The voltage generated in a noncentrosymmetric crystal due to the bulk photovoltaic effect (BPE) can greatly exceed the energy gap, however, the light energy conversion efficiency is extremely low. Here we show that the BPE is remarkably enhanced in the case of thin films. The measurements of the BPE in heteroepitaxial single domain ferroelectric ${\mathrm{BaTiO}}_3$ thin films reveal the enhancement of both photoinduced electric field and conversion efficiencies of the BPE by more than 4 orders of magnitude. Besides the fundamental aspect, our results indicate the potential for the use of the BPE in photovoltaic applications.

We investigate the possibility of many-body localization in translation-invariant Hamiltonian systems, which was recently brought up by several authors. A key feature of many-body localized disordered systems is recovered, namely the fact that resonant spots are rare and far-between. However, we point out that resonant spots are mobile, unlike in models with strong quenched disorder, and that these mobile spots constitute a possible mechanism for delocalization, albeit possibly only on very long timescales. In some models, this argument for delocalization can be made very explicit in first order of perturbation theory in the hopping. For models where this does not work, we present instead a nonperturbative argument that relies solely on ergodicity inside the resonant spots.

The very first dynamical implementation of the combined GW and dynamical mean-field scheme “$GW + \mathrm{DMFT}$” for a real material was achieved recently [Tomczak et al., Europhys. Lett. 100, 67001 (2012)], and applied to the ternary transition metal oxide ${\mathrm{SrVO}}_3$. Here, we review and extend that work, giving not only a detailed account of full $GW + \mathrm{DMFT}$ calculations, but also discussing and testing simplified approximate schemes. We give insights into the nature of exchange and correlation effects: dynamical renormalizations in the Fermi liquid regime of ${\mathrm{SrVO}}_3$ are essentially local, and nonlocal correlations mainly act to screen the Fock exchange term. The latter substantially widens the quasiparticle band structure, while the band narrowing induced by the former is accompanied by a spectral weight transfer to higher energies. Most interestingly, the exchange broadening is more pronounced in the unoccupied part of the spectrum than in the occupied one. In addition, shorter lifetimes for unoccupied states further contribute to making the corrections to the Kohn-Sham band structure asymmetric with respect to the chemical potential. As a result, the $GW + \mathrm{DMFT}$ electronic structure of ${\mathrm{SrVO}}_3$ resembles the conventional density functional based dynamical mean-field ($\mathrm{DFT} + \mathrm{DMFT}$) description for occupied states but is profoundly modified in the empty part. Our work leads to a reinterpretation of inverse photoemission spectroscopy (IPES) data. Indeed, we assign a prominent peak at about 2.7 eV dominantly to ${\mathrm{e}}_g$ states, rather than to an upper Hubbard band of ${\mathrm{t}}_{2g}$ character. Similar surprises can be expected for other transition metal oxides. This prediction urgently calls for more detailed investigations of conduction band states in correlated materials.

The unconventional (half-integer) quantum Hall effect for a single species of Dirac fermions is analyzed. We discuss possible experimental measurements of the half-integer Hall conductance $g_{xy}$ of topological insulator surface states and explain how to reconcile Laughlin's flux insertion argument with half-integer $g_{xy}$. Using a vortex state representation of Landau level wave functions, we calculate current density beyond linear response, which is in particular relevant to the topological image monopole effect. As a major result, the field theory describing the localization physics of the quantum Hall effect of a single species of Dirac fermions is derived. In this connection, the issue of (absent) parity anomaly is revisited. The renormalization group (RG) flow and the resulting phase diagram are extensively discussed. Starting values of the RG flow are given by the semiclassical conductivity tensor which is obtained from the Boltzmann transport theory of the anomalous Hall effect.

Under shock compression, most porous materials exhibit lower densities for a given pressure than that of a full-dense sample of the same material. However, some porous materials exhibit an anomalous, or enhanced, densification under shock compression. We demonstrate a molecular mechanism that drives this behavior. We also present evidence from atomistic simulation that silicon belongs to this anomalous class of materials. Atomistic simulations indicate that local shear strain in the neighborhood of collapsing pores nucleates a local solid-solid phase transformation even when bulk pressures are below the thermodynamic phase transformation pressure. This metastable, local, and partial, solid-solid phase transformation, which accounts for the enhanced densification in silicon, is driven by the local stress state near the void, not equilibrium thermodynamics. This mechanism may also explain the phenomenon in other covalently bonded materials.

