decoherence and relaxation of single-electron excitations induced by strong effective screened Coulomb interactions in Quantum Hall edge channels are an important challenge for the applications of electron quantum optics in quantum information and quantum sensing. In this paper, we study intrinsic single-electron decoherence within an ideal single-electron channel with long-range effective Coulomb interactions to determine the influence of the material and sample properties. We find that weak-coupling materials characterized by a high velocity of hot-electron excitations may offer interesting perspectives for limiting intrinsic decoherence due to electron/electron interactions. We discuss quantitively how extrinsic decoherence due to the coupling with the channel's electromagnetic environment can be efficiently inhibited in specifically designed samples at $\nu =2$ with one closed edge channel and we propose a realistic geometry for testing decoherence control in a Hong-Ou-Mandel experiment.

Symmetry breaking of the confinement in In(Ga)As quantum dots leads to mixing between the bright and dark exciton states and an increase in the oscillator strength of the dark exciton. We demonstrate here that the change in degree of linear polarization of the bright and dark excitons as a function of applied magnetic field, where both parallel and perpendicular magnetic field components are applied, can be used to quantify the dark-bright coupling. Experimental results for a dot where the coupling is weak and the dark exciton is undetectable in zero applied magnetic field are in good agreement with theoretical simulations based on atomistic calculations reported by M. Zieliński et al. [Phys. Rev. B 91, 085403 (2015)].

The spin relaxation time of localized charge carriers is a few orders of magnitude larger than that of free electrons and holes. Therefore, mutual conversion of spin polarization, charge current, and spin current turns out to be underlined in the hopping conductivity regime. We reveal different regimes of the coupled spin and charge dynamics depending on the relation between the spin relaxation time and the characteristic hopping time. We derive kinetic equations to describe electrical spin orientation, dc spin Hall effect, and spin-galvanic effect in the transverse magnetic field. The generalized macroscopic conductivities describing these effects are calculated using percolation theory supported by numerical simulation. The conductivities change the sign at least once as functions of magnetic field for all values of the spin relaxation time.

Interfaces are ubiquitous in materials science, and in devices in particular. As device dimensions are constantly shrinking, understanding the physical properties emerging at interfaces is crucial to exploit them for applications, here for spintronics. Using first-principles techniques and Monte Carlo simulations, we investigate the mutual magnetic interaction at the interface between graphene and an antiferromagnetic semiconductor ${\mathrm{BaMnO}}_3$. We find that graphene deeply affects the magnetic state of the substrate, down to several layers below the interface, by inducing an overall magnetic softening, and switching the in-plane magnetic ordering from antiferromagnetic to ferromagnetic. The graphene-${\mathrm{BaMnO}}_3$ system presents a Rashba gap 300 times larger than in pristine graphene, leading to a flavor of quantum anomalous Hall effect (QAHE), a hybrid QAHE, characterized by the coexistence of metallic and topological insulating states. These findings could be exploited to fabricate devices that use graphene to control the magnetic configuration of a substrate.

Nodal-line semimetals are characterized by symmetry-protected band crossing lines and are expected to exhibit nontrivial electronic properties. Connections of the multiple nodal lines, resulting in nodal nets, chains, or links, are envisioned to produce even more exotic quantum states. In this work, we propose a feasible approach to realize tunable nodal-line connections in real materials. We show that certain space group symmetries support the coexistence of the planar symmetry-enforced and accidental nodal lines, which are robust to spin-orbit coupling and can be tailored into intricate patterns by chemical substitution, pressure, or strain. Based on first-principles calculations, we identify nonsymmorphic centrosymmetric quasi-one-dimensional compounds, ${\mathrm{K}}_2\mathrm{SnBi}$ and $M{X}_3(M=\mathrm{Ti}$, Zr, Hf and $X=\mathrm{Cl}$, Br, I), as materials hosting such tunable two-dimensional (2D) Dirac nodal nets. Unique Landau levels are predicted for the nodal-line semimetals with the 2D Dirac nodal nets. Our results provide a viable approach to realize the novel physics of the nodal-line connections in practice.

In recent years, low-dimensional materials with tetragonal P4/nmm (orthorhombic Pnma) space group having square-net (chainlike) substructure of $p$-block elements have been studied extensively. By using a first-principles calculation and a two-sites $\otimes$ two-orbitals tight-binding model, we construct the unified low-energy effective Hamiltonian and the ${\mathbb{Z}}_2$ topological phase diagram for such materials with different filling factors. Near the chemical potential, we show that the staggered arrangement of ions at 2c (4c) site yields a virtual hopping that has the same form with the second nearest-neighbor hopping between the square-net (chainlike) ions. We show that this hybridization and low-symmetry of the chainlike structure protects the quantum spin Hall insulator phase. Finally, the second-order spin-orbit coupling on top of the atomic spin-orbit coupling is considered to clarify the origin of the nonzero Berry phase signals reported in recent quantum oscillation experiments.

