We devise a unitary transformation that replaces the fermionic degrees of freedom of lattice gauge theories by (hard-core) bosonic ones. The resulting theory is local and gauge invariant, with the same symmetry group. The method works in any spatial dimension and can be directly applied, among others, to the gauge groups $G=U\left(N\right)$ and $SU\left(2N\right)$, where $N\in \mathbb{N}$. For $SU(2N+1)$, one can also carry out the transformation after introducing an extra idle ${\mathbb{Z}}_2$ gauge field, so that the resulting symmetry group trivially contains ${\mathbb{Z}}_2$ as a normal subgroup. Those results have implications in the field of quantum simulations of high-energy physics models.

In this work we employ the $GW$ approximation in the framework of the SternheimerGW method to investigate the effects of many-body corrections to the band structures and Fermi surfaces of bulk and monolayer ${\mathrm{NbS}}_2$. For the bulk system, we find that the inclusion of these many-body effects leads to important changes in the band structure, especially in the low-energy regime around the Fermi level, and that our calculations are in good agreement with recent ARPES measurements. In the case of a free-standing monolayer ${\mathrm{NbS}}_2$, we observe a strong increase of the screened Coulomb interaction and the quasiparticle corrections as compared to bulk. In this case we also perform calculations to include the effect of screening by a substrate. We report in detail the results of our convergence tests and computational parameters, to serve as a solid basis for future studies.

Terahertz (THz) and sub-THz coherent acoustic phonons have been successfully used as probes of various quantum systems. Since their wavelength is in the nanometer range, they can probe nanostructures buried below a surface with nanometer resolution and enable control of electrical and optical properties on a picosecond time scale. However, coherent acoustic phonons have not yet been widely used to study van der Waals (vdW) two-dimensional (2D) materials and heterostructures. This class of 2D systems features strong covalent bonding of atoms in the layer planes and weak van der Waals attraction between the layers. The dynamical properties of the interface between the layers or between a layer and its supporting substrate are often omitted as they are difficult to probe. On the other hand, these play a crucial role in interpreting experiments and/or designing new device structures. Here, we use picosecond ultrasonic techniques to investigate phonon transport in vdW InSe nanolayers and InSe/hBN heterostructures. Coherent acoustic phonons are generated and detected in these 2D systems and allow us to probe elastic parameters of different layers and their interfaces. In particular, our study of the elastic properties of the interface between vdW layers reveals a strong coupling over a wide range of frequencies up to 0.1 THz, offering prospects for high-frequency electronics and technologies that require control over the charge and phonon transport across an interface. In contrast, we reveal a weak coupling between the InSe nanolayers and sapphire substrates, relevant in thermoelectrics and sensing applications, which can require quasi-suspended layers.

We performed low-temperature de Haas–van Alphen effect measurements on a ${\mathrm{Ce}}_{1-x}{\mathrm{Nd}}_x{\mathrm{CoIn}}_5$ series, for $x=0.02$, 0.05, 0.1, and 1, down to $T=40$ mK using torque magnetometry in magnetic fields up to 35 T. Our results indicate that a Fermi surface (FS) reconstruction occurs from a quasi-two-dimensional topology for $\mathrm{Nd}\text{−}2\%$ to a rather three-dimensional one for $\mathrm{Nd}\text{−}5\%$, thus reducing the possibility of perfect FS nesting. The FS evolves further with increasing Nd content with no observed divergence of the effective mass between $\mathrm{Nd}\text{−}2\%$ and $10\%$, consistent with the crossing of a spin density wave type of quantum critical point. Our results elucidate the origin of the $Q$ phase observed at the $5\%$ Nd-doping level [Raymond et al., J. Phys. Soc. Jpn. 83, 013707 (2014)].

