The quantum entanglement between two excitons in two-dimensional materials, embedded in an optical microcavity, was investigated. The energy eigenstates of a Jaynes-Cummings like Hamiltonian for two qubits coupled to a single cavity mode have been calculated. The quantum entanglement between such states was estimated by calculating the concurrence between two qubits in each of these eigenstates. According to the results of our calculations, if the system is allowed to decay only through the emission of cavity photons at low temperatures, then there is a maximally entangled eigenstate, protected from decay. We demonstrated that the existence of such a state results in the counterintuitive conclusion that, for some initial states of the system, the fact that the cavity is leaky can actually lead to an increase in the average concurrence on the timescales of the average photonic lifetime. By briefly analyzing the three-qubit model, we have demonstrated that the probability for the entanglement to be preserved is enhanced when the number of qubits is increased. In addition, we calculated the time evolution of the concurrence between a pair of excitons in a strained graphene monolayer.

The orbital Hall effect (OHE) designates the generation of a charge-neutral flow of orbital angular momentum transverse to an initial charge current. Recent theoretical investigations suggest that transition metals display sizable OHE, encouraging experimental search along this direction. Nonetheless, most of these theories assume that the orbital moment originates from the region centered on the atomic sites, adopting the so-called atom-centered approximation. In periodic crystals though, the motion of the wave packet between atoms provides a crucial contribution to the overall orbital moment, and neglecting it can lead to a severe misestimation of the OHE. By applying the “modern theory” of orbital magnetization to the OHE, we assess the relative importance of intra- and interatomic contributions in selected materials from first principles. We find that whereas the OHE is mostly of intra-atomic origin for wide band-gap semiconductors (e.g., ${\mathrm{MoS}}_2$), the interatomic contribution becomes crucial in narrow band-gap semiconductors (SnTe, PbTe) and transition metals (Pt, V etc.). These predictions invalidate the atom-centered approximation adopted in some of the previous papers and open perspectives for the realization of efficient sources of orbital currents.

${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ approximates the two-dimensional spin-$\frac{1}{2}$ triangular-lattice Heisenberg antiferromagnet. This compound displays magnetic-field-induced quantum phase transitions, including the $\frac{1}{3}$-magnetization plateau, but its magnetization processes for the magnetic field $H$ parallel and perpendicular to the $c$ axis are different due to the weak easy-plane anisotropy and the weak interlayer antiferromagnetic exchange interaction. To elucidate how the quantum phase transitions change between these two field directions, we measured the field-angle dependence of the magnetization process in ${\mathrm{Ba}}_3{\mathrm{CoSb}}_2{\mathrm{O}}_9$ using pulsed high-magnetic fields. We compared obtained magnetic-field–field-angle phase diagram with those obtained by the large-size cluster mean-field method combined with a scaling scheme and the semiclassical theory. We also found a narrow $\frac{1}{3}$-magnetization plateau and a high-field transition with a small magnetization jump for $H\parallel c$, not observed in the previous studies.

Computing the $GW$ quasiparticle band structure and Bethe-Salpeter equation (BSE) absorption spectra for materials with spin-orbit coupling have commonly been done by treating $GW$ corrections and spin-orbit coupling (SOC) as separate perturbations to density-functional theory. However, accurate treatment of materials with strong spin-orbit coupling (such as many topological materials of recent interest, and thermoelectrics) often requires a nonperturbative approach using spinor wave functions in the Kohn-Sham equation and $GW/\mathrm{BSE}$. Such calculations have only recently become available, in particular for the BSE. We have implemented this approach in the plane-wave pseudopotential $GW/\mathrm{BSE}$ code BerkeleyGW, which is highly parallelized and widely used in the electronic-structure community. We present reference results for quasiparticle band structures and optical absorption spectra of solids with different strengths of spin-orbit coupling, including Si, Ge, GaAs, GaSb, CdSe, Au, and ${\mathrm{Bi}}_2{\mathrm{Se}}_3$. The calculated quasiparticle band gaps of these systems are found to agree with experiment to within a few tens of meV. SOC splittings are found to be generally in better agreement with experiment, including quasiparticle corrections to band energies. The absorption spectrum of GaAs is not significantly impacted by the inclusion of spin-orbit coupling due to its relatively small value (0.2 eV) in the $\mathrm{\Lambda }$ direction, while the absorption spectrum of GaSb calculated with the spinor $GW/\mathrm{BSE}$ captures the large spin-orbit splitting of peaks in the spectrum. For the prototypical topological insulator ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, we find a drastic change in the low-energy band structure compared to that of DFT, with the spinorial treatment of the $GW$ approximation correctly capturing the parabolic nature of the valence and conduction bands after including off-diagonal self-energy matrix elements. We present the detailed methodology, approach to spatial symmetries for spinors, comparison against other codes, and performance compared to spinless $GW/\mathrm{BSE}$ calculations and perturbative approaches to SOC. This work aims to spur further development of spinor $GW/\mathrm{BSE}$ methodology in excited-state research software and enables a more accurate and detailed exploration of electronic and optical properties of materials containing elements with large atomic numbers.

