Collective electronic fluctuations in correlated materials give rise to various important phenomena, such as charge ordering, superconductivity, Mott insulating and magnetic phases, and plasmon and magnon modes. Unfortunately, the description of these correlation effects requires significant effort, since they almost entirely rely on strong local and nonlocal electron-electron interactions. Some collective phenomena, such as magnetism, can be sufficiently described by simple Heisenberg-like models that are formulated in terms of bosonic variables. This fact suggests that other many-body excitations can also be described by simple bosonic models in the spirit of Heisenberg theory. Here we derive an effective bosonic action for charge degrees of freedom for the extended Hubbard model and define a physical regime where the obtained action reduces to a classical Hamiltonian of an effective Ising model.

In this work, we investigate the effect of local dissipation on the presence of density-wave ordering in spinful fermions with both local and nearest-neighbor interactions as described by the extended Hubbard model. We find density-wave order to be robust against decoherence effects up to a critical point where the system becomes homogeneous with no spatial ordering. Our results will be relevant for future cold-atom experiments using fermions with nonlocal interactions arising from the dressing by highly excited Rydberg states, which have finite lifetimes due to spontaneous emission processes.

${\mathrm{LaTiO}}_3$ films on ${\mathrm{SrTiO}}_3$ single crystal substrates exhibit metallic behavior attributed to the ${\mathrm{LaTiO}}_3$ film, the interface as well as part of the ${\mathrm{SrTiO}}_3$. In the limit of ultrathin ${\mathrm{LaTiO}}_3$ films on ${\mathrm{SrTiO}}_3$, the contribution to the metallicity from strain-induced electronic structure modification of the ${\mathrm{LaTiO}}_3$ film is minimized so that the dominant contribution to metallicity is from the interface and part of the ${\mathrm{SrTiO}}_3$ due to charge transfer of $3d$ electrons from ${\mathrm{LaTiO}}_3$ to ${\mathrm{SrTiO}}_3$. In such a limit, we observe quantum oscillations whose angular dependence indicates a three-dimensional Fermi surface. Such angular dependence is observed in two sets of quantum oscillations—one low frequency and one high frequency—that we have attributed to an inner and outer Fermi surface associated with a Rashba-like spin split hybridized $d_{xz+yz}$ band.

In this paper, we demonstrate that the hybrid normal-superconducting-normal (NSN) structure has potential for a multifunctional thermal device which could serve for heat flux control and cooling of microstructures. By adopting the scattering matrix approach, we theoretically investigate thermal and electrical effects emerging in such structures due to the Cooper pair splitting (CPS) and elastic cotunneling phenomena. We show that a finite superconductor can, in principle, mediate heat flow between normal leads, and we further clarify special cases when this seems contradictory to the second law of thermodynamics. Among other things, we demonstrate that the CPS phenomenon can appear even in the simple case of a ballistic NSN structure.

In this paper we report on an x-ray absorption spectroscopy (XAS) and x-ray magnetic circular dichroism (XMCD) investigation of the Mn/GaAs(001) interface. The deposition of Mn in the submonolayer regime on the GaAs(001) surface at $390{}^{\circ }\mathrm{C}$ induces a ($2\times 1$) reconstruction which evolves to a ($2\times 2$) by increasing the Mn amount. The combined information from XAS and XMCD indicates that the Mn atoms are located on surface sites possessing an almost square symmetry with a limited hybridization of the Mn electronic levels with the GaAs ones. The Mn atoms present a magnetic moment ranging between 4.9 and $4.2{\mu }_B$ depending on the surface reconstruction. The Mn/GaAs(001) interface shows a paramagnetic behavior with an average magnetic moment which decreases by increasing the Mn amount. This variation of the surface magnetization is interpreted as an antiferromagnetic coupling of the Mn magnetic moments as the surface density of Mn atoms is increased. A further increase of the Mn amount after the ($2\times 2$) transition leads to the formation of Mn aggregates on the sample surface.

