A Luttinger liquid coupled to a quantum impurity describes a large number of physical systems. The Hamiltonian consists of left- and right-moving fermions interacting among themselves via a density-density coupling and scattering off a localized transmitting and reflecting impurity. We solve exactly the Hamiltonian by means of an incoming-outgoing scattering Bethe basis which properly incorporates all scattering processes. A related model, the weak-tunneling model, wherein the impurity is replaced by a tunnel junction, is solved by the same method. The consistency of the construction is established through a generalized Yang-Baxter relation. Periodic boundary conditions are imposed and the resulting Bethe ansatz equations are derived by means of the off-diagonal Bethe ansatz approach. We derive the spectrum of the model for all coupling constant regimes and calculate the impurity free energy. We discuss the low energy behavior of the systems for both repulsive and attractive interactions.

The martensitic transformation in ${\mathrm{Ni}}_2{\mathrm{Mn}}_{1+x}{\mathrm{Sn}}_{1-x}$ alloys has been investigated within ab initio density functional theory. The experimental trend of a martensitic transition happening beyond $x=0.36$ is captured within these calculations. The microscopic considerations leading to this are traced to increased Ni-Mn hybridization which results from the Ni atom experiencing a resultant force along a lattice vector and moving towards the Mn atoms above a critical concentration. The presence of the lone pair electrons on Sn forces the movement of Ni atoms away from Sn. While band Jahn Teller effects have been associated with this transition, we show quantitatively that at least in this class of compounds they have a minor role.

The strongly constrained and appropriately normed (SCAN) semilocal density functional [J. Sun, A. Ruzsinszky, and J. P. Perdew, Phys. Rev. Lett. 115, 036402 (2015)] obeys all 17 known exact constraints for meta-generalized-gradient approximations (meta-GGAs), and it includes some medium-range correlation effects. Long-range London dispersion interactions are still missing, but they can be accounted for via an appropriate correction scheme. In this study, we combine SCAN with an efficient London dispersion correction and show that lattice energies of simple organic crystals can be improved with the applied correction by 50%. The London-dispersion corrected SCAN meta-GGA outperforms all other tested London-dispersion corrected meta-GGAs for molecular geometries. Our method yields mean absolute deviations (MADs) for main group bond lengths that are consistently below 1 pm, rotational constants with MADs of 0.2%, and noncovalent distances with MADs below 1%. For a large database of general main group thermochemistry and kinetics ($\sim 800$ chemical species), one of the lowest weighted mean absolute deviations for long-range corrected meta-GGA functionals is achieved. Noncovalent interactions are of average quality, and hydrogen bonded systems in particular seem to suffer from overestimated polarization related to the self-interaction error of SCAN. We also discuss some consequences of numerical sensitivity encountered for meta-GGAs.

We consider the fate of a helical edge state of a spin Hall insulator and its topological transition in presence of a circularly polarized light when coupled to various forms of environments. A Lindblad-type equation is developed to determine the fermion occupation of the Floquet bands. We find by using analytical and numerical methods that nonsecular terms, corresponding to two-photon transitions, lead to a mixing of the band occupations, hence the light-induced photocurrent is in general not perfectly quantized in the presence of finite coupling to the environment, although deviations are small in the adiabatic limit. Sharp crossovers are identified at driving frequencies near the Rabi frequency $\mathrm{\Omega }$ (which is the strength of light-matter coupling) and at $\frac{1}{2}\mathrm{\Omega }$ with the former resembling a phase transition.

Iron chalcogenide superconductors are multiorbital materials with strong electron correlations. Here we use angle-resolved photoemission spectroscopy to study orbital dependent correlation effects in single-layer $\mathrm{FeSe}/\mathrm{Nb}:{\mathrm{BaTiO}}_3/{\mathrm{KTaO}}_3$, an iron chalcogenide superconductor with high interfacial superconductivity, nondegenerate electron pockets, and varied electron correlation compared with single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$. Signatures of polaronic behavior are observed over the whole temperature range, suggesting electron-boson interactions. Moreover, the nondegeneracy of the electron bands helps to resolve the temperature dependent evolution of different bands. The coherent spectral weight of one electron band significantly decreases above 115 K and is completely depleted at 200 K while that of the other one remains finite, giving direct evidence of an orbital selective Mott crossover. Correspondingly, the weight of the incoherent photoemission spectra is enhanced, indicating a coherent-incoherent crossover during the Mott crossover process. Compared with that in single-layer $\mathrm{FeSe}/{\mathrm{SrTiO}}_3$, the depletion temperature of the ${\mathrm{d}}_{xy}$ band is higher in single-layer $\mathrm{FeSe}/\mathrm{Nb}:{\mathrm{BaTiO}}_3/{\mathrm{KTaO}}_3$ due to the decreased correlation of the ${\mathrm{d}}_{xy}$ band in $\mathrm{FeSe}/\mathrm{Nb}:{\mathrm{BaTiO}}_3$. These phenomena help to construct a more complete picture of electron correlations in the FeSe family.