Understanding the properties of molecular solids at high pressure is a key element in the development of new solid-state theories. However, the most commonly used generalized gradient approximation of the density functional theory (DFT) often fails to describe the behavior of these systems correctly at high pressure. Here we utilize the hybrid DFT approach to model the properties of elemental bromine at high pressure. The calculations reproduce in very good agreement with experimentation the properties of molecular phase I ($Cmca$ symmetry) and its pressure-induced transition into nonmolecular phase II $\left(Immm\right)$. The experimentally yet unobserved transition into phase III $\left(I4/mmm\right)$ is predicted to occur at 128 GPa, followed by subsequent formation of an fcc lattice at 188 GPa. Analysis of the structure and electronic properties of the modeled phases indicates that the molecular $Cmca$ phase becomes metallic just at the borderline of its stability, and that both the $Immm$ and $I4/mmm$ phases are metallic and quasi–two-dimensional. Finally, we show that the incommensurate phases of bromine postulated from experimentation are transient species that can be viewed as intermediates in the dissociation process occurring at the boundary of the transition from phase I to phase II.

We discuss a quantum thermal machine that generates power from a thermally driven double quantum dot coupled to normal and superconducting reservoirs. Energy exchange between the dots is mediated by electron-electron interactions. We can distinguish three main mechanisms within the device operation modes. In the Andreev tunneling regime, energy flows in the presence of coherent superposition of zero- and two-particle states. Despite the intrinsic electron-hole symmetry of Andreev processes, we find that the heat engine efficiency increases with increasing coupling to the superconducting reservoir. The second mechanism occurs in the regime of quasiparticle transport. Here we obtain large efficiencies due to the presence of the superconducting gap and the strong energy dependence of the electronic density of states around the gap edges. Finally, in the third regime there exists a competition between Andreev processes and quasiparticle tunneling. Altogether, our results emphasize the importance of both pair tunneling and structured band spectrum for an accurate characterization of the heat engine properties in normal-superconducting coupled dot systems.

We study the spectral and transport properties of a Mott insulator driven by a static electric field into a nonequilibrium steady state. For the dissipation, we consider two mechanisms: wide band fermion reservoirs and phonons included within the Migdal approximation. The electron correlations are treated via nonequilibrium dynamical mean field theory with an impurity solver suitable for strong correlations. We find that dissipation via phonons is limited to restricted ranges of field values around Wannier-Stark resonances. To cover the full range of field strengths, we allow for a small coupling with fermionic baths, which stabilizes the solution. When considering both dissipation mechanisms, we find that phonons enhance the current for field strengths close to half of the gap while lowering it at the gap resonance as compared to the purely electronic dissipation used by Murakami and Werner [Murakami and Werner, Phys. Rev. B 98, 075102 (2018)]. Once phonons are the only dissipation mechanism, the current in the metallic phase is almost one order of magnitude smaller than the typical values obtained by coupling to a fermionic bath. In this case, the transport regime is characterized by an accumulation of charge in the upper Hubbard band described by two effective chemical potentials.