To further investigate the interplay between ferromagnetism and topological insulators, thin films of the low-carrier topological insulator ${\left({\mathrm{Bi}}_x{\mathrm{Sb}}_{1-x}\right)}_2{\mathrm{Te}}_3$ were deposited on the insulating ferromagnet EuS (100) in situ. AC susceptibility indicates magnetic anomalies between $T\approx 30\mathrm{K}$ and $T\approx 60\mathrm{K}$, well above the Curie temperature $T_C\approx 15\mathrm{K}$ of EuS. When the Fermi level is close to the Dirac point and the surface state dominates the electric conduction, sharp increases in resistance with decreasing temperatures were observed concurrently with the magnetic anomalies. Positive-negative magnetoresistance crossovers were observed at the Curie temperature, which seem only to appear when the sheet resistance exceeds the Mott-Ioffe-Regel limit $h/e^2$. A two-stage gap-opening process due to magnetic proximity is proposed.

In this paper, we study examples of systematic biases that can occur in quantum Monte Carlo methods due to the accumulation of nonlinear expectation values, and approaches by which these errors can be corrected. We begin with a study of the Krylov-projected full configuration interaction quantum Monte Carlo (KP-FCIQMC) approach, which was recently introduced to allow efficient, stochastic calculation of dynamical properties. This requires the solution of a sampled effective Hamiltonian, resulting in a nonlinear operation on these stochastic variables. We investigate the probability distribution of this eigenvalue problem to study both stochastic errors and systematic biases in the approach, and demonstrate that such errors can be significantly corrected by moving to a more appropriate basis. This is lastly expanded to include consideration of the correlation function quantum Monte Carlo (QMC) approach of Ceperley and Bernu, showing how such an approach can be taken in the full configuration interaction QMC (FCIQMC) framework.

Using the multiband $d\text{−}p$ model and unrestricted Hartree-Fock approximation we investigate the electronic structure and spin-orbital order in a three-dimensional ${\mathrm{VO}}_3$ lattice. The main aim of this paper is testing if a simple $d\text{−}p$ model with partly filled $3d$ orbitals (in vanadium ions) and $2p$ orbitals (in oxygen ions) is capable of reproducing correctly nontrivial coexisting spin-orbital order observed in the vanadium perovskites. We point out that the multiband $d\text{−}p$ model has to include partly filled $e_g$ orbitals in vanadium ions. The results suggest weak self-doping as an important correction beyond the ionic model and reproduce the possible ground states with broken spin-orbital symmetry in vanadium ions: either $C$-type alternating orbital order accompanied by $G$-type antiferromagnetic spin order or $G$-type alternating orbital order accompanied by $C$-type antiferromagnetic spin order. Both states are experimentally observed and compete with each other in ${\mathrm{YVO}}_3$ whereas only the latter was observed in ${\mathrm{LaVO}}_3$. Orbital order is induced and stabilized by particular patterns of oxygen distortions arising from the Jahn-Teller effect. In contrast to time-consuming ab initio calculations, the computations using the $d\text{−}p$ model are very quick and should be regarded as very useful in solid-state physics, provided the parameters are selected carefully.

We study the impact of Hund's-rule coupling on orbital excitations, as, e.g., measured in inelastic resonant x-ray scattering. We find that the interpretation in terms of spin-orbit separation, which has been derived for one-dimensional systems without Hund's rule, remains robust in its presence. Depending on whether or not the orbital excitation includes a spin excitation, Hund's rule leads to an attractive or repulsive interaction between spinon and orbiton. Attraction (repulsion) leave clear signatures through a transfer of spectral weight to the lower (upper) edge of the spectrum.

We present an implementation of the rotationally invariant slave boson technique as an impurity solver for density functional theory plus dynamical mean field theory (DFT+DMFT). Our approach provides explicit relations between quantities in the local correlated subspace treated with DMFT and the Bloch basis used to solve the DFT equations. In particular, we present an expression for the mass enhancement of the quasiparticle states in reciprocal space. We apply the method to the study of the electronic correlations in ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$ under anisotropic strain. We find that the spin-orbit coupling plays a crucial role in the mass enhancement differentiation between the quasi-one-dimensional $\alpha$ and $\beta$ bands, and on its momentum dependence over the Fermi surface. The mass enhancement, however, is only weakly affected by either uniaxial or biaxial strain, even across the Lifshitz transition induced by the strain.