Using Monte Carlo simulations, we study in detail the overlap distribution for individual samples for several spin-glass models including the infinite-range Sherrington-Kirkpatrick model, short-range Edwards-Anderson models in three and four space dimensions, and one-dimensional long-range models with diluted power-law interactions. We study three long-range models with different powers as follows: The first is approximately equivalent to a short-range model in three dimensions, the second to a short-range model in four dimensions, and the third to a short-range model in the mean-field regime. We study an observable proposed earlier by some of us which aims to distinguish the “replica symmetry breaking” picture of the spin-glass phase from the “droplet picture,” finding that larger system sizes would be needed to unambiguously determine which of these pictures describes the low-temperature state of spin glasses best, except for the Sherrington-Kirkpatrick model, which is unambiguously described by replica symmetry breaking. Finally, we also study the median integrated overlap probability distribution and a typical overlap distribution, finding that these observables are not particularly helpful in distinguishing the replica symmetry breaking and the droplet pictures.

Using Monte Carlo simulations and finite-size scaling, we study a three-state Potts antiferromagnet on a layered square lattice with two and four layers: $L_z=2$ and 4, respectively. As temperature decreases, the system develops quasi-long-range order via a Berezinskii-Kosterlitz-Thouless transition at finite temperature $T_{c1}$. For $L_z=4$, as temperature is further lowered, a long-range order breaking the $Z_6$ symmetry develops at a second transition at $T_{c2}<T_{c1}$. The transition at $T_{c2}$ is also Berezinskii-Kosterlitz-Thouless-like, but has a magnetic critical exponent $\eta =1/9$ instead of the conventional value $\eta =1/4$. The emergent $U\left(1\right)$ symmetry is clearly demonstrated in the quasi-long-range ordered region $T_{c2}\le T\le T_{c1}$.

We analyze how the magnetic disorder affects the properties of the two-band $s_{\pm }$ and $s_{++}$ models, which are the subject of discussions regarding iron-based superconductors and other multiband systems like ${\mathrm{MgB}}_2$. We show that there are several cases when the transition temperature $T_c$ is not fully suppressed by magnetic impurities in contrast to the Abrikosov-Gor'kov theory, but a saturation of $T_c$ takes place in the regime of strong disorder. These cases are: (1) the purely interband impurity scattering and (2) the unitary scattering limit. We show that in the former case the $s_{\pm }$ gap is preserved, while the $s_{++}$ state transforms into the $s_{\pm }$ state with increasing magnetic disorder. For case (2), the gap structure remains intact.

A Bose insulator composed of a low density of strongly localized Cooper pairs develops at the two-dimensional superconductor to insulator transition (SIT) in a number of thin film systems. Investigations of ultrathin amorphous PbBi films far from the SIT described here provide evidence that the Bose insulator gives way to a second insulating phase with decreasing film thickness. At a critical film thickness $d_c$ the magnetoresistance changes sign from positive, as expected for boson transport, to negative, as expected for fermion transport, signs of local Cooper-pair phase coherence effects on transport vanish, and the transport activation energy exhibits a kink. Below $d_c$ pairing fluctuation effects remain visible in the high-temperature transport while the activation energy continues to rise. These features show that Cooper pairing persists and suggest that the localized unpaired electron states involved in transport are interspersed among regions of strongly localized Cooper pairs in this strongly localized, low Cooper-pair density phase.

We investigate the spin Hall magnetoresistance (SMR) in $\mathrm{SrMn}{\mathrm{O}}_3\left(\mathrm{SMO}\right)/\mathrm{Pt}$ hybrids, where SMO is an antiferromagnetic (AFM) insulator. The AFM moments partially rotate with out-of-plane magnetic fields, producing room-temperature SMR. By manipulating the electron spins in Pt, we observe Larmor precession-induced oscillating SMR, reaffirming the spin current transport determined by the relative arrangement between the Pt electron spins and AFM moments. The use of the AFM with no net moments annihilates the magnetic proximity effect and thus confirms the SMR origination from AFM-controlled spin current transport, with significant spin mixing conductance of $\sim {10}^{17}{\mathrm{m}}^{-2}$. Our findings provide an interesting perspective to detecting AFM moments and represent a significant step towards AFM spintroincs.