Anisotropic bulk Dirac (or Weyl) cones in three-dimensional systems have recently gained intense research interest as they are examples of materials with tilted Dirac (or Weyl) cones indicating the violation of Lorentz invariance. In contrast, the studies on anisotropic surface Dirac cones in topological materials which contribute to anisotropic carrier mobility have been limited. By employing angle-resolved photoemission spectroscopy and first-principles calculations, we reveal the anisotropic surface Dirac dispersion in a tetradymite material ${\mathrm{Ti}}_2{\mathrm{Te}}_2\mathrm{P}$ on the (001) plane of the Brillouin zone. We observe quasielliptical Fermi pockets at the $\overline{M}$ point of the Brillouin zone forming the anisotropic surface Dirac cones. Our calculations of the ${\mathbb{Z}}_2$ indices confirm that the system is topologically nontrivial with multiple topological phases in the same material. In addition, the observed nodal-line-like feature formed by bulk bands makes this system topologically rich.

It is a well-known fact that the disorder has its most dramatic effects on the conventional quantum transport in one-dimensional systems. In flat-band (FB) systems, it is revealed that the conductivity at the FB energy is robust against the disorder and can even be tremendously boosted. Here, the disorder is due to randomly distributed vacancies. Furthermore, challenging our understanding of the physical phenomena, the giant increase occurs in the limit of low FB state density. The singular behavior of the quantum metric of the FB eigenstates is found to be at the heart of these unexpected and puzzling features. Additionally, it is shown that the compact localized eigenstates should extend over at least two unit cells to allow a boost. Our findings should have interesting fallout for other physical systems, and may as well open up engineering strategies to boost the critical temperature in two-dimensional superconducting FB materials.

Symmetry protected nontrivial states in chiral topological materials hold immense potential for fundamental science and technological advances. Here, we report electrical transport, quantum oscillations, and electronic structure results of a single crystal of chiral quantum material PtAl. Based on the de Haas-van Alphen (dHvA) oscillations, we show that the smallest Fermi pocket $\left(\alpha \right)$ possesses a nontrivial Berry phase $1.16\pi$. The band associated with this Fermi pocket carries a linear energy dispersion over a substantial energy window of $\sim 700\mathrm{meV}$ that is further consistent with the calculated optical conductivity. First principles calculations unfold that PtAl is a higher fold chiral fermion semimetal where structural chirality drives the chiral fermions to lie at the high symmetry $\mathrm{\Gamma }$ and $R$ points of the cubic Brillouin zone. In the absence of spin orbit coupling, the band crossings at $\mathrm{\Gamma }$ and $\mathrm{R}$ points are three and fourfold degenerate with a chiral charge of $-2$ and $+2$, respectively. The inclusion of spin orbit coupling transforms these crossing points into four and sixfold degenerate points with a chiral charge of $-4$ and $+4$. Nontrivial surface states on the (001) surface connect the bulk projected chiral points through the long helical Fermi arcs that spread over the entire surface Brillouin zone.

Rashba-like spin-orbit interaction at oxide heterostructures emerges as a much sought-after feature in the context of oxide spintronics and spin-orbitronics. ${\mathrm{KTaO}}_3$ (KTO) is one of the best substrates available for the purpose, owing to its strong spin-orbit interaction and alternating $+1|-1$ charged layers along the (001) direction. Employing first-principles calculations within density functional theory (DFT) and proposing a possible electrostatic model for charge transfer to the surfaces of KTO slabs, we comprehensively analyze Rashba-like spin-orbit interaction with the help of three-dimensional (3D) band dispersion, isoenergetic contours, and projected spin textures – all directly obtained from our DFT calculations – in a thin insulating slab and a conducting thick slab of KTO. Our results reveal reasonably strong linear Rashba interaction with no signature of Dresselhaus or higher order Rashba interactions in the systems considered here. The rigorous analysis presented here may be crucial for future developments in oxide spintronics.

