The anomalous thermodynamic properties of the paradigmatic frustrated spin-1/2 triangular-lattice Heisenberg antiferromagnet (TLH) has remained an open topic of research over decades, both experimentally and theoretically. Here, we further the theoretical understanding based on the recently developed, powerful exponential tensor renormalization group method on cylinders and stripes in a quasi-one-dimensional (1D) setup, as well as a tensor product operator approach directly in 2D. The observed thermal properties of the TLH are in excellent agreement with two recent experimental measurements on the virtually ideal TLH material ${\mathrm{Ba}}_8{\mathrm{CoNb}}_6{\mathrm{O}}_{24}$. Remarkably, our numerical simulations reveal two crossover temperature scales, at $T_l/J\sim 0.20$ and $T_h/J\sim 0.55$, with $J$ the Heisenberg exchange coupling, which are also confirmed by a more careful inspection of the experimental data. We propose that in the intermediate regime between the low-temperature scale $T_l$ and the higher one $T_h$, the “rotonlike” excitations are activated with a strong chiral component and a large contribution to thermal entropies. Bearing remarkable resemblance to the renowned roton thermodynamics in liquid helium, these gapped excitations suppress the incipient ${120}^{\circ }$ order that emerges for temperatures below $T_l$.

Artificial ice systems have been designed to replicate paradigmatic phenomena observed in frustrated spin systems. Here, we present a detailed theoretical analysis based on Monte Carlo simulations of the low-energy phases in an artificial colloidal ice system, a recently introduced ice system where an ensemble of repulsive colloids are two-dimensionally confined by gravity to a lattice of double wells at a one-to-one filling. Triggered by recent results obtained by Brownian dynamics simulations [A. Libál et al., Phys. Rev. Lett. 120, 027204 (2018)], we analyze the energetics and the phase transitions that occur in the honeycomb geometry (realizing the analog of a spin-ice system on a kagome lattice) when decreasing the temperature. When the particles are restricted to occupy the two minima of the potential well, we recover the same phase diagram as the dipolar spin-ice system, with a long-range-ordered chiral ground state. In contrast, when considering the particle motion and their relaxation within the traps, we observe ferromagnetic ordering at low temperature. This observation highlights the fundamental role played by the continuous motion of colloids in artificial ice systems.

We study theoretically the response of group III-V compound semiconductors (AlAs, AlP, GaAs, GaP, GaSb) to free-electron laser irradiation, identifying their damage thresholds. The employed hybrid code XTANT is capable of modeling both thermal and nonthermal effects under ultrafast electronic excitation. It allowed us to reveal common trends in the studied materials: all but the AlAs III-V compounds studied here exhibit a phase transition into a metallic disordered state of lower density than the solid phase via a thermal phase transition. This transition is instigated by electron-ion coupling at doses below the nonthermal melting. Irradiated AlAs showed two possible phases produced: low-density and high-density liquid. We demonstrate that the transferrable tight-binding method within the Born-Oppenheimer approximation significantly overestimates the damage threshold predicting only nonthermal melting in comparison to a non-Born-Oppenheimer scheme, which accounts for both effects and their interplay.

The geometric or Berry phase, a characteristic of quasiparticles, is fundamental to the underlying quantum materials. The discoveries of new materials at a rapid pace nowadays call for efficient detection of the Berry phase. Utilizing the $\alpha -T_3$ lattice as a paradigm, we find that, in the Dirac electron optics regime, the semiclassical decay of the quasiparticles from a chaotic cavity can be effectively exploited for detecting the Berry phase. In particular, we demonstrate a one-to-one correspondence between the exponential decay rate and the geometric phase for the entire family of $\alpha -T_3$ materials. This chaos-based detection scheme represents an experimentally feasible way to assess the Berry phase and to distinguish the quasiparticles.

Starting from our previous work in which we obtained a system of coupled integrodifferential equations for acoustic sound waves and phonon density fluctuations in two-dimensional (2D) crystals, we derive here the corresponding hydrodynamic equations, and we study their consequences as a function of temperature and frequency. These phenomena encompass propagation and damping of acoustic sound waves, diffusive heat conduction, second sound, and Poiseuille heat flow, all of which are characterized by specific transport coefficients. We calculate these coefficients by means of correlation functions without using the concept of relaxation time. Numerical calculations are performed as well in order to show the temperature dependence of the transport coefficients and of the thermal conductivity. As a consequence of thermal tension, mechanical and thermal phenomena are coupled. We calculate the dynamic susceptibilities for displacement and temperature fluctuations and study their resonances. Due to the thermomechanical coupling, the thermal resonances such as the Landau-Placzek peak and the second-sound doublet appear in the displacement susceptibility, and conversely the acoustic sound wave doublet appears in the temperature susceptibility, Our analytical results not only apply to graphene, but they are also valid for arbitrary 2D crystals with hexagonal symmetry, such as 2D hexagonal boron nitride, 2H-transition-metal dichalcogenides, and oxides.