We report a photoemission and x-ray absorption study on a ${\mathrm{Au}}_{1-x}{\mathrm{Pt}}_x{\mathrm{Te}}_2$ ($x=0$ and 0.35) triangular lattice in which superconductivity is induced by Pt substitution for Au. Au $4f$ and Te $3d$ core-level spectra of ${\mathrm{AuTe}}_2$ suggest a valence state of ${\mathrm{Au}}^{2+}$(${\mathrm{Te}}_2$)${}^{2-}$, which is consistent with its distorted crystal structure with Te-Te dimers and compressed ${\mathrm{AuTe}}_6$ octahedra. On the other hand, valence-band photoemission spectra and preedge peaks of the Te $3d$ absorption edge indicate that Au $5d$ bands are almost fully occupied and that Te $5p$ holes govern the transport properties and the lattice distortion. The two apparently conflicting pictures can be reconciled by strong Au $5d$/Au $6s$–Te $5p$ hybridization. The absence of a core-level energy shift with Pt substitution is inconsistent with the simple rigid band picture for hole doping. The Au $4f$ core-level spectrum gets slightly narrow with Pt substitution, indicating that the small Au $5d$ charge modulation in distorted ${\mathrm{AuTe}}_2$ is partially suppressed.

We show that to understand the orthogonality catastrophe in the half-filled lattice model of spinless fermions with repulsive nearest-neighbor interaction and a local impurity in its Luttinger liquid phase one has to take into account (i) the impurity scaling, (ii) unusual finite-size $L$ corrections of the form $ln\left(L\right)/L$, as well as (iii) the renormalization group flow of the umklapp scattering. The latter defines a length scale $L_u$, which becomes exceedingly large the closer the system is to its transition into the charge density wave phase. Beyond this transition umklapp scattering is relevant in the renormalization group sense. Field theory can only be employed for length scales larger than $L_u$. For small to intermediate two-particle interactions, for which the regime $L>L_u$ can be accessed, and taking into account the finite-size corrections resulting from (i) and (ii) we provide strong evidence that the impurity backscattering contribution to the orthogonality exponent is asymptotically given by $1/16$. While further increasing the two-particle interaction leads to a faster renormalization group flow of the impurity towards the cut chain fixed point, the increased bare amplitude of the umklapp scattering renders it virtually impossible to confirm the expected asymptotic value of $1/16$ given the accessible system sizes. We employ the density matrix renormalization group.

We investigate the sign problem for full configuration interaction quantum Monte Carlo (FCIQMC), a stochastic algorithm for finding the ground-state solution of the Schrödinger equation with substantially reduced computational cost compared with exact diagonalization. We find $k$-space Hubbard models for which the solution is yielded with storage that grows sublinearly in the size of the many-body Hilbert space, in spite of using a wave function that is simply a linear combination of states. The FCIQMC algorithm is able to find this sublinear scaling regime without bias and with only a choice of the Hamiltonian basis. By means of a demonstration we solve for the energy of a 70-site half-filled system (with a space of ${10}^{38}$ determinants) in 250 core hours, substantially quicker than the $\sim {10}^{36}$ core hours that would be required by exact diagonalization. This is the largest space that has been sampled in an unbiased fashion. The challenge for the recently developed FCIQMC method is made clear: Expand the sublinear scaling regime while retaining exact-on-average accuracy. We comment upon the relationship between this and the scaling law previously observed in the initiator adaptation (i-FCIQMC). We argue that our results change the landscape for the development of FCIQMC and related methods.

A joint theoretical and experimental study of the optoelectronic properties of ${\mathrm{Ta}}_3{\mathrm{N}}_5$ was conducted by means of ab initio calculations and ellipsometry measurements. Previous experimental work on ${\mathrm{Ta}}_3{\mathrm{N}}_5$ has not been conclusive regarding the direct or indirect nature of light absorption. Our work found excellent agreement between the optical spectrum computed using the Bethe-Salpeter equation and the measured one, with two prominent features occurring at 2.1 and 2.5 eV assigned to direct transitions between N and Ta states. The computed optical gap, obtained from the $G_0{W}_0$ direct photoemission gap, including spin-orbit coupling, electron-phonon renormalization of the conduction band, and exciton binding energy, was found to be in excellent agreement with measurements. Our results also showed that ${\mathrm{Ta}}_3{\mathrm{N}}_5$ is a highly anisotropic material with heavy holes in several directions, suggesting low hole mobilities, consistent with low measured photocurrents in the ${\mathrm{Ta}}_3{\mathrm{N}}_5$ literature.

