We show that driving a two-dimensional electron gas by an in-plane electric field oscillating at the frequency $\omega$ gives rise to an electric current at $2\omega$ flowing near the edge of the system. This current has both parallel and perpendicular to the edge components, which emit electromagnetic waves at $2\omega$ with different polarizations. We develop a microscopic theory of such an edge second harmonic generation and calculate the edge current at $2\omega$ in different regimes of electron transport and electric-field screening. We also show that at high frequencies the spatial profile of the edge current contains oscillations caused by excitation of plasma waves.

${\mathrm{NaYF}}_4$ nanoparticles (NPs) form in two crystal structures, cubic ($\alpha$-phase) and hexagonal ($\beta$-phase), each one presenting a different crystal electric field (CEF) Stark effect, that affects the upconversion (UC) light emission of the NPs when doped with rare-earth elements. Therefore, the knowledge of the CEF parameters, the wave functions, and energy levels of the rare earth (RE) $J$-multiplet is expected to be of great help for the understanding and improvement of the UC light emission. In this work, $\alpha$-phase ${\mathrm{NaYF}}_4$ NPs doped with ${\mathrm{Dy}}^{3+}, {\mathrm{Er}}^{3+}$, or ${\mathrm{Yb}}^{3+}$ were investigated by means of magnetization, electron spin resonance (ESR), and optical spectroscopy techniques. Fittings of the temperature- and magnetic-field-dependent magnetization were performed to determine the fourth- and sixth-order cubic CEF parameters, $B_4$ and $B_6$. The ground state of ${\mathrm{Er}}^{3+}, {\mathrm{Yb}}^{3+}$, and ${\mathrm{Dy}}^{3+}$ in these $\alpha \text{−}{\mathrm{NaYF}}_4$ NPs was confirmed by low-temperature ESR experiments. The obtained CEF parameters were used to write down a total Hamiltonian that allows to determine the CEF Stark splitting for all energy levels of the REs $4f$ unfilled shell. We give details of how the Stark effect affects the overall energy splitting of the various $J$-multiplets and may explain the fine structure of the UC light emission in these cubic ${\mathrm{NaY}}_{1-\delta }{\mathrm{RE}}_{\delta }{\mathrm{F}}_4$ NPs.

We study the role of the edges in determining the features of the topological phase in a quasicrystalline higher-order topological insulator. We consider a specific model consisting of two stacked Haldane models with opposite Chern number and a ${30}^{\circ }$ twist, whose structure is crystallographically equivalent to that of the graphene quasicrystal. We find that the gap-opening in the low-energy spectrum of the higher-order topological insulator occurs at different energies when different kinds of edges are considered. Crucially, bearded bonds appear to be necessary for the gap to appear close to the charge neutrality point. In the more realistic case of zigzag edges, the gap opens symmetrically in the electron and hole sectors, away from zero energy. We explain our findings by inspecting the edge bands of the decoupled bilayer, in the approximation of quasiperiodicity.

Linking structure is a new concept characterizing topological semimetals, which indicates the interweaving of gap-closing nodes at the Fermi energy ($E_F$) with other nodes below $E_F$. As the number of linked nodes can be changed only via pair creation or pair annihilation, a linked node is more stable and robust than ordinary nodes without linking. Here we propose a type of linked nodal structure between a nodal line (nodal surface) at $E_F$ with another nodal line (nodal surface) below $E_F$ in two-dimensional (three-dimensional) spinless fermion systems with $\mathcal{IT}$ symmetry where $\mathcal{I}$ and $\mathcal{T}$ indicate inversion and time-reversal symmetries, respectively. Because of additional chiral and rotational symmetries, in our system, a double band inversion creates a pair of linked nodes carrying the same topological charges, and thus the pair is unremovable via a Lifshiftz transition, which is clearly distinct from the cases of the linked nodes reported previously. A realistic tight-binding model and effective theory are developed for such a linking structure. Also, using density-functional-theory calculations, we propose a class of materials, composed of stacked bilayer graphene with Kekulé texture, as a candidate system hosting the linked nodal structure.