A room-temperature skyrmion-hosting compound ${\mathrm{Co}}_8{\mathrm{Zn}}_8{\mathrm{Mn}}_4$ has been examined by means of soft x-ray absorption spectroscopy, resonant small-angle scattering, and extended reference holography. An element-selective study was performed by exciting the $2p$-to-$3d$ transitions near Co and Mn $L_{2,3}$ absorption edges. By utilizing the coherence of soft x-ray beams the element-specific real-space distribution of local magnetization at different temperatures has been reconstructed using iterative phase retrieval and holography with extended reference. It was shown that the magnetic moments of Co and Mn are ferromagnetically coupled and exhibit similar magnetic patterns. Both imaging methods provide a real-space resolution of 30 nm and allowed us to record a magnetic texture in the temperature range between $T=20$ K and $T=120$ K, demonstrating the elongation of the skyrmions along the principal crystallographic axes at low temperatures. Micromagnetic simulations have shown that such deformation is driven by a decreasing ratio of symmetric exchange interaction to antisymmetric Dzyaloshinskii-Moriya interaction in the system and effect of the cubic anisotropy.

We measure the low-temperature magnetic transport in a recently discovered type-II Dirac semimetal ${\mathrm{NiTe}}_2$ and successfully observe the planar Hall effect (PHE) which is often attributed to the chiral anomaly of topological electrons. The planar Hall signals oscillate with the in-plane angle with a π period and reach the extremum at 45° and 135°, which can be ideally described by the theoretical formulas. However, by analyzing the in-plane anisotropic magnetoresistance, we find no negative longitudinal magnetoresistance. In addition, the ${\rho }_{xx}-{\rho }_{yx}$ parametric plot exhibits a “shock-wave” pattern. All the evidence show that the presented PHE in ${\mathrm{NiTe}}_2$ originates from the trivial orbital magnetoresistance rather than the topological-nontrivial chiral anomaly.

The hydrodynamic description of the Fermi arc surface states is proposed. In view of the strong suppression of scattering on impurities, the hydrodynamic regime for Fermi arc states should be, in principle, plausible. By using the kinetic theory, the Fermi arc hydrodynamics is derived and the corresponding effects on the bulk flow and surface collective modes are studied. For the bulk flow, the key effect of the proposed Fermi arc hydrodynamics is the modification of the corresponding boundary conditions. In a slab geometry, it is shown that, depending on the transfer rates between the surface and bulk, the hydrodynamic flow of the electron fluid inside the slab could be significantly altered and even enhanced near the surfaces. As to the spectrum of the surface collective modes, in agreement with earlier studies, it is found that the Fermi arcs allow for an additional gapless spectrum branch and a strong anisotropy of the surface plasmon dispersion relations in momentum space. The gapped modes are characterized by closed elliptic contours of constant frequency in momentum space.

An intrinsic electron injection model for two-dimensional (2D) Dirac materials, like graphene, is presented and its coupling to a recently developed quantum time-dependent Monte Carlo simulator for electron devices, based on the use of stochastic Bohmian conditional wave functions, is explained. The simulator is able to capture the full (dc, ac, transient, and noise) performance of 2D electron devices. In particular, we demonstrate that the injection of electrons with positive and negative kinetic energies is mandatory when investigating high-frequency performance of Dirac materials with Klein tunneling, while traditional models dealing with holes (defined as the lack of electrons) can lead to unphysical results. We show that the number of injected electrons is bias dependent, implying that an extra charge is required to get self-consistent results. Interestingly, we provide a successful comparison with experimental dc data. Finally, we predict that a genuine high-frequency signature due to a roughly constant electron injection rate in 2D linear band electron devices (which is missing in 2D parabolic band ones) can be used as a band structure tester.

In quantum field theories defined on a space-time with boundaries, the entanglement entropy exhibits subleading, boundary-induced corrections to the ubiquitous area law. At critical points described by conformal field theories (CFTs), and when the entangling surface intersects the physical boundary of the space, new universal terms appear in the entropy and encode valuable information about the boundary CFT. In $2+1$ dimensions, the universal subleading boundary term is logarithmic with coefficient $b\left(\theta \right)$ depending on the angle $\theta$ at which the entangling surface intersects the boundary, as well as on the boundary conditions (BCs). In this paper, we conduct a numerical study of $b\left(\theta \right)$ for free bosons on finite-size square lattices. We find a surprisingly accurate fit between our lattice results and the corresponding holographic function available in the literature. We also comment on the ratio $b^{\prime \prime }(\pi /2)/A_T$, where $A_T$ is the central charge in the near boundary expansion of the stress tensor, for which a holographic analysis suggests that it may be a universal quantity. Though we show evidence that this ratio is violated for the free boson with Dirichlet BCs, we conjecture its validity for free bosons evenly split between Dirichlet and Neumann BCs.