We discuss the electronic waiting-time distribution of a quantum-dot spin valve, i.e., a single-level quantum dot coupled to two ferromagnetic electrodes with magnetizations that can point in arbitrary directions. We demonstrate that the rich transport physics of this setup, such as dynamical channel blockade and spin precession in an interaction-driven exchange field, shows up in the waiting-time distribution, and we analyze the conditions necessary to observe the various effects.

Adsorption of Fe atom and ${\mathrm{Fe}}_n$ ($n=2-7$) clusters on graphene was studied by first-principles calculation and chemical bonding analysis. Various adsorption configurations, i.e., Fe clusters placed at different lateral positions and with different orientations with respect to the graphene lattice, have been optimized to locate the ground-state geometry of the Fe clusters on graphene. The calculation shows that Fe on graphene prefers a three-dimensional growth mode with a critical size of two atoms. The calculation also shows that ${\mathrm{Fe}}_n$ clusters on graphene exhibit ferromagnetic order and large magnetic moment. However, due to charge redistribution caused by the interaction with graphene, some ${\mathrm{Fe}}_n$ clusters on graphene have smaller magnetic moments compared to the corresponding free-standing clusters. Adsorption of ${\mathrm{Fe}}_n$ clusters also induces magnetic moments in graphene, and the induced magnetic moment on each carbon atom in graphene is correlated with the distortion of the graphene lattice.

We present a theoretical procedure with numerical demonstration of a workable and efficient method to evaluate the surface Green's function of semi-infinite leads connected to a device. Such a problem always occurs in quantum transport calculations but also in the study of surfaces and heterojunctions. We show here that these semi-infinite leads can be properly described by real-energy Green's functions obtained analytically by a smart solution of the Riccati matrix equation. The performance of our method is demonstrated in the case of a multichain two-dimensional electron-gas system, composed of a central ribbon connected to two semi-infinite leads, pierced by two opposite magnetic fields.

We study the collective modes of a low-energy continuum model of the quasi-two-dimensional electron liquid in a layer of the organic compound $\alpha -{\mathrm{(BEDT-TTF)}}_2{\mathrm{I}}_3$ in a perpendicular magnetic field. As testified by zero magnetic field transport experiments and ab initio theory, this material hosts both massless and massive low-energy carriers, the former being described by two tilted and anisotropic Dirac cones. The polarizability of these cones is anisotropic, and two sets of magnetoplasmon modes occur between any two cyclotron resonances. We show that the tilt of the cones causes a unique intervalley damping effect: the upper hybrid mode of one cone is damped by the particle-hole continuum of the other cone in generic directions. We analyze how the presence of massive carriers affects the response of the system, and demonstrate how doping can tune $\alpha -{\mathrm{(BEDT-TTF)}}_2{\mathrm{I}}_3$ between regimes of isotropic and anisotropic screening.

We propose a scheme based on topological insulators to generate Kramers pairs of Majorana fermions or parafermions in the complete absence of magnetic fields. Our setup consists of two topological insulators whose edge states are brought close to an $s$-wave superconductor. The resulting proximity effect leads to an interplay between a nonlocal crossed Andreev pairing, which is dominant in the strong electron-electron interaction regime, and usual superconducting pairing, which is dominant at large separation between the two topological insulator edges. As a result, there are zero-energy bound states localized at interfaces between spatial regions dominated by the two different types of pairing. Due to the preserved time-reversal symmetry, the bound states come in Kramers pairs. If the topological insulators carry fractional edge states, the zero-energy bound states are parafermions, otherwise, they are Majorana fermions.

BiTeI is a polar semiconductor with gigantic Rashba spin-split bands in bulk. We have investigated the effect of pressure on the electronic structure of this material via magnetotransport. Periods of Shubunikov–de Haas (SdH) oscillations originating from the spin-split outer Fermi surface and inner Fermi surface show disparate responses to pressure, while the carrier number derived from the Hall effect is unchanged with pressure. The associated parameters which characterize the spin-split band structure are strongly dependent on pressure, reflecting the pressure-induced band deformation. We find the SdH oscillations and transport response are consistent with the theoretically proposed pressure-induced band deformation leading to a topological phase transition. Our analysis suggests the critical pressure for the quantum phase transition near $P_{\mathrm{c}}=3.5$ GPa.