We study the thermal drag between two mesoscopic quantum Hall (QH) circuits. Each circuit consists of an ohmic contact perfectly coupled to quantum Hall edge states. The drag is caused by strong capacitive coupling between ohmic contacts. The nonequilibirum conditions and the electron-electron interaction are considered by using the bosonization technique in the combination with the scattering theory or, equivalently, with the help of the Langevin equations. The thermal drag current in the passive circuit, the noise power of the corresponding heat current, and the Fano factor are calculated and analyzed for different coupling strengths.

We introduce and investigate an open many-body quantum system in which kinetically constrained coherent and dissipative processes compete. The form of the incoherent dissipative dynamics is inspired by that of epidemic spreading or cellular-automaton-based computation related to the density-classification problem. It features two nonfluctuating absorbing states as well as a ${\mathcal{Z}}_2$-symmetric point in parameter space. The coherent evolution is governed by a kinetically constrained ${\mathcal{Z}}_2$-symmetric many-body Hamiltonian which is related to the quantum XOR-Fredrickson-Andersen model. We show that the quantum coherent dynamics can stabilize a fluctuating state and we characterize the transition between this active phase and the absorbing states. We also identify a rather peculiar behavior at the ${\mathcal{Z}}_2$-symmetric point. Here the system approaches the absorbing-state manifold with a dynamics that follows a power law whose exponent continuously varies with the relative strength of the coherent dynamics. Our work shows how the interplay between coherent and dissipative processes as well as symmetry constraints may lead to a highly intricate nonequilibrium evolution and may stabilize phases that are absent in related classical problems.

The lattice thermal $\mathrm{conductivity}({\kappa }_{\mathrm{L}}$) of most single crystals usually follows $T^{-1}$ temperature dependent relation at high temperature region. Here, we investigate the microscopic mechanism of the significant deviation of ${\kappa }_{\mathrm{L}}\sim T^{-1}$ rule in Cu-based chalcopyrite compounds $\mathrm{CuIn}{X}_2$($X$=Se, Te) by performing first-principles calculations and solving Peierls-Boltzmann transport equation with phonon anharmonic renormalization. The temperature sensitive second-order force constants lead to the dramatically softened optical phonon bands, which contributes to the enlarged phonon scattering phase space and consequent enhanced scattering rate. The strong temperature dependent thermal conductivity in $\mathrm{CuIn}{X}_2$($X$=Se, Te) can be well reproduced when calculating thermal conductivity with renormalization force constants. The motion of Cu contributes the most to the large anharmonicity at high temperature, which makes the anharmonic renormalization non-negligible for estimating accurate lattice dynamic and thermal conductivity. Our work will deepen the understanding of thermal transport property of Cu-based chalcopyrite, which is helpful for designing better thermoelectric or photovoltaic devices.

Underlying disorder in skyrmion materials may both inhibit and facilitate skyrmion reorientations and changes in topology. The identification of these disorder-induced topologically active regimes is critical to realizing robust skyrmion spintronic implementations, yet few studies exist for disordered bulk samples. Here, we employ small-angle neutron scattering (SANS) and micromagnetic simulations to examine the influence of skyrmion order on skyrmion lattice formation, transition, and reorientation dynamics across the phase space of a disordered polycrystalline ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ bulk sample. Our measurements reveal a disordered-to-ordered skyrmion square lattice transition pathway characterized by the promotion of fourfold order in SANS and accompanied by a change in topology of the system, reinforced through micromagnetic simulations. Pinning responses are observed to dominate skyrmion dynamics in the metastable triangular lattice phase, enhancing skyrmion stabilization through a remarkable skyrmion memory effect which reproduces previous ordering processes and persists in zero field. These results uncover the cooperative interplay of anisotropy and disorder in skyrmion formation and restructuring dynamics, establishing tunable pathways for skyrmion manipulation.