Although there are many techniques that can detect bandgap states associated with point defects in the lattice, it is not routinely possible to determine the type of defect at submicron spatial resolution. Here we show that high-resolution electron energy loss spectroscopy (EELS) in a scanning transmission electron microscope can locate and identify point defects with a resolution of about 10 nm in a wide-bandgap BAlN semiconductor. B interstitials, N vacancies, as well as other point defects have been experimentally detected using EELS and have been identified using density functional theory.

We experimentally study negative fluctuations of stochastic entropy production in an electronic double dot operating in nonequilibrium steady-state conditions. We record millions of random electron tunneling events at different bias points, thus collecting extensive statistics. We show that for all bias voltages, the experimental average values of the minima of stochastic entropy production lie above $-k_{\mathrm{B}}$, where $k_{\mathrm{B}}$ is the Boltzmann constant, in agreement with recent theoretical predictions for nonequilibrium steady states. Furthermore, we also demonstrate that the experimental cumulative distribution of the entropy production minima is bounded, at all times and for all bias voltages, by a universal expression predicted by the theory. We also extend our theory by deriving a general bound for the average value of the maximum heat absorbed by a mesoscopic system from the environment and compare this result with experimental data. Finally, we show by numerical simulations that these results are not necessarily valid under nonstationary conditions.

We present a scheme to obtain antichiral edge states in an exciton-polariton honeycomb lattice with strip geometry, where the modes corresponding to both edges propagate in the same direction. Under resonant pumping the effect of a polariton condensate with nonzero velocity in one linear polarization is predicted to tilt the dispersion of polaritons in the other, which results in an energy shift between two Dirac cones, and the otherwise flat edge states become tilted. Our simulations show that due to the spatial separation from the bulk modes the edge modes are robust against disorder.

We study the adiabatic dynamics of the charge, spin, and energy of a quantum dot with a Coulomb interaction under two-parameter driving, associated to time-dependent gate voltage and magnetic field. The quantum dot is coupled to a single reservoir at temperature $T=0$ and the dynamical Onsager matrix is fully symmetric, hence, the net energy dynamics is fully dissipative. However, in the presence of many-body interactions, other interesting mechanisms take place, like the net exchange of work between the two types of forces and the nonequilibrium accumulation of charge with different spin orientations. The latter has a geometric nature. The dissipation takes place in the form of an instantaneous Joule law with the universal resistance $R_0=h/2e^2$. We show the relation between this Joule law and instantaneous fluctuation-dissipation relations. The latter lead to generalized Korringa-Shiba relations, valid in the Fermi-liquid regime.

In this paper, we revisit collapse and revival oscillations in superfluids suddenly quenched by strong local interactions for the case of a one-dimensional Bose-Hubbard model. As the main result, we identify the inherent nonequilibrium quantum many-body character of these oscillations by revealing that they are controlled by a sequence of underlying dynamical quantum phase transitions in the real-time evolution after the quench, which manifest as temporal nonanalyticities in return probabilities or Loschmidt echos. Specifically, we find that the timescale of the collapse and revival oscillations is, first, set by the frequency at which dynamical quantum phase transitions appear, and is, second, of emergent nonequilibrium nature, since it is not only determined by the final Hamiltonian but also depends on the initial condition.

We introduce a McMillan-Ginzburg-Landau theory to describe the cooperative coexistence of charge density and superconducting order in two-dimensional crystals. With a free energy that explicitly accounts for the competition between commensurate and incommensurate ground states, we are able to map the transition between these phases and monitor the development of discommensurations in the near-commensurate regime. Attributing the enhancement of superconducting order to density wave fluctuations, we propose a coupling scheme that yields a phase diagram in qualitative agreement with experiments in conducting transition-metal dichalcogenides. The model predicts the development of nonuniform superconductivity similar to that arising from a pair density wave, with a spatial texture driven by the underlying charge density wave fluctuations.