In the conventional theory of density wave ordering in metals, the onset of spin density wave (SDW) order coincides with the reconstruction of the Fermi surfaces into small “pockets.” We present models which display this transition, while also displaying an alternative route between these phases via an intermediate phase with topological order, no broken symmetry, and pocket Fermi surfaces. The models involve coupling emergent gauge fields to a fractionalized SDW order, but retain the canonical electron operator in the underlying Hamiltonian. We establish an intimate connection between the suppression of certain defects in the SDW order and the presence of Fermi surface sizes distinct from the Luttinger value in Fermi liquids. We discuss the relevance of such models to the physics of the hole-doped cuprates near optimal doping.

Our recently proposed theoretical formulation [presented in D. Novko et al., Phys. Rev. B 93, 125413 (2016)] is used to study optical absorption and transmission in molybdenum disulfide $\left({\mathrm{MoS}}_2\right)$ monolayer as a function of incident photon energy and angle. The investigation is not focused on exploration of well-documented spin-orbit split excitons around optical absorption onset, but rather on the most intensive features in absorption spectrum in the visible and near-ultraviolet photon energy range $\left(1.7–4\mathrm{eV}\right)$. It is shown that three most intensive peaks, at 2.7, 3.1, and 3.7 eV, result from transitions between $\text{Mo}\left(d\right)$ and $\text{S}\left(p\right)$ valence and conduction bands and that the character of their charge/current density fluctuations is intrinsically in plane, located in the molybdenum plane. This also implies that ${\mathrm{MoS}}_2$ monolayer is completely transparent when illuminated by grazing incidence $p$-polarized light. The validity of the presented results is supported by our effective two-band tight-binding model and finally by good agreement with some recent experimental results.

The self-consistent $\mathrm{GW}\mathrm{\Gamma }$ method satisfies the Ward-Takahashi identity (i.e., the gauge invariance or the local charge continuity) for arbitrary energy ($\omega$) and momentum ($\mathit{q}$) transfers. Its self-consistent first-principles treatment of the vertex $\mathrm{\Gamma }={\mathrm{\Gamma }}_v$ or ${\mathrm{\Gamma }}_W$ is possible to first order in the bare ($v$) or dynamically screened ($W$) Coulomb interaction. It is developed within a linearized scheme and combined with the Bethe-Salpeter equation (BSE) to accurately calculate photoabsorption spectra (PAS) and photoemission (or inverse photoemission) spectra (PES) simultaneously. The method greatly improves the PAS of Na, ${\mathrm{Na}}_3, {\mathrm{B}}_2$, and ${\mathrm{C}}_2{\mathrm{H}}_2$ calculated using the standard one-shot $G_0{W}_0+\mathrm{BSE}$ method that results in significantly redshifted PAS by 0.8–3.1 eV, although the PES are well reproduced already in $G_0{W}_0$.

In this paper, we investigate the edge Majorana modes in the simplest possible $p{}_x+i{p}_y$ superconductor defined on surfaces with different geometries, the annulus, the cylinder, the Möbius band, and a cone (by cone we mean a cone with the tip cut away so it is topologically equivalent to the annulus and cylinder), and with different configurations of magnetic fluxes threading holes in these surfaces. In particular, we shall address two questions: Given that, in the absence of any flux, the ground state on the annulus does not support Majorana modes while the one on the cylinder does, how is it possible that the conical geometry can interpolate smoothly between the two? Given that in finite geometries edge Majorana modes have to come in pairs, how can a $p{}_x+i{p}_y$ state be defined on a Möbius band, which has only one edge? We show that the key to answering these questions is that the ground state depends on the geometry, even though all the surfaces are locally flat. In the case of the truncated cone, there is a nontrivial holonomy, while the nonorientable Möbius band must necessarily support a domain wall.