Hydrogen and its isotopes, absorbed in metals, induce local stress on the atomic structure, which generates a global expansion in proportion to the concentration of hydrogen. The dipole force tensor and its interaction with the stress fields give rise to an effective attractive nonlocal potential between hydrogen atoms—the elastic hydrogen-hydrogen interaction—which is a key quantity governing the phase transitions of hydrogen in metals. While the dipole tensor and the elastic interaction have been researched in crystalline materials, they remain experimentally unexplored in metallic glasses and it is unclear how these quantities are affected by the lack of point group symmetries. Here, we investigate both experimentally and theoretically the volume changes, the components of the force dipole tensor, and ultimately the elastic hydrogen-hydrogen interaction in the metallic glass ${\mathrm{V}}_{80}{\mathrm{Zr}}_{20}$. In situ neutron reflectometry was used to determine the deuterium-induced volume changes as a function of deuterium concentration. The one-dimensional volume expansion is found to change by more than 14% without any structural degradation, up to concentrations of one deuterium atom per metal atom. From the expansion, we determine that the out-of-plane component of the elastic dipole tensor is remarkably similar to a composition weighted sum of the ones found in crystalline vanadium and zirconium. Via ab initio calculations of both free and biaxially constrained expanded metallic structures, we determine that the trace of the dipole tensor varies with hydrogen concentration and is essentially invariant of global elastic boundary conditions. As a consequence, the elastic hydrogen-hydrogen interaction energy is found to be concentration-dependent as well, illustrating that the disordered nature of a metallic glass does not impede the mediation of the elastic attraction, but rather allows it to vary with hydrogen content.

Anderson localization is a general phenomenon that applies to a variety of disordered physical systems. Recently, a manifestation of Anderson localization for wave packets launched with a finite average velocity was proposed, the quantum boomerang effect (QBE). This phenomenon predicts that the disorder-averaged center of mass of a particle initially moves ballistically, then makes a U-turn, and finally slowly returns to its initial position. The QBE has been predicted to take place in several Hermitian models with Anderson localization and has been experimentally observed in the paradigmatic quantum kicked rotor model. In this paper, we investigate the emergence of the QBE in non-Hermitian systems and clarify the importance of symmetries of the Hamiltonian and the initial state. We generalize the analytical arguments available in the literature and show that even in the case of complex spectrum a boomeranglike behavior can appear in a non-Hermitian system. We confirm our analytical results through a careful numerical investigation of the dynamics for several non-Hermitian models. We find that non-Hermiticity leads to the breakdown of the dynamical relation, though the QBE is preserved. This paper opens avenues for future investigations in Anderson localized systems. The models studied here may be implemented using cold atoms in optical lattices.

One-way quantum steering is of importance for quantum technologies, such as secure quantum teleportation. In this paper, we study the generation of one-way quantum steering between two distant yttrium iron garnet (YIG) microspheres in chiral waveguide electromagnonics. We consider that the magnon mode with the Kerr nonlinearity in each YIG sphere is chirally coupled to left- and right-propagating guided photons in the waveguide. We find that quantum steering between the magnon modes is absent with nonchirality but is present merely in the form of one way (i.e., one-way steering) when the chirality occurs. The maximal achievable steering is obviously improved as the chirality degree increases. We further find that when the waveguide's outputs are subjected to continuous homodyne detection, the steering can be considerably enhanced and asymmetric steering with strong entanglement can also be achieved by tuning the chirality. Our study shows that chirality can be explored to effectively realize one-way quantum steering. Compared to other studies on achieving asymmetric steering via controlling intrinsic dissipation, e.g., cavity loss rates, our scheme merely depends on the chirality enabled via positioning the micromagnets in the waveguide and is continuously adjustable and experimentally more feasible.

The Yu-Shiba-Rusinov (YSR) state appears as a bound state of a quasiparticle at a magnetic atom embedded in a superconductor. We discuss why the YSR state has energy below the superconducting gap and why the pair potential changes the sign at the magnetic atom. Although a magnetic atom in a superconductor has been considered as a pair breaker since the 1960s, we propose an alternative physical picture to explain these reasons. The analytical expression of the Green's function indicates that a magnetic atom converts a spin-singlet $s$-wave Cooper pair into odd-frequency Cooper pairs rather than breaking it and that the odd-frequency pairing correlations coexist with the YSR states below the gap. The relationships among the free-energy density, the amplitudes of pairing correlation functions, and the sign change of the pair potential at a magnetic impurity are discussed utilizing the self-consistent solution of the Eilenberger equation. We conclude that the sign change of the pair potential happens only when the amplitudes of odd-frequency pairing correlations are dominant at the magnetic impurity. In the presence of the local $\pi$-phase shift in the pair potential, odd-frequency pairs can decrease the free-energy density there because their response to a magnetic field is paramagnetic.

