The entanglement entropy (EE) can measure the entanglement between a spatial subregion and its complement, which provides key information about quantum states. Here, rather than focusing on specific regions, we study how the entanglement entropy changes with small deformations of the entangling surface. This leads to the notion of entanglement susceptibilities. These relate the variation of the EE to the geometric variation of the subregion. We determine the form of the leading entanglement susceptibilities for a large class of scale invariant states, such as the ground states of conformal field theories, and systems with Lifshitz scaling, which includes fixed points governed by disorder. We then use the susceptibilities to derive the universal contributions that arise due to nonsmooth features in the entangling surface: corners in two dimensions, as well as cones and trihedral vertices in three dimensions. We finally discuss the generalization to Rényi entropies.

We evaluate the nonequilibrium single-particle Green's functions in the steady state of the interacting resonant level model (IRLM) under the effect of an applied bias voltage. Employing the so-called auxiliary master equation approach, we present accurate nonperturbative results for the nonequilibrium spectral and effective distribution functions, as well as for the current-voltage characteristics. We find a drastic change of these spectral properties between the regimes of low- and high-bias voltages and discuss the relation of these changes to the negative differential conductance (NDC), a prominent feature in the nonequilibrium IRLM. The anomalous evolution of the effective distribution function next to the impurity shown by our calculations suggests a mechanism whereby the impurity gets effectively decoupled from the leads at voltages where the NDC sets in, in agreement with previous renormalization group approaches. This scenario is qualitatively confirmed by a Hartree-Fock treatment of the model.

Carbon nanotubes (CNTs) are a one-dimensional material system with intriguing physical properties that lead to emerging applications. While CNTs are unusually strain resistant compared to bulk materials, their optical-absorption spectrum is highly strain dependent. It is an open question, as to what extent this is attributed to strain-dependent (i) electronic single-particle transitions, (ii) dielectric screening, or (iii) atomic geometries including CNT radii. We use cutting-edge theoretical spectroscopy to explain strain-dependent electronic structure and optical properties of an (8,0) CNT. Quasiparticle effects are taken into account using Hedin's $GW$ approximation and excitonic effects are described by solving a Bethe-Salpeter-equation for the optical polarization function. This accurate first-principles approach allows us to identify an influence of strain on screening of the Coulomb electron-electron interaction and to quantify the impact on electronic structure and optical absorption of one-dimensional systems. We interpret our thoroughly converged results using an existing scaling relation and extend the use of this relation to strained CNTs. We show that it captures optical absorption with satisfactory accuracy, as long as screening, quasiparticle gap, and effective electron and hole masses of the strained CNT are known.

We present a study of longitudinal thermal transport in the Kitaev spin model on the honeycomb lattice, focusing on the role of anisotropic exchange to cover both gapless and gapped phases. Employing a complementary combination of exact diagonalization on small systems and an average gauge configuration approach for up to $\sim O\left({10}^4\right)$ spinful sites, we report results for the dynamical energy current autocorrelation function as well as the dc thermal conductivity over a wide range of temperatures and exchange anisotropies. Despite a pseudogap in the current correlation spectra, induced by emergent thermal gauge disorder on any finite system, we find that in the thermodynamic limit, gapless and gapful phases both feature normal dissipative transport, with a temperature dependence crossing over from power law to exponentially activated behavior upon gap opening.

We report that the local Ising anisotropy in pyrochlore oxides—the crucial requirement for realizing the spin-ice state—can be broken by means of high magnetic fields. For the case of the well-established classical spin-ice compound ${\mathrm{Ho}}_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ the magnetization exceeds the angle-dependent saturation value of the Ising limit using ultrahigh fields up to 120 T. However, even under such extreme magnetic fields full saturation cannot be achieved. Crystal-electric-field calculations reveal that a level crossing for two of the four ion positions leads to magnetization steps at 55 and 100 T. In addition, we show that by using a field sweep rate in the range of the spin-relaxation time the dynamics of the spin system can be probed. Exclusively at 25 ns/T, a new peak of the susceptibility appears around 2 T. We argue, this signals the crossover between spin-ice and polarized correlations.

We theoretically study the spin-orbit crossed susceptibility of topological Dirac semimetals. Because of strong spin-orbit coupling, the orbital motion of electrons is modulated by Zeeman coupling, which contributes to orbital magnetization. We find that the spin-orbit crossed susceptibility is proportional to the separation of the Dirac points and it is highly anisotropic. The orbital magnetization is induced only along the rotational symmetry axis. We also study the conventional spin susceptibility. The spin susceptibility exhibits anisotropy and the spin magnetization is induced only along the perpendicular to the rotational symmetry axis in contrast to the spin-orbit crossed susceptibility. We quantitatively compare the two susceptibilities and find that they can be comparable.