Using the perturbative scaling equations and the numerical renormalization group, we study the characteristic energy scales in the Kondo impurity problem as a function of the exchange coupling constant $J$ and the conduction-band electron density. We discuss the relation between the energy gain (impurity binding energy) $\mathrm{\Delta }E$ and the Kondo temperature $T_K$. We find that the two are proportional only for large values of $J$, whereas in the weak-coupling limit the energy gain is quadratic in $J$, while the Kondo temperature is exponentially small. The exact relation between the two quantities depends on the detailed form of the density of states of the band. In the limit of low electron density the Kondo screening is affected by the strong particle-hole asymmetry due to the presence of the band-edge van Hove singularities. We consider the cases of one- (1D), two- (2D), and three-dimensional (3D) tight-binding lattices (linear chain, square lattice, cubic lattice) with inverse-square-root, step-function, and square-root onsets of the density of states that are characteristic of the respective dimensionalities. We always find two different regimes depending on whether $T_K$ is higher or lower than $\mu$, the chemical potential measured from the bottom of the band. For 2D and 3D, we find a sigmoidal crossover between the large-$J$ and small-$J$ asymptotics in $\mathrm{\Delta }E$ and a clear separation between $\mathrm{\Delta }E$ and $T_K$ for $T_K<\mu$. For 1D, there is, in addition, a sizable intermediate-$J$ regime where the Kondo temperature is quadratic in $J$ due to the diverging density of states at the band edge. Furthermore, we find that in 1D the particle-hole asymmetry leads to a large decrease of $T_K$ compared to the standard result obtained by approximating the density of states to be constant (flat-band approximation), while in 3D the opposite is the case; this is due to the nontrivial interplay of the exchange and potential scattering renormalization in the presence of particle-hole asymmetry. The 2D square-lattice density of states behaves to a very good approximation as a band with constant density of states.

Extensive studies have demonstrated the promising capability of the organic-inorganic hybrid halide perovskite ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ in solar cells with a high power conversion efficiency exceeding 20%. However, the intrinsic as well as extrinsic instabilities of this material remain the major challenge to the commercialization of perovskite-based solar cells. Mixing halides is expected to resolve this problem. Here, we investigate the effect of chemical substitution in the position of the halogen atom on the structural, electronic, and optical properties of mixed halide perovskites ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{\left({\mathrm{I}}_{1-x}{\mathrm{Br}}_x\right)}_3$ with a pseudocubic phase using the virtual crystal approximation method within density functional theory. With an increase of Br content $x$ from 0.0 to 1.0, the lattice constant decreases in proportion to $x$ with the function of $a\left(x\right)=6.420-0.333x$ (Å), while the band gap and the exciton binding energy increase with the quadratic function of $E_g\left(x\right)=1.542+0.374x+0.185x^2$ (eV) and the linear function of $E_b\left(x\right)=0.045+0.057x$ (eV), respectively. The photoabsorption coefficients are also calculated, showing a blueshift of the absorption onsets for higher Br contents. We calculate the phase decomposition energy of these materials and analyze the electronic charge density difference to estimate the material stability. Based on the calculated results, we suggest that the best match between efficiency and stability can be achieved at $x\approx 0.2$ in ${\mathrm{CH}}_3{\mathrm{NH}}_3\mathrm{Pb}{\left({\mathrm{I}}_{1-x}{\mathrm{Br}}_x\right)}_3$ perovskites.

We present a theory of quantum oscillations in insulators that are particle-hole symmetric and nontopological but with arbitrary band dispersion, at both zero and nonzero temperature. At temperatures $T$ less than or comparable to the gap, the dependence of oscillations on $T$ is markedly different from that in metals and depends crucially on the position of the chemical potential $\mu$ in the gap. If $\mu$ is in the middle of the gap, oscillations do not change with $T$; however, if $\mu$ is asymmetrically positioned in the gap, surprisingly, oscillations go to zero at a critical value of the inverse field determined by $T$ and $\mu$ and then change their phase by $\pi$ and grow again. Additionally, the temperature dependence is different for quantities derived from the grand canonical potential, such as magnetization and susceptibility, and those derived from the density of states, such as resistivity. However, the nontrivial features arising from asymmetric $\mu$ are present in both.

