The study of quantum frustrated systems remains one of the most challenging subjects of quantum magnetism, as they can hold quantum spin liquids, whose characterization is quite elusive. The presence of gapped quantum spin liquids possessing long-range entanglement while being locally indistinguishable often demands highly sophisticated numerical approaches for their description. Here we propose an easy computational method based on exact diagonalization with engineered boundary conditions in very small plaquettes. We apply the method to study the quantum phase diagram of diverse antiferromagnetic frustrated Heisenberg models in the triangular lattice. Our results are in qualitative agreement with previous results obtained by means of sophisticated methods like density matrix renormalization group (2D-DMRG) or variational quantum Monte Carlo.

Recently observed magnetophonon resonances in the magnetoresistance of graphene are investigated using the Kubo formalism. This analysis provides a quantitative fit to the magnetophonon resonances over a wide range of carrier densities. It demonstrates the predominance of carrier scattering by low-energy transverse acoustic (TA) mode phonons: the magnetophonon resonance amplitude is significantly stronger for the TA modes than for the longitudinal acoustic (LA) modes. We demonstrate that the LA and TA phonon speeds and the electron-phonon coupling strengths determined from the magnetophonon resonance measurements also provide an excellent fit to the measured dependence of the resistivity at zero magnetic field over a temperature range of 4–150 K. A semiclassical description of magnetophonon resonance in graphene is shown to provide a simple physical explanation for the dependence of the magneto-oscillation period on carrier density. The correspondence between the quantum calculation and the semiclassical model is discussed.

Accurate modeling of electronic properties of nanostructures is a challenging theoretical problem. Methods making use of continuous media approximation, such as $\mathit{k}·\mathit{p}$, sometimes struggle to reproduce results obtained with more accurate atomistic approaches. On the contrary, atomistic schemes generally come with a substantially larger cost of computation. Here, we bridge these two approaches by taking the eight-band $\mathit{k}·\mathit{p}$ method augmented with nonlinear strain terms fit to reproduce $s{p}^3{d}^5{s}^{*}$ tight-binding results. We illustrate this method in the example of electron and hole states confined in quantum wells and quantum dots of photonics applications relevant InAs/GaAs material system, and demonstrate a good agreement of a nonlinear $\mathit{k}·\mathit{p}$ scheme with the empirical tight-binding method. We discuss limits of our procedure as well as provide nonlinear eight-band $\mathit{k}·\mathit{p}$ parameter sets for InAs and GaAs. Finally, we propose a parametrization for effective term used to improve the accuracy of the standard effective-mass method.

Luttinger semimetals are three-dimensional electron systems with a parabolic band touching and an effective total spin $J=3/2$. In this paper, we present an analytical theory of dielectric screening of inversion-symmetric Luttinger semimetals with an arbitrary carrier density and conduction-valence effective mass asymmetry. Assuming a spherical approximation for the single-particle Luttinger Hamiltonian, we determine analytically the dielectric screening function in the random phase approximation for arbitrary values of the wave vector and frequency, the latter in the complex plane. We use this analytical expression to calculate the dispersion relation and Landau damping of the collective modes in the charge sector (i.e., plasmons).

We study charge transport through $N$-lead junctions ($N\ge 3$) of spinless Luttinger-liquid wires with bias voltages applied to Fermi-liquid reservoirs. In particular, we consider a Y junction, which is a setup characteristic of the tunneling experiment. In this setup, the strength of electron-electron interactions in one of the arms (“tunneling tip”) is different from that in the other two arms (which form together the “main wire”). For a generic single-particle $S$ matrix of the junction, we find that the bias voltage $V$ applied, even symmetrically, to the main wire generates a current proportional to $\left|V\right|$ in the tip wire. We identify two mechanisms of this nonequilibrium-induced “emergent chirality” in a setup characterized by the time-reversal- and parity-symmetric Hamiltonian of the junction. These are (i) the emergence of an effective magnetic flux, which breaks time-reversal symmetry, and (ii) the emergence of parity-breaking asymmetry of the setup, both proportional to the interaction strength and the sign of the voltage. The current in the tip wire generated by mechanism (i) is reminiscent of the Hall current in the linear response of a system the Hamiltonian of which breaks time-reversal symmetry; however, in the absence of any magnetic field or a local magnetic moment. Similarly, mechanism (ii) can be thought of as an emergent “photogalvanic effect,” however, in the presence of inversion symmetry within the main wire. The nonequilibrium chirality implies a rectification of the current in the tip when the main wire is biased by ac voltage.