Electronic phase separation has been increasingly recognized as an important phenomenon in understanding many of the intriguing properties displayed in transition metal oxides. It is believed to produce fascinating functional properties in otherwise chemically homogenous electronic systems, e.g., colossal magnetoresistance manganites and high-$T_{\mathrm{c}}$ cuprates. While many well-known electronically phase-separated systems are oxides, it has been argued that the same phenomenon should occur in other electronic systems with strong competing interactions. Here we report the observation of electronic phase separation in molecular $({\mathrm{ND}}_4{)}_2{\mathrm{FeCl}}_5·{\mathrm{D}}_2\mathrm{O}$, a type-II multiferroic. We show that two magnetic phases, one of which is commensurate and the other of which is incommensurate, coexist in this material. Their evolution under applied magnetic field produces emergent properties. In particular, our measurements reveal a field-induced exotic state linked to a direct transition from a paraelectric/paramagnetic phase to a ferroelectric/antiferromagnetic phase, a collective phenomenon that hasn't been seen in other magnetic multiferroics.

We consider the structure of anisotropic exchange interactions in ytterbium-based insulating rare-earth magnets built from edge-sharing octahedra. We argue the features of trivalent ytterbium and this structural configuration allow for a semiquantitative determination of the different anisotropic exchange regimes that may manifest themselves in such compounds. The validity of such superexchange calculations is tested through comparison to the well-characterized breathing pyrochlore compound ${\mathrm{Ba}}_3{\mathrm{Yb}}_2{\mathrm{Zn}}_5{\mathrm{O}}_{11}$. With this in hand, we then consider applications to three-dimensional pyrochlore spinels as well as two-dimensional honeycomb and triangular lattice systems built from such edge-sharing octahedra. We find an extended regime of robust emergent weak anisotropy with dominant antiferromagnetic Heisenberg interactions as well as smaller regions with strong anisotropy. We discuss the implications of our results for known compounds with the above structures, such as the spinels $A{\mathrm{Yb}}_2{X}_4$ ($A=\mathrm{Cd}$, Mg, $X=\mathrm{S}$, Se) and the triangular compound ${\mathrm{YbMgGaO}}_4$, which have recently been put forth as promising candidates for exploring unconventional magnetic phenomena. Finally, we speculate on implications of our calculations for the $R_2{M}_2{\mathrm{O}}_7$ pyrochlore compounds and some honeycomb ytterbium magnets.

Recent experiments show strong evidences of nematic triplet superconductivity in doped ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and in ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ thin film on a superconducting substrate, but with varying identifications of the direction of the $d$ vector of the triplet that is essential to the topology of the underlying superconductivity. Here we show that the $d$ vector can be directly visualized by scanning tunneling measurements: At subgap energies the $d$ vector is along the leading peak wave vector in the quasiparticle-interference (QPI) pattern for potential impurities, and counterintuitively along the elongation of the local density-of-state profile of the vortex. Our results provide a useful guide to experiments. The $d$ vectors visualized from QPI and vortex elongation should agree, posing a stringent constraint on the pairing symmetry.

We study the connection between the phase behavior of quantum dimers and the dynamics of classical stochastic dimers. At the so-called Rokhsar-Kivelson (RK) point a quantum dimer Hamiltonian is equivalent to the Markov generator of the dynamics of classical dimers. A less well understood fact is that away from the RK point the quantum-classical connection persists: in this case the Hamiltonian corresponds to a nonstochastic “tilted” operator that encodes the statistics of time-integrated observables of the classical stochastic problem. This implies a direct relation between the phase behavior of quantum dimers and properties of ensembles of stochastic trajectories of classical dimers. We make these ideas concrete by studying fully packed dimers on the square lattice. Using transition path sampling—supplemented by trajectory umbrella sampling—we obtain the large deviation statistics of dynamical activity in the classical problem, and show the correspondence between the phase behavior of the classical and quantum systems. The transition at the RK point between quantum phases of distinct order corresponds, in the classical case, to a trajectory phase transition between active and inactive dynamical phases. Furthermore, from the structure of stochastic trajectories in the active dynamical phase we infer that the ground state of quantum dimers has columnar order to one side of the RK point. We discuss how these results relate to those from quantum Monte Carlo, and how our approach may generalize to other problems.