MXene-based heterostructures have received considerable interest owing to their unique properties. Herein, we examine various heterostructures of the prototypical MXene $\mathrm{T}{\mathrm{i}}_3{C}_2{T}_2$ ($T=\mathrm{O}$, OH, F; terminal groups) and graphene using density-functional theory. We find that the adhesion energy, charge transfer, and band structure of these heterostructures are sensitive not only to the surface functional group, but also to the stacking order. Due to its greatest difference in work function with graphene, $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$ has the strongest interaction with graphene, followed by ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2$ and then ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2$. Electron transfers from $\mathrm{T}{\mathrm{i}}_3{\mathrm{C}}_2{\left(\mathrm{OH}\right)}_2$ to graphene but from graphene to ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{O}}_2$ and ${\mathrm{Ti}}_3{\mathrm{C}}_2{\mathrm{F}}_2$, which causes a shift in the Dirac point of the graphene bands in the heterostructures of monolayer graphene and monolayer MXene. In the heterostructures of bilayer graphene and monolayer MXene, the interface breaks the symmetry of the bilayer graphene; in the case of the AB-stacking bilayer, the electron transfer leads to an interfacial electric field that opens up a gap in the graphene bands at the $K$ point. This internal polarization strengthens both the interfacial adhesion and the cohesion between the two graphene layers. The MXene-graphene-MXene and graphene-MXene-graphene sandwich structures behave as two mirror-symmetric MXene-graphene interfaces. Our first-principles studies provide a comprehensive understanding for the interaction between a typical MXene and graphene.

In this work we investigate the phenomena associated with the new thresholds in the spectrum of excitations arising when different one-dimensional strongly interacting systems are voltage biased and weakly coupled by tunneling. We develop the perturbation theory with respect to tunneling and derive an asymptotic behavior of physical quantities close to threshold energies. We reproduce earlier results for the electron relaxation at the edge of an integer quantum Hall system and for the nonequilibrium Fermi edge singularity phenomenon. In contrast to previous works, our analysis does not rely on the free-fermionic character of local tunneling; therefore, we are able to extend our theory to a wider class of systems, without well-defined electron excitations, such as spinless Luttinger liquids and chiral quantum Hall edge states at fractional filling factors.

We study all the possible different two-terminal configurations of Josephson junctions containing wires of time-reversal-invariant topological superconductors and ordinary superconductors, including combinations with an interacting quantum dot between both wires in the junction. We introduce simple effective Hamiltonians which explain the different qualitative behaviors obtained. We analyze a wide range of phenomena, including occurrence and quenching of the so-called $0-\pi$ transition, anomalous periodicity, and jumps of the Josephson current as a function of the phase difference, and finite Josephson current in the absence of magnetic flux.

Generation of high harmonics in novel two-dimensional (2D) nanostructures such as silicene, germanene, and stanene initiated by strong coherent electromagnetic radiation of arbitrary polarization, taking into account the spin-orbit coupling and the buckling of two Bravais lattices, is investigated. The buckled hexagonal lattice system is described by the four-band second-nearest-neighbor tight-binding model. The developed theory of the interaction of massive (nonzero effective mass) carriers with a strong driving wave field covers the full Brillouin zone of a 2D hexagonal nanostructure. The wave-matter interaction is taken in the length gauge that provides proper inclusion of inter- and intraband transitions with nonzero Berry curvatures. The closed set of differential equations for the single-particle density matrix of massive carriers is solved numerically. The obtained results show that novel 2D nanostructures can serve as an effective medium for the generation of even and odd high harmonics of arbitrary polarization. Moreover, for the nanostructures under consideration, the role of the band topology is significant at harmonic generation.

We experimentally show that the angular distribution of diffusely transmitted light from highly scattering dense media, such as photonic glasses, deviates significantly from Fresnel's law. We model a system of a photonic glass on a substrate by introducing an optical boundary layer that is sandwiched between the bulk effective medium and the substrate. Based on the model, we calculate the extrapolation length ratio, the photon transport/scattering mean-free paths, and the angular distribution of transmitted light intensity. The internal reflectance of the photonic glass is accurately determined by considering the diffuse scattering in the boundary layer. The modeling results agree well with the experimentally measured angular distribution and transport mean-free path. Our modeling also reaffirms that the real physical process of light scattering near the interface between photonic glass and the substrate cannot be simply described as internal reflection from a flat surface. The extrapolation length ratio of a photonic glass calculated from our boundary layer model is considerably larger than that from Fresnel's law. This indicates that the transport mean-free paths determined in other reported experiments for highly scattering dense media may have included significant errors and that the boundary layer analysis could be a good replacement to accurately describe the internal reflectance for photonic glasses.