Motivated by the recent x-ray absorption spectroscopy and resonant inelastic x-ray scattering (RIXS) experiments, we use a detailed impurity model to explore the nature of the parent compound and hole-doped states of (La, Nd, $\mathrm{Pr}){\mathrm{NiO}}_2$ by including the crystal field splitting, the $\mathrm{Ni}\text{−}3d$ multiplet structure, and the hybridization between $\mathrm{Ni}\text{−}3d, \mathrm{O}\text{−}2p$, and $\mathrm{Nd}\text{−}5d$ orbitals. For simplicity and stimulated by the recent electronic structure calculations, the latter are formally replaced with symmetric orbitals centered at the missing O sites in the Nd layer, forming a two-dimensional band strongly hybridizing with the $\mathrm{Ni}\text{−}3d_{z^2}^9$ state. This hybridization pushes the main part of the $3d_{z^2}^9$ spectral function up in energy by several eV and stabilizes the singlet with considerable $d_{z^2}^9$ and other configurational components. For the parent compound, we find that states of $\mathrm{Ni}\text{−}3d_{z^2}^9$ character spread over a large energy range in the spectra, and cannot and should not be represented by a single orbital energy, as suggested in other approximations. This is qualitatively consistent with the RIXS measurements showing a broad distribution of the $\mathrm{Ni}\text{−}3d_{z^2}^9$ hole state, although the shape of the $\mathrm{Ni}\text{−}3d_{z^2}^9$ related structure is much more complicated requiring reinterpretations of the RIXS data. For the hole-doped systems, we show that adding these additional ingredients can still result in the lowest-energy hole-doped state having a singlet character.

Gallium nitride (GaN) is a material with important applications in optoelectronics, wireless communications, and power electronics. Devices for such applications are normally made with GaN single-crystal wafers. Yet an infinitesimal amount of atomic defects in these single crystals considerably hinders the electronic performance of GaN. One kind of atomic defect in GaN crystals that has been theoretically predicted but eludes direct experimental observation is nitrogen (N) vacancies. Here, we unambiguously identify a single N vacancy on a cleaved $m$-plane surface of GaN by direct visualization in real space with scanning tunneling microscopy (STM) and atomic force microscopy (AFM). The presence of a vacancy is established by AFM imaging and force spectroscopy measurements. The identification is accomplished by the analysis of STM images, tunneling current spectroscopy, and comparison with the outcomes from the quantification of band bending near the surface, current calculations, and first-principles simulations. All this information provides insight into the electronic perturbation of a single N vacancy at the surface band structure of GaN. Our results provide further understanding of the effect that point defects have on GaN, and will hopefully contribute to tune the behavior of this technologically relevant material in electronic devices.

In this work we analyze the finite-size effects on the structural properties and on the polarization of th e rhombohedral phase of ${\mathrm{HfO}}_2$ subjected to a biaxial compressive strain. We show how the presence of surface charges affects the polarization, leading to a strong reduction with respect to its bulk value. This reduction can be ascribed to two mechanisms: (i) the coupling between compressive strain and the phase-transition order parameter; (ii) the changes in the ferroelectric distortion. We give two alternative explanations of this phenomenon: From an atomistic point of view, analyzing the evolution of the bond lengths, and from a symmetryanalysis point of view, considering the changes in the amplitude of the symmetry-allowed distortions, when a slab configuration is considered. These results are in dependent of the slabthickness in the considered range, suggesting the absence of a critical thickness for ferroelectricity in ${\mathrm{HfO}}_2$, in agreement with the proposed improper nature of hafnia ferroelectricity.