Using polarization analysis of linearly polarized synchrotron radiation we demonstrate the existence of a giant magneto-optical Faraday effect at the carbon $1s$ edge of single-layer graphene on Co, reaching Faraday rotation angles of $2.9\times {10}^5\mathrm{deg}/\mathrm{mm}$. This value is of the order of those observed at the Co $3p$ and $2p$ edges. Using element-selective magnetic hysteresis curves we find that graphene on Co exhibits ferromagnetic order. The magnetism in graphene is found to be carried by and be strongly enhanced by aligned π orbitals of carbon atoms. It is induced by hybridization with the Co $3d_{z}2$ orbitals while carbon σ bonds show negligible magnetism due to insignificant hybridization with Co. From additional x-ray magnetic circular dichroism and transversal magneto-optical Kerr effect spectra a magnetic moment of $0.14{\mu }_{\mathrm{B}}$ is estimated for graphene. From Faraday spectra the complete set of x-ray magneto-optical constants of graphene has been deduced which allows for future modeling of magneto-optical devices based on graphene. The strong magnetism in graphene results from hybridization of carbon $p_{\mathrm{z}}$ and metal $3d$ orbitals. Atoms of the graphene sublattice A, placed on top of Co, lead to strongest hybridization with Co $3{d_z}^2$ orbitals. Carbon atoms of sublattice B, and those of rotated graphene domains without Co atoms beneath, hybridize with each other and with $3d_{\mathrm{xz}}$ and $3d_{\mathrm{yz}}$ orbitals of neighboring Co atoms forming tilted $p_{\mathrm{z}}$ bonds. We show that the related reduction of A-B symmetry leads to a splitting of the spin-polarized density of conduction-band states which is responsible for the strong magneto-optical Faraday effect.

We develop a model for the angle-dependent magnetoresistance of ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ in the underdoped regime where the Fermi surface is thought to be reconstructed by an ordered state such as a charge-density wave. We show that such measurements can be employed to unambiguously distinguish the form of the Fermi surface's interlayer warping, placing severe constraints on the symmetry and nature of the reconstructing order. We describe experimentally accessible conditions in which our calculations can be put to the test.

We explore the ground-state properties of the two-band Hubbard model with degenerate electronic bands, parametrized by nearest-neighbor hopping $t$, intra- and interorbital on-site Coulomb repulsions $U$ and $U^{\prime }$, and Hund coupling $J$, focusing on the case with $J>0$. Using Jastrow-Slater wave functions, we consider both states with and without magnetic/orbital order. Electron pairing can also be included in the wave function, in order to detect the occurrence of superconductivity for generic electron densities $n$. When no magnetic/orbital order is considered, the Mott transition is continuous for $n=1$ (quarter filling); instead, at $n=2$ (half filling), it is first order for small values of $J/U$, while it turns out to be continuous when the ratio $J/U$ is increased. A significant triplet pairing is present in a broad region around $n=2$. By contrast, singlet superconductivity (with $d$-wave symmetry) is detected only for small values of the Hund coupling and very close to half filling. When including magnetic and orbital order, the Mott insulator acquires antiferromagnetic order for $n=2$; instead, for $n=1$ the insulator has ferromagnetic and antiferro-orbital orders. In the latter case, a metallic phase is present for small values of $U/t$ and the metal-insulator transition becomes first order. In the region with $1<n<2$, we observe that ferromagnetism (with no orbital order) is particularly robust for large values of the Coulomb repulsion and that triplet superconductivity is strongly suppressed by the presence of antiferromagnetism. The case with $J=0$, which has an enlarged SU(4) symmetry due to the interplay between spin and orbital degrees of freedom, is also analyzed.