Amorphous carbon (a-C) occurs as a tribologically induced phase in diamond or diamond-like carbon coatings. The interaction of ambient gases (${\mathrm{H}}_2$, ${\mathrm{N}}_2$, ${\mathrm{O}}_2$, ${\mathrm{H}}_2\mathrm{O}$, ${\mathrm{CO}}_2$) with a-C of varying mass density is studied by means of ab initio thermodynamics simulations. Different scenarios such as moist air or pure gases are investigated under different pressure and temperature conditions. Equilibrium concentrations of chemisorbed and fragmented final states are found to only exhibit a minor dependence on the specific conditions of the gas phase reservoir. The differences in local structure of the a-C samples with varying mass density such as pore size and coordination and dangling bonds, as well as the competition among different gas molecules for a-C atoms as reactants, affect the equilibrium concentrations to a greater degree. The availability and reactivity of a-C atoms are thus found to mainly control the chemical composition of a-C interacting with ambient gases in thermodynamic equilibrium. Trends found in the analysis of chemical groups occurring in equilibrium can possibly be transferred to a more realistic nonequilibrium tribological scenario.

Schwinger Keldysh field theory is a widely used paradigm to study nonequilibrium dynamics of quantum many-body systems starting from a thermal state. We extend this formalism to describe nonequilibrium dynamics of quantum systems starting from arbitrary initial many-body density matrices. We show how this can be done for both bosons and fermions, and for both closed and open quantum systems, using additional sources coupled to bilinears of the fields at the initial time, calculating Green's functions in a theory with these sources, and then taking appropriate set of derivatives of these Green's functions with respect to initial sources to obtain physical observables. The set of derivatives depends on the initial density matrix. The physical correlators in a dynamics with arbitrary initial conditions do not satisfy Wick's theorem, even for noninteracting systems. However, our formalism constructs intermediate “$n-\mathrm{particle}$ Green's functions” that obey Wick's theorem and provide a prescription to obtain physical correlation functions from them. This allows us to obtain analytic answers for all physical many-body correlation functions of a noninteracting system even when it is initialized to an arbitrary density matrix. We use these exact expressions to obtain an estimate of the violation of Wick's theorem and relate it to presence of connected multiparticle initial correlations in the system. We illustrate this new formalism by calculating density and current profiles in many-body fermionic and bosonic open quantum systems initialized to nontrivial density matrices. We have also shown how this formalism can be extended to interacting many-body systems.

The fully relativistic disordered local moment (DLM) theory is used to perform calculations of the magnetic torque of tetragonally distorted Fe and fully disordered $\left(A2\right){\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x\left(0\le x\le 0.2\right)$ alloys to describe the temperature dependence of their magnetoelasticity. The finite-temperature magnetoelasticity, in particular the magnetoelastic constant $B_1$, is obtained within DLM theory by studying the response of the magnetic torque generated by the magnetocrystalline anisotropy to the application of a tetragonal strain. Calculations of $B_1$ have been performed on bcc Fe across its ferromagnetic temperature range. Our results show good qualitative agreement with experiment, in particular reproducing the anomalous, nonmonotonic thermal behavior of bcc Fe's magnetostriction, which has been largely unexplained for more than 50 years. The method has also been used to calculate the finite-temperature magnetoelasticity of the $A2$ phase of ${\mathrm{Fe}}_{1-x}{\mathrm{Ga}}_x$ alloys as a starting point for further investigations into the giant magnetostriction of Galfenol alloys. Our calculations show that homogeneously doping bcc Fe with Ga does not produce an enhancement in magnetostriction and that the nonmonotonic temperature dependence and significant volume dependence are suppressed by increasing Ga content.

We study the low temperature static and dynamical properties of the classical bond-disordered antiferromagnetic Heisenberg model on the kagome lattice. This model has recently been shown to host a new type of spin liquid exhibiting an exponentially large number of discrete ground states. Surprisingly, despite the rigidity of the ground states, we establish the vanishing of the corresponding spin stiffness. Locally, the low-lying eigenvectors of the Hessian appear to exhibit a fractal inverse participation ratio. Its spin dynamics resembles that of Coulomb Heisenberg spin liquids but exhibits a new low-temperature dynamically arrested regime, which however gets squeezed out with increasing system size. We also probe the properties of the energy landscape underpinning this behavior and find energy barriers between distinct ground states vanishing with system size. In turn the local minima appear highly connected and the system tends to lose memory of its initial state in an accumulation of soft directions.