Control of electron dynamics in graphene nanoribbons (GNRs) is critically important for the future applications of graphene-based nanoelectronic devices. Based on real-time simulations, we investigate the manipulation of electronic transport by femtosecond laser pulses in armchair GNRs. The simulation results show that the multiphoton absorption process can drive the photon-excited electron flow along a GNR. The corresponding transferred charges are determined by the order of multiphoton absorption and depend on the central frequency and energy distribution of few-cycle laser pulses. Furthermore, the transferred charge can be controlled by the carrier-envelope phase of pulses; this dependence vanishes as the duration time of laser pulses increases. These results also provide insights into the manipulation of electron dynamics using lasers and the design of optoelectronic devices based on graphene materials.

We present a model compound that forms a mixed-spin ferromagnetic chain. Our material design, based on the organic radicals, affords a verdazyl-based complex $(p\text{−}\mathrm{Py}\text{−}{\mathrm{V})}_2\left[\mathrm{Mn}{\left(\mathrm{hfac}\right)}_2\right]$. The molecular orbital calculations of the compound indicate the formation of a mixed-spin-(1/2, 1/2, 5/2) ferromagnetic chain. The temperature dependence of magnetic susceptibility reveals its ferromagnetic behavior. The magnetic specific heat exhibits a double-peak structure and indicates a phase transition at the low-temperature peak. The observed characteristics are explained using the quantum Monte Carlo calculations. Furthermore, the modified spin-wave theory verifies that the double-peak structure of the specific heat significantly reflects the relative ration of the acoustic excitation band and the optical excitation gap.

Quasi-one-dimensional systems exhibit many-body effects elusive in higher dimensions. A prime example is spin-orbital separation, which has been measured by resonant inelastic x-ray scattering (RIXS) in ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$. Here, we theoretically analyze the time-resolved RIXS spectrum of ${\mathrm{Sr}}_2{\mathrm{CuO}}_3$ under the action of a time-dependent electric field. We show that the external field can reversibly modify the parameters in the effective $t\text{−}J$ model used to describe spinon and orbiton dynamics in the material. For strong driving amplitudes, we find that the spectrum changes qualitatively as a result of reversing the relative spinon to orbiton velocity. The analysis shows that in general, the spin-orbital dynamics in Mott insulators in combination with time-resolved RIXS should provide a suitable platform to explore the reversible control of many-body physics in the solid with strong laser fields.

Quantum spin liquids are fascinating phases of matter, hosting fractionalized spin excitations and unconventional long-range quantum entanglement. These exotic properties, however, also render their experimental characterization challenging, and finding ways to diagnose quantum spin liquids is therefore a pertinent challenge. Here, we numerically compute the spectral function of a single hole doped into the half-filled Hubbard model on the triangular lattice using techniques based on matrix product states. At half-filling the system has been proposed to realize a chiral spin liquid at intermediate interaction strength, surrounded by a magnetically ordered phase at strong interactions and a superconducting/metallic phase at weak interactions. We find that the spectra of these phases exhibit distinct signatures. By developing appropriate parton mean-field descriptions, we gain insight into the relevant low-energy features. While the magnetic phase is characterized by a dressed hole moving through the ordered spin background, we find indications of spinon dynamics in the chiral spin liquid. Our results suggest that the hole spectral function, as measured by angle-resolved photoemission spectroscopy, provides a useful tool to characterize quantum spin liquids.

The skyrmion-substrate interaction is an important issue to consider when studying particle-like skyrmions in nanostructures, which may strongly affect the skyrmion dynamics and lead to complex dynamic phenomena. Here, we report the current-driven dynamic behaviors of particle-like skyrmions interacting with the funnel obstacle in a ferromagnetic layer. We computationally demonstrate that a funnel obstacle with enhanced perpendicular magnetic anisotropy can be used to modify the dynamics of skyrmions due to the repulsive skyrmion-funnel interaction, which could generate several nontrivial effects, including the compression, clogging, annihilation, guided motion, and capturing of skyrmions. It is also found that the funnel obstacle can separate an incoming flow of skyrmions into two asymmetric flows. Moreover, we show that it is possible to capture skyrmions by the funnel obstacle in both static and dynamic ways. Our results are useful for understanding the skyrmion dynamics interacting with the substrate and could provide guidelines for the control of skyrmion flows in a transmission channel.