We investigate lattice dynamics and spin-phonon coupling as well as the magnetic and electronic properties of $\beta \text{−}{\mathrm{Na}}_{0.33}{\mathrm{V}}_2{\mathrm{O}}_5$ using temperature-dependent Raman scattering, dc magnetization and dc resistivity, x-ray photoemission, and absorption spectroscopy. The Rietveld refinement of the x-ray diffraction pattern with space group $C2/m$ confirms the monoclinic structure. The analysis of temperature-dependent Raman spectra in a temperature range of 13–673 K reveals an anharmonic dependence of the phonon frequency and full width at half maximum, which is accredited to the symmetric phonon decay. However, $<\sim 40\mathrm{K}$, the hardening of the phonon frequency beyond anharmonicity is attributed to the spin-phonon coupling. Interestingly, the estimated effective magnetic moment ${\mu }_{\mathrm{eff}}= 0.63{\mathrm{\mu }}_{\mathrm{B}}$ from the magnetization data manifests a mixed-valence state of V ions in $4+(18\pm 1\%)$ and $5+(82\pm 1\%$). A similar ratio of ${\mathrm{V}}^{4+}$ to ${\mathrm{V}}^{5+}$ is also observed in the x-ray photoemission and x-ray absorption near-edge spectra and found to be consistent with the sample stoichiometry. The analysis of extended x-ray absorption fine structure at the V $K$-edge gives the corresponding V–O bond lengths, which are utilized in the detailed analysis of Raman modes. Moreover, the temperature-dependent resistivity resembles a semiconducting behavior where the charge carrier transport is facilitated by the band conduction at higher temperatures and via hopping $\le 260\mathrm{K}$.

There has been considerable recent progress in identifying candidate materials for the transverse-field Ising chain (TFIC), a paradigmatic model for quantum criticality. Here, we study the local spin dynamical structure factor of different spin components in the quantum disordered region of the TFIC. We show that the low-frequency local dynamics of the spins in the Ising- and transverse-field directions have strikingly distinctive temperature dependencies. This leads to the thermal-activation gap for the secular term of the nuclear magnetic resonance $1/T_2^{\prime }$ relaxation rate to be half of that for the $1/T_1$ relaxation rate. Our findings reveal a surprise in the nonzero-temperature dynamics of the venerable TFIC model and uncover a means to evince the material realization of the TFIC universality.

We study a continuum model of the interface of charge neutral graphene and vacuum in the quantum Hall regime via a sign-problem-free quantum Monte Carlo, allowing us to investigate the interplay of topology and strong interactions in a graphene quantum Hall edge for large system sizes. We focus on the topological phase transition from the spin-polarized state with symmetry-protected gapless helical edges to the fully charge gapped canted-antiferromagnet state with spontaneous symmetry breaking, driven by the Zeeman energy. Our large system size simulations allow us to detail the behavior of various quantities across this transition that are amenable to being probed experimentally, such as the spatially and energy-resolved local density of states and the local compressibility. We find peculiar kinks in the branches of the edge dispersion, and also an unexpectedly large charge susceptibility in the bulk of the canted antiferromagnet associated with its Goldstone mode.

Chiral edge modes inherent to the topological quantum anomalous Hall (QAH) effect are a pivotal topic of contemporary condensed matter research aiming at future quantum technology and application in spintronics. A large topological gap is vital to protecting against thermal fluctuations and thus enabling a higher operating temperature. From first-principles calculations, we propose ${\mathrm{Al}}_2{\mathrm{O}}_3$ as an ideal substrate for atomic monolayers consisting of Bi and group-III elements, in which a large-gap quantum spin Hall effect can be realized. Additional half-passivation with nitrogen then suggests a topological phase transition to a large-gap QAH insulator. By effective tight-binding modeling, we demonstrate that Bi-III monolayer/${\mathrm{Al}}_2{\mathrm{O}}_3$ is dominated by $p_x,p_y$ orbitals, with subdominant $p_z$ orbital contributions. The topological phase transition into the QAH is induced by Zeeman splitting, where the off-diagonal spin exchange does not play a significant role. The effective model analysis promises utility far beyond Bi-III monolayer/${\mathrm{Al}}_2{\mathrm{O}}_3$, as it should generically apply to systems dominated by $p_x,p_y$ orbitals with a band inversion at $\mathrm{\Gamma }$.