We propose an optical gating scheme for quantum computing based on crystal-phase type II double quantum dots in an InP nanowire. The qubit is encoded on the electron spin and the gate operations are performed using stimulated Raman adiabatic passage (STIRAP), using the orbital degree of freedom in double quantum dots to form an auxiliary ground state. Successful STIRAP gating processes require an efficient coupling of both qubit ground states of the double quantum dot to the gating auxiliary ground state, and we demonstrate that this can be achieved using a charged exciton state. Crucially, by using type II dots, the hole is localized between the two spatially separated electrons in the charged exciton complex, thereby efficiently coupling the electron ground-state orbitals. By taking advantage of the high-fidelity state transfer by means of STIRAP in type II double quantum dots, we propose a protocol for coherently manipulating the spin-orbital quantum state of confined electrons in a quantum dot chain of an InP nanowire. We subsequently exploit the protocol to realize single- and two-qubit gates with fidelity above 0.99.

Impurities embedded in electronic systems induce bound states which under certain circumstances can hybridize and lead to impurity bands. Doping of insulators with impurities has been identified as a promising route toward engineering electronic topological states of matter. In this paper we show how to realize tuneable Chern insulators starting from a three-dimensional topological insulator whose surface is gapped and intentionally doped with magnetic impurities. The main advantage of the protocol is that it is robust and in particular not very sensitive to the impurity configuration. We explicitly demonstrate this for a square lattice of impurities as well as a random lattice. In both cases we show that it is possible to change the Chern number of the system by one through manipulating its topological state. We also discuss how this can be used to engineer circuits of edge channels.

Recently, it has been shown that multiterminal superconducting nanostructures may possess topological properties that involve Berry curvatures in the parametric space of the superconducting phases of the terminals, and associated Chern numbers that are manifested in quantized transconductances of the nanostructure. In this paper, we investigate how the continuous spectrum that is intrinsically present in superconductors, affects these properties. We model the nanostructure within scattering formalism deriving the action and the response function that permits a redefinition of Berry curvature for continuous spectrum. We have found that the redefined Berry curvature may have a nontopological phase-independent contribution that adds a nonquantized part to the transconductances. This contribution vanishes for a time-reversible scattering matrix. We have found compact expressions for the redefined Berry curvature for the cases of weak energy dependence of the scattering matrix and investigated the vicinity of Weyl singularities in the spectrum.

We present an analytical investigation of the exciton optical absorption in a narrow armchair graphene nanoribbon (AGNR) in the presence of a longitudinal external electric field directed parallel to the ribbon axis. The two-body two-dimensional Dirac equation for the massless electron and hole subject to the ribbon confinement, Coulomb interaction, and electric field is employed. The ribbon confinement is assumed to be much stronger than the internal exciton electric field which in turn considerably exceeds the external electric field. In the single subband approximation of the isolated size-quantized subbands induced by the ribbon confinement, the exciton electroabsorption coefficient is determined in an explicit form. The pronounced dependencies of the exciton peak positions, widths, and intensities on the ribbon width and electric field strength are traced. The electron-hole exciton attraction enhances considerably the Franz-Keldysh electroabsorption in the frequency region below and significantly modifies it above the edges determined by the size-quantized energy levels. The ionization by increasing the ribbon width is shown to occur. In the double-subband approximation of the interacting ground and first excited subbands the total peak widths associated with the exciton electro- and autoionization caused by the electric field and intersubband coupling, respectively, are determined analytically. Our analytical results are in agreement with those obtained by numerical methods. Estimates of the expected experimental values for the typically employed AGNR show that for a weak electric field the exciton quasidiscrete states remain sufficiently stable to be observed in optical experiments, while relatively strong fields free the captured carriers to further restore their contribution to the transport.

One-dimensional electronic states are critical to many fascinating phenomena in low dimensions. We show that a series of interfacial states can be induced by applying an in-plane bending to a graphene racetrack geometry, an effective superlattice consisting of multiple graphene and hexagonal boron nitride ($h\mathrm{BN}$) domains. These local states, robust against the detailed atomic configuration along interfaces, display a unique coupling of their energies and their spatial locations. This way, it is possible to identify such a state with certain energy by merely referring to its location, mimicking well the transverse magnetic focusing. Further, these local states are ballistically conducting with different group velocities, indicating that each interfacial transport channel has a unique electric conduction. These findings may have implications for the design of nanoelectronics with heterostructures.