Transition-metal dichalcogenides (TMDCs) have offered experimental access to quantum confinement in one dimension. In recent years, metallic TMDCs like ${\mathrm{NbSe}}_2$ have taken center stage with many of them exhibiting interesting temperature-dependent properties such as charge density waves and superconductivity. In this paper, we perform a comprehensive optical analysis of ${\mathrm{NbSe}}_2$ by utilizing Raman spectroscopy, differential reflectance contrast, and spectroscopic ellipsometry. These analyses, when coupled with Kramers-Kronig analysis, allow us to extract the dielectric functions of bulk and atomically thin ${\mathrm{NbSe}}_2$ and relate them to the resonant behavior of the Raman spectra.

Tunable plasmons in the infrared energy range are of critical importance in designing novel plasmonic materials with fascinating uses in various solid-state applications, such as plasmonics, transformation optics, and biochemical sensors. Perovskite oxides are promising building-block materials for plasmon generation because they have wide and tunable band gaps as well as the possibility of having low plasmonic loss. Here, using a combination of high-resolution spectroscopic ellipsometry, x-ray-absorption spectroscopy both at $\mathrm{Ti}{L}_{3,2}$ and O $K$ edges, and transport measurements, we report a highly tunable quasilocal plasmon at room temperature in the long-wavelength infrared range (<1 eV) in a Mott-type insulator $\mathrm{E}{\mathrm{u}}_{0.30}\mathrm{B}{\mathrm{a}}_{0.70}\mathrm{T}{\mathrm{i}}_{(1-x)}\mathrm{N}{\mathrm{b}}_x{\mathrm{O}}_3\left(0\le x\le 0.10\right)$. Interestingly, the quasilocal plasmon, which can be controlled by varying Nb substitution, is originated by the effect of screening of Eu $4f$ electrons accompanied by band modulation that alters the relative strength of charge-transfer and Mott-transition interactions, yielding an insulator-metal transition. These quasilocal plasmons in Mott-type perovskites may open a strategy for device fabrication in the infrared-to-visible region.

We perform explicit time-dependent classical and quantum propagation of a spatially indirect exciton (SIX) driven by surface acoustic waves (SAWs) in a semiconductor heterostructure device. We model the SIX dynamics at different levels of description, from the Euler-Lagrange propagation of structureless classical particles to unitary Schrödinger propagation of an electron-hole wave packet in a mean field and to the full quantum propagation of the two-particle complex. A recently proposed beyond mean-field self-energy approach, adding internal virtual transitions to the c.m. dynamics, has been generalized to time-dependent potentials and turns out to describe very well full quantum calculations, while being orders of magnitude numerically less demanding. We show that SAW-driven SIXs are a sensitive probe of scattering potentials in the devices originating, for example, from single impurities or metallic gates, due to competing length and energy scales between the SAW elastic potential, the scattering potential, and the internal electron-hole dynamic of the SIX. Comparison between different approximations allow us to show that internal correlation of the electron-hole pair is crucial in scattering from shallow impurities, where tunneling plays a major role. On the other hand, scattering from broad potentials, i.e., with length scales exceeding the SIX Bohr radius, is well described as the classical dynamics of a pointlike SIX. Recent experiments are discussed in light of our calculations.

Motivated by the experimental difficulty to produce topological insulators (TIs) of the ${\text{Bi}}_2{\text{Se}}_3$ family with pure surface-state conduction, we study the effect that the bulk can have on the low-temperature transport properties of gated thin TI films. In particular, we focus on interference corrections, namely weak localization (WL) or weak antilocalization (WAL), and conductance fluctuations (CFs) based on an effective low-energy Hamiltonian. Utilizing diagrammatic perturbation theory, we first analyze the bulk and the surface separately and subsequently discuss WL/WAL and CFs when a tunneling-coupling is introduced. We identify the relevant soft diffusion modes of the coupled system and use this insight to make simultaneous predictions for both interference corrections and conductance fluctuations in various parameter regimes. The results strongly suggest that the combined measurement of both quantities can provide an improved understanding of the physics underlying the low-temperature transport processes in thin TI films with bulk-surface coupling.

All forms of water are essential for life and a wide range of scientific and technological processes. When adsorbed on metal surfaces, water usually forms a variety of clusters with a broad size distribution. Here we report the monodisperse water clusters formed on the semimetallic Bi(111) surface. High-resolution scanning tunneling microscopy images combined with density functional theory calculations demonstrate that the monodisperse clusters are built exclusively from hexagonal arrangement of six water molecules with slight buckling. All the water hexamers are oriented at the same direction and reveal the identical motif: three-lobed protrusions at high bias and a trigonal hole at low bias. The monodispersity of cyclic water clusters can be rationalized by the fact that hydrogen bonding is the dominant interaction responsible for water cluster stability on Bi(111). The water hexamer is the most favorable because of the maximum strength of H bonds. The decrease of H bonds (the presence of double acceptors) makes it difficult for water molecules to form a smaller (larger) cluster.