Although topological invariants have been introduced to classify the appearance of protected electronic states at surfaces of insulators, there are no corresponding indices for Weyl semimetals whose nodal points may appear randomly in the bulk Brillouin zone (BZ). Here we use a well-known result that every Weyl point acts as a Dirac monopole and generates integer Berry flux to search for the monopoles on rectangular BZ grids that are commonly employed in self-consistent electronic structure calculations. The method resembles data mining technology of computer science and was demonstrated on locating the Weyl points in known Weyl semimetals. It was subsequently used in high-throughput screening of several hundreds of compounds and predicting a dozen materials hosting nodal Weyl points and/or lines.

Vacancy-induced magnetization of a graphene layer is investigated by means of a first-principles DFT method. Calculations of the formation energy and the magnetization by creating the different number of vacancies in a supercell show that a clustering with a big number of vacancies in the cluster is rather favorable to that of isolated vacancies, homogeneously distributed in the layer. The magnetic moment of a cluster with a big number of vacancies is shown to not be proportional with the vacancy concentration, which is in good agreement with the recent experimental results. Our studies support the idea that, although the vacancies in graphene create a magnetic moment, they do not produce a magnetic ordering. It is shown that, although the Lieb's rule for the magnetization in a hexagonal structure violates, two vacancies, including a di-vacancy, in the supercell generates a quasilocalized state when they belong to the different sublattices and, instead, two vacancies generate an extended state when they belong to the same sublattices. Analytical investigation of the dynamics of carbon atom and vacancy concentrations according to the nonlinear continuity equations shows that the vacancies, produced by irradiation at the middle of a graphene layer, migrate to the edge of the sample, resulting in a specific “segregation” of the vacancy concentration and self-healing of the graphene.

The band structure of high carrier density metal CrP features an interesting crossing at the $Y$ point of the Brillouin zone. The crossing, which is protected by the nonsymmorphic symmetry of the space group, results in a hybrid, semi-Dirac-like energy-momentum dispersion relation near $Y$. The linear energy-momentum dispersion relation along $Y-\mathrm{\Gamma }$ is reminiscent of the observed band structure in several semimetallic extremely large magnetoresistance (XMR) materials. We have measured the transverse magnetoresistance of CrP up to 14 T at temperatures as low as $\sim 16$ mK. Our data reveal a nonsaturating, quadratic magnetoresistance as well as the behavior of the so-called “turn-on” temperature in the temperature dependence of resistivity. Despite the difference in the magnitude of the magnetoresistance and the fact that CrP is not a semimetal, these features are qualitatively similar to the observations reported for XMR materials. Thus, the high-field electrical transport studies of CrP offer the prospect of identifying the possible origin of the nonsaturating, quadratic magnetoresistance observed in a wide range of metals.

Hyperhoneycomb iridate $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ is a three-dimensional analog of two-dimensional honeycomb iridates, such as $\alpha -{\mathrm{Li}}_2{\mathrm{IrO}}_3$, which recently appeared as another playground for the physics of Kitaev-type spin liquid. $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ shows a noncollinear spiral ordering of spin-orbital-entangled $J_{\mathrm{eff}}=1/2$ moments at low temperatures below 38 K, which is known to be suppressed under a pressure of $\sim 2$ GPa. In addition, a structural transition is observed at $P_{\mathrm{S}}\sim 4$ GPa at room temperature. Using the neutron powder diffraction technique, the crystal structure in the high-pressure phase of $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$ above $P_{\mathrm{S}}$ was refined, which indicates the formation of ${\mathrm{Ir}}_2$ dimers on the zigzag chains, with an Ir-Ir distance of $\sim 2.66\AA$, even shorter than that of metallic Ir. We argue that the strong dimerization stabilizes the bonding molecular-orbital state comprising the two local $d_{zx}$ orbitals in the Ir-${\mathrm{O}}_2$-Ir bond plane, which conflicts with the equal superposition of $d_{xy}, d_{yz}$, and $d_{zx}$ orbitals in the $J_{\mathrm{eff}}=1/2$ wave function produced by strong spin-orbit coupling. The results of resonant inelastic x-ray scattering measurements and the electronic structure calculations are fully consistent with the collapse of the $J_{\mathrm{eff}}=1/2$ state. The competition between the spin-orbital-entangled $J_{\mathrm{eff}}=1/2$ state and molecular-orbital formation is most likely universal in honeycomb-based Kitaev materials.