We formulate a classical theory $({\omega }_c\tau \lesssim 1$ with ${\omega }_c$ as the cyclotron frequency and $\tau$ as the relaxation time) to study the influence of perpendicular magnetic field on the electron-impurity scattering process in two-dimensional electron gas. To describe the curved incoming and outgoing trajectories, we introduce a general recipe based on an abstraction of the actual impurity scattering process to define the scattering parameters such as the incoming and outgoing momentum and coordinate jump. In this picture, we can conveniently describe the skew scattering and coordinate jump, which will eventually modify the Boltzmann equation. We find an anomalous Hall resistivity different from the conventional Boltzmann-Drude result and a negative magnetoresistivity as a parabolic function of magnetic field. The origin of these results is analyzed. The relevance between our theory and recent simulation and experimental works is also discussed.

Magnetic systems with frustration often have large classical degeneracy. We show that their low-energy physics can be understood as dynamics within the space of classical ground states. We demonstrate this mapping in a family of quantum spin clusters where every pair of spins is connected by an $XY$ antiferromagnetic bond. The dimer with two spin-$S$ spins provides the simplest example—it maps to a quantum particle on a ring $\left(S^1\right)$. The trimer is more complex, equivalent to a particle that lives on two disjoint rings $\left(S^1\otimes {\mathbb{Z}}_2\right)$. It has an additional subtlety for half-integer $S$ values, wherein both rings must be threaded by $\pi$ fluxes to obtain a satisfactory mapping. This is a consequence of the geometric phase incurred by spins. For both the dimer and the trimer, the validity of the effective theory can be seen from a path-integral-based derivation. This approach cannot be extended to the quadrumer which has a nonmanifold ground-state space, consisting of three tori that touch pairwise along lines. In order to understand the dynamics of a particle in this space, we develop a tight-binding model with this connectivity. Remarkably, this successfully reproduces the low-energy spectrum of the quadrumer. For half-integer spins, a geometric phase emerges which can be mapped to two $\pi$-flux tubes that reside in the space between the tori. The nonmanifold character of the space leads to a remarkable effect—the dynamics at low energies is not ergodic as the particle is localized around singular lines of the ground-state space. The low-energy spectrum consists of an extensive number of bound states formed around singularities. Physically, this manifests as an order-by-disorder-like preference for collinear ground states. However, unlike order-by-disorder, this “order by singularity” persists even in the classical limit. We discuss consequences for field theoretic studies of magnets.

We consider the reduced density matrix for a disk in the ground state of the interacting fixed points of the large-$NO\left(N\right)$ vector model in $2+1$ dimensions. Using the map to the free energy on ${\mathbb{H}}_2\times S^1$, we show that there is an instability in the Rényi entropies at $n=1$ in the $N\to \infty$ limit, indicating a phase transition at or near this Rényi parameter. We close with a discussion of the finite-$N$ case.

A composite high-pressure phase diagram for silicon has been predicted for up to 4 TPa. This diagram has been built using a combination of evolutionary algorithm-based structure searches, electronic density functional theory, lattice dynamics of perfect crystals, and anharmonic corrections for the solid state in conjunction with molecular dynamics for evaluating the melt curve. The anharmonic corrections to free energy, arising from both finite-temperature multiphonon interactions and temperature dependence of the lattice axial ratios, play a critical role in properly identifying the solid-solid transition boundaries of the orthorhombic structures. A double hexagonal close-packed structure has been found to be thermodynamically and dynamically stable, sandwiched between the experimentally observed base-centered orthorhombic and hexagonal close-packed structures. In addition, beyond 2.8 TPa, there exists a sequence of face-centered cubic-to-body-centered cubic-to-simple cubic transitions that are accompanied by the localization of electrons in the interstitial spaces between the ions. Supplementing the structural calculations, second- and third-order interatomic force constants were evaluated to compute the phonon vibration modes and linewidths, respectively. This allowed an elaborate analysis of the Raman and infrared spectra for all of the structures of silicon identified so far.