The cubic pyrochlore $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ develops a metalliclike state below the antiferromagnetic transition at $T_{\mathrm{N}}\approx 90\mathrm{K}$, in contrast to the sharp transition to an insulating state below $T_{\mathrm{N}}$ of the $5d$-analog ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. We have synthesized polycrystalline $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ under high pressure, and investigated the responses of its electronic behavior to hydrostatic pressure and isovalent substitutions of ${\mathrm{Ca}}^{2+}$ for ${\mathrm{Cd}}^{2+}$. Three characteristic anomalies are identified in the electrical transport and magnetic properties of $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$, signaling an intimate correlation between charge and spin degrees of freedom. Interestingly, we found that the metalliclike state of $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ below $T_{\mathrm{N}}$ is very fragile and can be suppressed by a small hydrostatic pressure of $\le 1\mathrm{GPa}$ or substitution of 5–10% ${\mathrm{Ca}}^{2+}$ for ${\mathrm{Cd}}^{2+}$, resulting in a resistivity behavior similar to that of ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. In addition, the resultant insulating state below $T_{\mathrm{N}}$ is very robust against pressure, and the resistivity evolves gradually into two distinct activated regions under higher pressures. We constructed a temperature-pressure phase diagram for $\mathrm{C}{\mathrm{d}}_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7$ and discussed its peculiar metalliclike state in terms of the electronic itinerancy/localization dichotomy via side-by-side comparisons with the related compounds $A_2\mathrm{R}{\mathrm{u}}_2{\mathrm{O}}_7\left(A=\mathrm{Ca},\mathrm{Hg}\right)$ and ${\mathrm{Cd}}_2{\mathrm{Os}}_2{\mathrm{O}}_7$. Our results demonstrate that these cubic $4d/5d$ pyrochlore oxides offer an important paradigm for studying the exotic physics of correlated electrons on the border of (de)localization in the presence of strong geometrical frustration.

In recent years, there have been numerous observations of large vacuum level shifts, often attributed to the presence of mid-gap states, at many different metal/organic and organic/organic interfaces. Many of the same interfaces are found in bulk-heterojunction (BHJ) solar cells, where the large vacuum level shifts may alter the energy level alignment compared to common assumption. In this work, we have used a two-dimensional drift-diffusion simulation to calculate the vacuum level landscape for BHJ solar cells. We see that the large concentrations of mid-gap states required for experimentally observed vacuum level shifts to occur completely change the energetics of the device. When such mid-gap states are present, at thermal equilibrium, we find that the vacuum level landscape is dominated by abrupt changes in the vacuum level at interfaces and a constant value for the vacuum level within the donor and acceptor phases. Under illumination, for mid-gap-state depths larger than a certain threshold, the population of mid-gap states becomes dominated by the capture of free charge carriers. This behavior sets an upper limit for the electric field within the donor and acceptor phases which is much smaller than typical values for the electric field in the device under operational conditions. Our results show that, in general, simply adding experimental values for vacuum level shifts obtained for the individual interface will not produce the real vacuum level landscape in the device. The magnitude of the vacuum level shift at any one interface depends not only on the surface density and energy of the mid-gap states at that interface but also on the alignment at other interfaces throughout the device.

Within the context of dry friction, the ultralow friction regime has been reported at various atomically small contacts. Realization of large superlubric contacts under ordinary environmental conditions would greatly impact daily life and technology. Here, we focus on the multiatomic nature of a finite-size nanoparticle sliding on an atomically clean surface. The particle is subject to an effective field propagated by the surface and intermediated by the contact layer of the particle. The structural parameters, including the size, rigidity, and atomic configuration of the contact layer are taken into account to study the friction of the sliding nanoparticle. Several collective features show up once the particle contact layer is incommensurate with the surface potential, and the intralayer atomic coupling is strong. For a rigid layer, a considerable reduction of the friction is predicted at some particular sizes. In addition to these superlubric sizes that are determined by the lattice mismatch ratio only, enhancing the multiatomic feature by increasing the layer size and/or the intralayer coupling strength results in a friction reduction, which is essentially exponential for different values of the normal load (represented by the interaction amplitude). The latter kind of superlubricity is attributed to the increase of the number and synchronization of the intermediate slips of individual atoms in the contact layer that prevents the formation of high potential barriers. The edge atoms, on the other hand, are shown to be determinant in increasing the friction when they refuse to contribute to the collective slips.