The magnetic and electronic properties of the magnetically doped topological insulator ${\mathrm{Bi}}_{2-x}{\mathrm{Mn}}_x{\mathrm{Te}}_3$ were studied using electron-spin resonance (ESR) and measurements of static magnetization and electrical transport. The investigated high-quality single crystals of ${\mathrm{Bi}}_{2-x}{\mathrm{Mn}}_x{\mathrm{Te}}_3$ show a ferromagnetic phase transition for $x\ge 0.04$ at $T_{\mathrm{C}}\approx 12$ K. The Hall measurements reveal a $p$-type finite charge-carrier density. Measurements of the temperature dependence of the ESR signal of Mn dopants for different orientations of the external magnetic field give evidence that the localized Mn moments interact with the mobile charge carriers leading to Ruderman-Kittel-Kasuya-Yosida-type ferromagnetic coupling between the Mn spins of order 2–3 meV. Furthermore, ESR reveals a low-dimensional character of magnetic correlations that persist far above the ferromagnetic ordering temperature.

We show by spatially and time-resolved photoluminescence that the application of an electric field transverse to the plane of an intrinsic GaAs (111) quantum well (QW) allows the transport of photogenerated electron spins polarized along the direction perpendicular to the QW plane over distances exceeding 10 $\mu \mathrm{m}$. We attribute the long spin transport lengths to the compensation of the in-plane effective magnetic field related to the intrinsic spin-orbit (SO) interaction by means of the electrically generated SO field. Away from SO compensation, the precession of the spin vector around the SO field decreases the out-of-plane polarization of the spin ensemble as the electrons move away from the laser generation spot. The results are reproduced by a model for two-dimensional drift diffusion of spin polarized charge carriers under weak SO interaction.

We study coherent optics in topological insulator surface states with broken time-reversal symmetry and develop a theory for the dynamical Hall effect driven by an intense electromagnetic field. The influence of the optical Stark effect enters as a nonlinear dependence on the optical field in the resulting Faraday ${\theta }_F$ and Kerr ${\theta }_K$ rotations. This nonlinear correction is found to decrease ${\theta }_F$ with the strength of the a.c. electric field, whereas ${\theta }_K$ exhibits a nonmonotonic behavior. We also discuss the effects of relaxation and dephasing on the Hall and magneto-optical responses when the frequency detuning is comparable to the inverse lifetime of the conduction electrons.

Metamaterials possess artificial bulk and surface electromagnetic states. Tamed dispersion properties of surface waves allow one to achieve a controllable super-Planckian radiative heat transfer (RHT) process between two closely spaced objects. We numerically demonstrate enhanced RHT between two two-dimensional grooved metal plates by a full-wave scattering approach. The enhancement originates from both transverse-magnetic spoof surface-plasmon polaritons and a series of transverse-electric bonding- and anti-bonding-waveguide modes at surfaces. The RHT spectrum is frequency selective and highly geometrically tailorable. Our simulation also reveals thermally excited nonresonant surface waves in constituent metallic materials may play a prevailing role for RHT at an extremely small separation between two metal plates, rendering metamaterial modes insignificant for the energy-transfer process.

We synthesized single crystals of composition ${\mathrm{Ba}}_2{\mathrm{CuSi}}_2{\mathrm{O}}_6{\mathrm{Cl}}_2$ and investigated their quantum magnetic properties. The crystal structure is closely related to that of the quasi-two-dimensional (2D) dimerized magnet ${\mathrm{BaCuSi}}_2{\mathrm{O}}_6$ also known as Han purple. ${\mathrm{Ba}}_2{\mathrm{CuSi}}_2{\mathrm{O}}_6{\mathrm{Cl}}_2$ has a singlet ground state with an excitation gap of $\mathrm{\Delta }/k_{\mathrm{B}}=20.8$ K. The magnetization curves for two different field directions almost perfectly coincide when normalized by the $g$ factor except for a small jump anomaly for a magnetic field perpendicular to the $c$ axis. The magnetization curve with a nonlinear slope above the critical field is in excellent agreement with exact-diagonalization calculations based on a 2D coupled spin-dimer model. Individual exchange constants are also evaluated using density functional theory (DFT). The DFT results demonstrate a 2D exchange network and weak frustration between interdimer exchange interactions, supported by weak spin-lattice coupling implied from our magnetostriction data. The magnetic-field-induced spin ordering in ${\mathrm{Ba}}_2{\mathrm{CuSi}}_2{\mathrm{O}}_6{\mathrm{Cl}}_2$ is described as the quasi-2D Bose-Einstein condensation of triplets.