Despite the progress in understanding the formation and migration of oxygen vacancies in strongly correlated complex oxides, few studies focused on the dynamic behaviors of the oxygen vacancies under the transient and nonequilibrium states. Here we report a series of multitimescale ultrafast x-ray diffraction experiments using picosecond synchrotron and femtosecond table-top x-ray sources to monitor the structural dynamics in oxygen-vacancy-ordered $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ thin films excited by photons. A giant photoinduced structure distortion in the $c$-axis direction (with strain $\mathrm{\Delta }c/c>1\%$) was observed, higher than any previously reported data in the other transition metal oxide films. The femtosecond x-ray diffraction reveals the formation and propagation of the coherent acoustic phonons, indicating an instantaneous and a much larger photoinduced stress within the first 100 ps. Density functional theory reproduced the photostrictive responses and the strong dependence on excitation wavelength as observed in the experiments. The combined experimental and theoretic results demonstrate that the photoexcitation of the bonding to antibonding states via charge transfer is the dominant mechanism in the $\mathrm{SrCo}{\mathrm{O}}_{2.5}$ thin films, distinct from the depolarization effect by photoinduced carriers in the other perovskite oxides.

The band structure of two coupled one-dimensional (1D) bicomponent magnonic crystals is constructed from theory. The model takes into account both the exchange interaction and the long-range dipolar interaction between the magnonic crystals, for which different coupling configurations are analyzed systematically. In comparison with a monolayer 1D bicomponent magnonic crystal, the band structure of the bilayer magnonic crystal is significantly modified and depends on the geometric positioning between the layers. The widths of the band gaps may be altered considerably to induce a flattening of the high-frequency modes or nonreciprocity in frequency of two counterpropagating collective spin waves. In the latter case, the nonreciprocity is mainly observed in modes with frequencies above the first band gap, whereas the low-frequency modes remain almost reciprocal. These theoretical results show that two coupled magnonic crystals are excellent candidates to assist in controlling the collective spin waves, thereby permitting great versatility in designing reconfigurable spin-based devices.

Transport properties of the classical antiferromagnetic XXZ model on the square lattice have been theoretically investigated, putting emphasis on how the occurrence of a phase transition is reflected in spin and thermal transports. As is well known, the anisotropy of the exchange interaction $\mathrm{\Delta }\equiv J_z/J_x$ plays a role to control the universality class of the transition of the model, i.e., either a second-order transition at $T_N$ into a magnetically ordered state or the Kosterlitz-Thouless (KT) transition at $T_{\text{KT}}$, which respectively occur for the Ising-type ($\mathrm{\Delta }>1$) and XY-type ($\mathrm{\Delta }<1$) anisotropies, while for the isotropic Heisenberg case of $\mathrm{\Delta }=1$, a phase transition does not occur at any finite temperature. It is found by means of the hybrid Monte Carlo and spin-dynamics simulations that the spin current probes the difference in the ordering properties, while the thermal current does not. For the XY-type anisotropy, the longitudinal spin-current conductivity ${\sigma }_{xx}^s$ ($={\sigma }_{yy}^s$) exhibits a divergence at $T_{\text{KT}}$ of the exponential form ${\sigma }_{xx}^s\propto exp\left[B/\sqrt{T/T_{\text{KT}}-1}\right]$ with $B=O\left(1\right)$, while for the Ising-type anisotropy, the temperature dependence of ${\sigma }_{xx}^s$ is almost monotonic without showing a clear anomaly at $T_N$ and such a monotonic behavior is also the case in the Heisenberg-type spin system. The significant enhancement of ${\sigma }_{xx}^s$ at $T_{\text{KT}}$ is found to be due to the exponential rapid growth of the spin-current relaxation time toward $T_{\text{KT}}$, which can be understood as a manifestation of the topological nature of a vortex whose lifetime is expected to get longer toward $T_{\text{KT}}$. Possible experimental platforms for the spin-transport phenomena associated with the KT topological transition are discussed.

It was recently shown that BiTeI, a semiconductor with polar crystal structure, possesses a giant spin splitting of electrons, which has been interpreted in terms of Rashba-type spin-orbit coupling. Here we use high field magneto-optical spectroscopy to quantify the deviations of the conduction-band profile from this appealing, but at the same time, strongly simplifying model. We find that the optical response—comprising a series of inter-Landau level excitations—can be described by the Rashba model only at low magnetic fields. In contrast, the high-field response appears to be more consistent with a simple picture of massless electrons in a conical band. This points towards more linear rather than parabolic dispersion at energies well above the bottom of the conduction band.