We analyze the possibility of odd-frequency pairing near a quantum critical point(QCP) in a metal. We consider a model with dynamical pairing interaction $V\left({\mathrm{\Omega }}_n\right)\sim 1/{\left|{\mathrm{\Omega }}_n\right|}^{\gamma }$ (the $\gamma$ model). This interaction gives rise to a non-Fermi liquid in the normal state and is attractive for pairing. The two trends compete with each other. We search for odd-frequency solutions for the pairing gap $\mathrm{\Delta }\left({\omega }_m\right)=-\mathrm{\Delta }\left({\omega }_m\right)$. We show that for $\gamma <1$, odd-frequency superconductivity loses the competition with a non-Fermi liquid and does not develop. We show that the pairing does develop in the extended model in which interaction in the pairing channel is larger than the one in the particle-hole channel. For $\gamma >1$, we argue that the original model is at the boundary towards odd-frequency pairing and analyze in detail how superconductivity is triggered by a small external perturbation. In addition, we show that for $\gamma >2$, the system gets frozen at the critical point towards pairing in a finite range in the parameter space. This gives rise to highly unconventional phase diagrams with flat regions.

Recently, compounds with noncentrosymmetric crystal structure have attracted much attention for providing a rich playground in search for unconventional superconductivity. ${\mathrm{NbIr}}_2{\mathrm{B}}_2$ is a new member to this class of materials harboring superconductivity below $T_{\mathrm{c}}=7.3\left(2\right)$ K and a very high upper critical field that exceeds Pauli limit. Here we report on muon spin rotation ($\mu \mathrm{SR}$) experiments probing the temperature and field dependence of effective magnetic penetration depth in this compound. Our transverse-field-$\mu \mathrm{SR}$ results suggest a fully gapped $s$-wave superconductivity. Furthermore, the estimated high value of the upper critical field is also supplemented by high-field transport measurements. Remarkably, the ratio $T_{\mathrm{c}}/{\lambda }^{-2}\left(0\right)$ obtained for ${\mathrm{NbIr}}_2{\mathrm{B}}_2$ ($\sim 2$) is comparable to those of unconventional superconductors. Zero-field $\mu \mathrm{SR}$ data reveal no significant change in the muon spin relaxation rate above and below $T_{\mathrm{c}}$, evincing that time-reversal symmetry is preserved in the superconducting state. The presented results will stimulate theoretical investigations to obtain a microscopic understanding of the origin of superconductivity with preserved time-reversal symmetry in this unique noncentrosymmetric system.

Quantum information theoretical measures are useful tools for characterizing quantum dynamical phases. However, employing them to study excited states of random spin systems is a challenging problem. Here, we report results for the entanglement entropy (EE) scaling of excited eigenstates of random XX antiferromagnetic spin chains with long-range (LR) interactions decaying as a power law with distance with exponent $\alpha$. To this end, we extend the real-space renormalization group technique for excited states (RSRG-X) to solve this problem with LR interactions. For comparison, we perform numerical exact diagonalization (ED) calculations. From the distribution of energy-level spacings, as obtained by ED for up to $N\sim 18$ spins, we find indications of a delocalization transition at ${\alpha }_c\approx 1$ in the middle of the energy spectrum. With RSRG-X and ED, we show that for $\alpha >{\alpha }^{*}$ the EE of excited eigenstates retains a logarithmic divergence similar to the one observed for the ground state of the same model, while for $\alpha <{\alpha }^{*}$ EE displays an algebraic growth with the subsystem size $l, S_l\sim l^{\beta }$, with $0<\beta <1$. We find that ${\alpha }^{*}\approx 1$ coincides with the delocalization transition ${\alpha }_c$ in the middle of the many-body spectrum. An interpretation of these results based on the structure of the RG rules is proposed, which is due to rainbow proliferation for very long-range interactions $\alpha \ll 1$. We also investigate the effective temperature dependence of the EE, allowing us to study the half-chain EE of eigenstates at different energy densities, where we find that the crossover in EE occurs at ${\alpha }^{*}<1$.