Zeeman spectra, dichroic signals, and neutron Bragg diffraction patterns generated by copper ions in magnetically ordered copper metaborate $\left(\mathrm{Cu}{\mathrm{B}}_2{\mathrm{O}}_4\right)$ are investigated within a minimal model of Cu atomic states. A theory platform, common to understanding optical spectra and neutron diffraction patterns, affords the immediate benefit of a unified description of the experimental probes in terms of electronic multipoles. Results for dichroic signals illustrate a nontrivial use of a general, quantum mechanical theory of photon absorption couched in terms of Dirac multipoles that are magnetic and polar. Anapoles (Dirac dipoles) are predicted to generate Bragg spots in magnetic neutron diffraction that are not indexed by the motif of conventional (axial) magnetic-dipole moments. The minimal model of Cu states is informed by magnetic symmetry, derived from an established commensurate antiferromagnetic order, with a sparse number of parameters that comply with available empirical evidence.

Ferrites with (inverse) spinel structure display a large variety of electronic and magnetic properties, making some of them interesting for potential applications in spintronics. We investigate the thermally induced interdiffusion of ${\mathrm{Ni}}^{2+}$ ions out of NiO into ${\mathrm{Fe}}_3{\mathrm{O}}_4$ ultrathin films, resulting in off-stoichiometric nickel ferrite–like thin layers. We synthesized epitaxial ${\mathrm{Fe}}_3{\mathrm{O}}_4$/NiO bilayers on Nb-doped ${\mathrm{SrTiO}}_3$(001) substrates by means of reactive molecular beam epitaxy. Subsequently, we performed an annealing cycle comprising three steps at temperatures of 400 ${}^{\circ }\mathrm{C}$, 600 ${}^{\circ }\mathrm{C}$, and 800 ${}^{\circ }\mathrm{C}$ under an oxygen background atmosphere. We studied the changes of the chemical and electronic properties as result of each annealing step with help of hard x-ray photoelectron spectroscopy and found a rather homogeneous distribution of Ni and Fe cations throughout the entire film after the overall annealing cycle. For one sample we observed a cationic distribution close to that of the spinel ferrite ${\mathrm{NiFe}}_2{\mathrm{O}}_4$. Further evidence comes from low-energy electron diffraction patterns indicating a spinel-type structure at the surface after annealing. Site- and element-specific hysteresis loops performed by x-ray magnetic circular dichroism uncovered the antiferrimagnetic alignment between the octahedral coordinated ${\mathrm{Ni}}^{2+}$ and ${\mathrm{Fe}}^{3+}$ ions and the ${\mathrm{Fe}}^{3+}$ ion in tetrahedral coordination. We find a quite low coercive field of 0.02 T, indicating a rather low defect concentration within the thin ferrite films.

To investigate the impact of both the CoFe ferromagnetic layer thickness and the capping paramagnetic layer on the tunnel magnetoresistance (TMR), we performed first-principles simulations on epitaxial magnetic tunnel junctions contacted with either CoFe or Ta paramagnetic capping layers. We observed a strong oscillation of the TMR amplitude with respect to the thickness of the ferromagnetic layer. The TMR is found to be amplified whenever the MgO spin tunnel barrier is thickened. Quantization of the electronic structure of the ferromagnetic layers is found to be at the origin of this oscillatory behavior. Metals such as Ta contacting the magnetic layer are found to enhance the amplitude of the oscillations due to the occurrence of an interface dipole. The latter drives the band alignment and tunes the nature of the spin channels that are active during the tunneling process. Subsequently, the regular transmission spin channels are modulated in the magnetic tunnel junction stack and other complex ones are being activated.