A novel scheme of beating for the generation of the temporal pulse train, both with periodical density dip and corresponding $\pi$ phase difference, is theoretically investigated in a quasi-one-dimensional (quasi-1D) exciton-polariton condensate. Polariton particles, pumped both by the nonresonant field and coherent field, form two distinct sources with different frequency for beating to induce the interference in the temporal domain to generate the temporal pulse train. The oscillating period of the temporal pulse train is determined mainly by the intensity of the nonresonant pump fields but not the frequency. The temporal pulse train carries all the information of interference such as the phase difference and the density relationship between two polariton sources, which could be stored and recalled on demand with high efficiency and fidelity by simply switching off and on the coherent field.

Using a qubit to probe non-Gaussian noise environments is theoretically studied in the context of classical random telegraph processes. Protocols for control pulses are developed to effectively scan higher noise correlations, offering valuable information on the charge environment of the qubit. Specifically, the noise power spectrum and trispectrum are reconstructed simultaneously for a wide range of qubit-fluctuator coupling strengths, demonstrating the method's robustness. These protocols are readily testable in various qubit systems with well-developed quantum control, including quantum dot spins, superconducting qubits, and nitrogen-vacancy centers in diamond.

Transition-metal monochalcogenides comprise a class of two-dimensional materials with electronic band gaps that are highly sensitive to material thickness and chemical composition. Here, we explore the tunability of the electronic excitation spectrum in GaSe by using angle-resolved photoemission spectroscopy. The electronic structure of the material is modified by in situ potassium deposition as well as by forming ${\mathrm{GaS}}_x{\mathrm{Se}}_{1-x}$ alloy compounds. We find that potassium-dosed samples exhibit a substantial change of the dispersion around the valence-band maximum (VBM). The observed band dispersion resembles that of a single tetralayer and is consistent with a transition from the direct-gap character of the bulk to the indirect-gap character expected for monolayer GaSe. Upon alloying with sulfur, we observe a phase transition from AB to ${\text{AA}}^{\prime }$ stacking. Alloying also results in a rigid energy shift of the VBM towards higher binding energies, which correlates with a blueshift in the luminescence. The increase of the band gap upon sulfur alloying does not appear to change the dispersion or character of the VBM appreciably, implying that it is possible to engineer the gap of these materials while maintaining their salient electronic properties.

${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy can probe several local structural, electronic, and magnetic properties of Fe-containing systems. However, to establish a direct relationship between these properties and a system's geometric structure, the experimental Mössbauer parameters need to be analyzed via electronic structure calculations. Herein, structural, electronic, and magnetic effects of iron substituents in the topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, as uniquely probed by ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy, have been determined via spin-polarized electronic structure calculations. The iron ion substituents, of nominal ${\mathrm{Fe}}^{3+}(S=5/2)$ oxidation and spin state, are unequivocally shown to substitute ${\mathrm{Bi}}^{3+}$ sites in epitaxial ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin-films used for Mössbauer measurements. Concomitant with iron substitution, localized structural rearrangements take place whereby the longer Bi-Se bonds of the native system are replaced by significantly shorter Fe-Se counterparts in the Fe-containing system. The resulting distorted-octahedral environment about substituent iron ions gives rise to characteristic Mössbauer parameters $({\delta }_{\mathrm{Fe}}\approx 0.51$ mm/s, $\mathrm{\Delta }{E}_{\mathrm{Q}}\approx 0.20$ mm/s) which have been calculated to be in excellent agreement with measured values for Fe-doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ thin films. Consistent with a substituent ${\mathrm{Fe}}^{3+}$ ion's nominal high-spin electronic configuration $\left(t_{2\mathrm{g}}^{\uparrow \uparrow \uparrow }{e}_{\mathrm{g}}^{\uparrow \uparrow }\right)$, an Fe-centered spin density has been established which, nevertheless, extends toward neighboring Se atoms via direct Fe-Se bonding and concomitant $\text{Fe}\left(d\right)\text{−}\text{Se}\left(p\right)$ hybridization.