The surface integral equation (SIE) method, discretized with the method of moments, is a well-established methodology for the scattering analysis of subwavelength plasmonic nanoparticles. SIEs are usually discretized with low-order basis functions that preserve the normal continuity of the surface currents across the edges arising in the meshed boundary, such as Rao-Wilton-Glisson (RWG) functions. However, the plasmonic enhancement modeling on sharp-edged particles is an extremely challenging task, especially due to the singular fields exerted at sharp corners, exposing a slow (or no) convergence in the computation of the scattering and absorption spectra. In this paper, we propose an alternative discretization strategy based on a discontinuous basis function set in conjunction with a volumetric-tetrahedral testing scheme. We demonstrate the potential of the proposed discretization scheme by studying scattering and absorption spectra of three canonical plasmonic polyhedra, i.e., a hexahedral, an octahedral, and a tetrahedral silver inclusion. The results expose an improved accuracy and faster convergence in both far-field and near-field regions when compared to the standard RWG implementation. The proposed discretization scheme can offer faster and more accurate routes towards the exploration and design of the plasmonic resonant spectrum of sharp-edged nanoparticles and nanoantennas.

We report on the ultrafast dynamics of charge order and structural response during the photoinduced suppression of charge and orbital order in a mixed-valence manganite. Employing femtosecond time-resolved resonant x-ray diffraction below and at the Mn $K$ absorption edge, we present a method to disentangle the transient charge order and structural dynamics in thin films of ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{MnO}}_3$. Based on the static resonant scattering spectra, we extract the dispersion correction of charge-ordered ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ ions, allowing us to separate the transient contributions of purely charge order from structural contributions to the scattering amplitude after optical excitation. Our finding of a coherent structural mode at around $2.3\mathrm{THz}$, which primarily modulates the lattice but does not strongly affect the charge order, supports the picture of the charge order being the driving force of the combined charge, orbital, and structural transition.

We study the magnetic field-dependence of the thermal conductivity due to magnetic excitations in frustrated spin-1/2 Heisenberg chains. Near the saturation field, the system is described by a dilute gas of weakly interacting fermions (free-fermion fixed point). We show that in this regime the thermal conductivity exhibits a nonmonotonic behavior as a function of the ratio $\alpha =J_2/J_1$ between second- and first-nearest-neighbor antiferromagnetic exchange interactions. This result is a direct consequence of the splitting of the single-particle dispersion minimum into two minima that takes place at the Lifshitz point $\alpha =1/4$. Upon increasing $\alpha$ from zero, the inverse mass vanishes at $\alpha =1/4$ and it increases monotonically from zero for $\alpha \ge 1/4$. By deriving an effective low-energy theory of the dilute gas of fermions, we demonstrate that the Drude weight $K_{\mathrm{th}}$ of the thermal conductivity exhibits a similar dependence on $\alpha$ near the saturation field. Moreover, this theory predicts a transition between a two-component Tomonaga-Luttinger liquid and a vector-chiral phase at a critical value $\alpha ={\alpha }_c$ that agrees very well with previous density matrix renormalization group results. We also show that the resulting curve $K_{\mathrm{th}}\left(\alpha \right)$ is in excellent agreement with exact diagonalization (ED) results. For the low-magnetic field regime, our ED results show that $K_{\mathrm{th}}\left(\alpha \right)$ has a pronounced minimum at $\alpha \simeq 0.7$. We also demonstrate that the thermal conductivity is significantly affected by the presence of magnetothermal coupling.

Detailed information on the electronic states of both V and Mn $3d$ electrons in the ferrimagnet $\mathrm{M}{\mathrm{n}}_2\mathrm{VAl}$ is obtained by bulk sensitive resonant inelastic soft x-ray scattering (SX-RIXS) excited with circularly polarized light under an external magnetic field. The results under the V $L$-edge excitation have revealed the negligible partial density of states (PDOS) of the V $3d$ states around the Fermi energy as well as their rather localized character. Under the Mn $L$-edge excitation, on the other hand, the spectra are dominated by fluorescence with clear magnetic circular dichroism with noticeable excitation photon energy dependence. Compared with the theoretical prediction of the RIXS spectra based on the density-functional-theory band structure calculation, an itinerant, spin-dependent character of the Mn $3d$ states and decays of the Mn $2p$ core states are confirmed in consistence with the half-metallicity of the Mn $3d$ states.