Many-body perturbation theory within the $GW$ approach has been established as a quantitatively accurate approach for predicting the quasiparticle and excited-state properties of a wide variety of materials. However, the successful application of the method is often complicated by the computational complexity associated with the evaluation and inversion of the frequency-dependent dielectric matrix $\varepsilon \left(\omega \right)$. Here, we describe an approach to speed up the evaluation of the frequency-dependent part of $\varepsilon \left(\omega \right)$ in the traditional sum-over-states $GW$ framework based on the low-rank approximation of the static dielectric matrix, a technique often used in $GW$ implementations that are based on a starting mean field within density-functional perturbation theory. We show that the overall accuracy of the approach, independently from other calculation parameters, is solely determined by the threshold on the eigenvalues of the static dielectric matrix, $\varepsilon (\omega =0)$, and that it can yield orders-of-magnitude speed-ups in full-frequency $GW$ calculations. We validate our implementation with several benchmark calculations ranging from bulk materials to systems with reduced dimensionality, and show that this technique allows one not only to study larger systems, but also to carefully consider the convergence of computationally demanding systems, such as ZnO, without relying on plasmon-pole models.

The band structure of some translationally invariant lattice Hamiltonians contains strictly dispersionless flat bands (FB). These are induced by destructive interference and typically host compact localized eigenstates (CLS) which occupy a finite number $U$ of unit cells. FBs are important due to macroscopic degeneracy and consequently due to their high sensitivity and strong response to different types of weak perturbations. We use a recently introduced classification of FB networks based on CLS properties, and extend the FB Hamiltonian generator introduced in Phys. Rev. B 95, 115135 (2017) to an arbitrary number $\nu$ of bands in the band structure, and arbitrary size $U$ of a CLS. The FB Hamiltonian is a solution to equations that we identify with an inverse eigenvalue problem. These can be solved only numerically in general. By imposing additional constraints, e.g., a chiral symmetry, we are able to find analytical solutions to the inverse eigenvalue problem.

We investigate properties of a spin-1 Heisenberg model with extended and biquadratic interactions, which captures crucial aspects of the low energy physics in FeSe. While we show that the model exhibits a rich phase diagram with four different magnetic ordering tendencies, we identify a parameter regime with strong competition between Néel, staggered dimer, and stripelike magnetic fluctuations, accounting for the physical properties of FeSe. We evaluate the spin and Raman responses using exact diagonalization. Through comparison with experiments we find enhanced magnetic frustration between Néel and collinear stripe ordering tendencies, which increases with increasing temperature. The explanation of these spectral behaviors with this frustrated spin model supports the idea of local spin interactions in FeSe.

Among two-dimensional lattices, both kagome and Lieb lattices have been extensively studied, showing unique physics related to their exotic flat and Dirac bands. Interestingly, we realize that the two lattices are in fact interconvertible by applying strains along the diagonal direction, as they share the same structural configuration in the unit cell, i.e., one corner-site and two edge-center states. We study phase transitions between the two lattices using the tight-binding approach and propose one experimental realization of the transitions using photonic devices. The evolution of the band structure demonstrates a continuous evolution of the flat band from the middle of the Lieb band to the top/bottom of the kagome band. Though the flat band is destroyed during the transition, the topological features are conserved due to the retained inversion symmetry, as confirmed by Berry curvature, Wannier charge center, and edge state calculations. Meanwhile, the triply degenerate Dirac point ($M$) in the Lieb lattice transforms into two doubly degenerate Dirac points, one of which moves along $M-\mathrm{\Gamma }$ and the other moves along $M-K/K^{\prime }$ directions that form the kagome band eventually. Interestingly, the Dirac cones in the transition states are strongly tilted, showing a coexistence of type-I and type-II Dirac points. We finally show that these transitions can be experimentally realized in photonic lattices using waveguide arrays.