We have investigated the thermodynamic and local magnetic properties of the Mott insulating system ${\mathrm{Ag}}_3{\mathrm{LiRu}}_2{\mathrm{O}}_6$ containing ${\mathrm{Ru}}^{4+}$ ($4d^4$) for novel magnetism. The material crystallizes in a monoclinic $C2/m$ structure with ${\mathrm{RuO}}_6$ octahedra forming an edge-shared two-dimensional honeycomb lattice with limited stacking order along the $c$ direction. The large negative Curie-Weiss temperature (${\theta }_{CW}=-57$ K) suggests antiferromagnetic interactions among ${\mathrm{Ru}}^{4+}$ ions, though magnetic susceptibility and heat capacity show no indication of magnetic long-range order down to 1.8 and 0.4 K, respectively. ${}^7\mathrm{Li}$ nuclear magnetic resonance (NMR) shift follows the bulk susceptibility between 120 and 300 K and levels off below 120 K. Together with a power-law behavior in the temperature-dependent spin-lattice relaxation rate between 0.2 and 2 K, it suggest dynamic spin correlations with gapless excitations. Electronic structure calculations suggest an $S=1$ description of the Ru moments and the possible importance of further neighbor interactions as well as biquadratic and ring-exchange terms in determining the magnetic properties. Analysis of our muon spin rotation data indicates spin freezing below 5 K, but the spins remain on the borderline between static and dynamic magnetism even at 20 mK.

Using a realistic tight-binding Hamiltonian based on maximally localized Wannier functions, we investigate the two-ion magnetocrystalline anisotropy energy (MAE) in $L{1}_0$ transition metal compounds. MAE contributions from throughout the Brillouin zone are obtained using magnetic force theorem calculations with and without perturbation theory. The results from both methods agree with each other, and both reflect features of the Fermi surface. The intrasublattice and intersublattice contributions to MAE are evaluated using a Green's function method. We find that the sign of the intersublattice contribution varies among compounds, and that its amplitude may be significant, suggesting MAE can not be resolved accurately in a single-ion manner. The results are further validated by scaling spin-orbit-coupling strength in density functional theory. Overall, this realistic tight-binding method provides an effective approach to evaluate and analyze MAE while retaining the accuracy of corresponding first-principles methods.

The static magnetization and dynamic susceptibility responses of the cluster system within a Griffiths phase of the magnetocaloric compound ${\mathrm{Tb}}_{5-x}{\mathrm{La}}_x{\mathrm{Si}}_2{\mathrm{Ge}}_2$ ($x=0.075$) have been investigated. A novel cluster-glass state within the Griffiths phase is formed at a characteristic freezing temperature where short-range ferromagnetic correlations set in the paramagnetic regime. Ferromagneticlike correlations are built up at around 155 K, which suddenly become frozen at a lower temperature $\sim 140\mathrm{K}$, thus in analogy with a reentrant spin glass behavior. The ac susceptibility near the freezing temperature follows a critical slowing down process characterized by ${\tau }_0={10}^{-13}s$ and dynamic exponents $\mathrm{z}\nu \sim 6$ and $\beta \sim 0.4$, similar to well-known spin glass systems. The nonlinear ac susceptibility analysis shows clearly the existence of a transition associated to the reentrant behavior. The origin of the intermediate cluster-glass phase inside the Griffiths phase is proposed to be the result of a combination of short-ranged RKKY intralayer positive exchange interactions between rare-earth ${\mathrm{Tb}}^{3+}$ ions and antiferromagnetic exchange between adjacent interlayers involving Si and Ge atoms in connection to the ${\mathrm{Tb}}^{3+}$ atoms.

We observe that an rf microwave field strongly influences the transport of incoherent thermal magnons in yttrium iron garnet. Ferromagnetic resonance in the nonlinear regime suppresses thermal magnon transport by $95\%$. The transport is also modulated at nonresonant conditions in two cases, both related to the magnon band minimum. Firstly, a strong enhancement of the nonlocal signal appears at a static magnetic field below the resonance condition. This increase only occurs at one field polarity and can be as large as $800\%$. We attribute this effect to magnon kinetic processes, which give rise to band-minimum magnons and high-energy chiral surface modes. Secondly, the signal increases at a static field above the resonance condition, where the rf frequency coincides with the magnon band minimum. Our study gives insight into the interplay between coherent and incoherent spin dynamics: the rf field modifies the occupation of relevant magnon states and, via kinetic processes, the magnon spin transport.