A systematic ab initio study of the influence of local magnetism on the generalized stacking fault energy (GSFE) surface in pure fcc iron at 0 K has been performed. In the calculations we considered ferro- and antiferro- (single- and double-layer) magnetic order of local moments as well as their complete disorder, corresponding to paramagnetic (PM) state. We have shown that local magnetism is one of the most important factors stabilizing austenitic structure in iron (with respect to more stable at 0 K hcp) and that the perturbation of magnetic structure by the formation of stacking fault is a short-range effect. Local magnetism also strongly influences the GSFE surface topology and, therefore, the material's plasticity by reducing the energetic barriers that need to be overcome to form the intrinsic stacking fault (ISF) or return from the ISF structure to fcc. The influence of atomic relaxations on such barriers is moderate and does not exceed 15%. In addition, a methodology to evaluate the PM ISF energy using a superposition of the ISF energies obtained for ordered magnetic structures is proposed to overcome computational impediments arising when dealing with disorder in the PM state. The complications of the proposed methodology together with the ways to overcome them are also discussed.

We present a model of the lattice dynamics of the rare-earth titanate pyrochlores $R_2{\mathrm{Ti}}_2{\mathrm{O}}_7$ $(R=\mathrm{Tb},\text{Dy},\text{Ho})$, which are important materials in the study of frustrated magnetism. The phonon modes are obtained by density functional calculations, and these predictions are verified by comparison with scattering experiments. Single crystal inelastic neutron scattering is used to measure acoustic phonons along high symmetry directions for $R=\mathrm{Tb}$, Ho; single crystal inelastic x-ray scattering is used to measure numerous optical modes throughout the Brillouin zone for $R=\mathrm{Ho}$; and powder inelastic neutron scattering is used to estimate the phonon density of states for $R=\mathrm{Tb}$, Dy, Ho. Good agreement between the calculations and all measurements is obtained, allowing confident assignment of the energies and symmetries of the phonons in these materials under ambient conditions. Knowledge of the phonon spectrum is important for understanding spin-lattice interactions, and can be expected to be transferred readily to other members of the series to guide the search for unconventional magnetic excitations.

We combine the ideas of intensity interferometry, polarization optics, and Bell's measurement into an experimental proposal which is hosted in a $\nu =2$ quantum Hall (QH) edge state. Our interferometer comprises of a single gate, that separates the spin-resolved edge states of $\nu =2$ state. An analog of wave plate (from polarization optics) is realized by exposing the individual edges to nanomagnets over a finite length which facilitates coherent manipulation of the electron spin on the edge. We show that the in-plane rotation of magnetization direction of the nanomagnets results in oscillations observed solely in the cross-correlated noise which arises from two-particle interference. Through numerical simulations we demonstrate that our proposal is well within the reach of recent experimental developments of spin manipulations on the QH edge.

We define the entropy operator as the negative of the logarithm of the density matrix, give a prescription for extracting its thermodynamically measurable part, and discuss its dynamics. For an isolated system we derive the first, second, and third laws of thermodynamics. For weakly coupled subsystems of an isolated system, an expression for the long-time limit of the expectation value of the rate of change of the thermodynamically measurable part of the entropy operator is derived and interpreted in terms of entropy production and entropy transport terms. The interpretation is justified by comparison to the known expression for the entropy production in an aged classical Markovian system with Gaussian fluctuations and by a calculation of the current-induced entropy production in a conductor with electron-phonon scattering.

We introduce a two-orbital Hamiltonian on a square lattice that contains on-site attractive interactions involving the two $e_g$ orbitals. Via a canonical mean-field procedure similar to the one applied to the well-known negative-$U$ Hubbard model, it is shown that the model develops $d$-wave ($B_{1g}$) superconductivity with nodes along the diagonal directions of the square Brillouin zone. This result is also supported by exact diagonalization of the model in a small cluster. The expectation is that this relatively simple attractive model could be used to address the properties of multiorbital $d$-wave superconductors in the same manner that the negative-$U$ Hubbard model is widely applied to the study of the properties of $s$-wave single-orbital superconductors. In particular, we show that by splitting the $e_g$ orbitals and working at three-quarters filling, such that the $x^2-y^2$ orbital dominates at the Fermi level but the $3z^2-r^2$ orbital contribution is nonzero, the $d$-wave pairing state found here phenomenologically reproduces several properties of the superconducting state of the high $T_c$ cuprates.

We analyze the impact of strong correlations on the surface states of a three-dimensional topological Kondo insulator. An important observation is that correlations are strongly increased at the surface, which leads to an energetical confinement of the surface $f$ electrons into a narrow window around the Fermi energy at low enough temperature. This correlation effect in combination with the nontrivial topology has two remarkable consequences: (i) coexistence of light and heavy surface states at low temperatures. While heavy surface states are formed directly in the surface layer, light surface states are formed in the next-nearest surface layer. Furthermore, (ii) with increasing the temperature, the heavy surface states become incoherent and only light surface states can be observed. The coexistence of light and heavy surface states is thereby a remarkable characteristic of the combination of strong correlations and nontrivial topology. We believe that these results are applicable to the candidate topological Kondo insulator ${\mathrm{SmB}}_6$.

We study the quantum fidelity (ground-state overlap) near quantum phase transitions of the Ising universality class in one-dimensional (1D) systems of finite-size $L$. Prominent examples occur in magnetic systems (e.g., spin-Peierls, the anisotropic $XY$ model) and in 1D topological insulators of any topologically nontrivial Altland-Zirnbauer-Kitaev universality class. The rescaled fidelity susceptibility is a function of the only dimensionless parameter $LM$, where $2M$ is the gap in the fermionic spectrum. We present analytic expressions for the fidelity susceptibility for periodic and open boundaries conditions with zero, one, or two edge states. The latter are shown to have a crucial impact and alter the susceptibility both quantitatively and qualitatively. We support our analytical solutions with numerical data.

We study spin-orbit torques and charge pumping in magnetic quasi-one-dimensional zigzag nanoribbons with a hexagonal lattice, in the presence of large intrinsic spin-orbit coupling. Such a system experiences a topological phase transition from a trivial band insulator to a quantum spin Hall insulator by tuning of either the magnetization direction or the intrinsic spin-orbit coupling. We find that the spin-charge conversion efficiency (i.e., spin-orbit torque and charge pumping) is dramatically enhanced at the topological transition, displaying a substantial angular anisotropy.

Atomistic details of the interaction of impurities with quasi-one-dimensional charge density wave (CDW) are revealed by scanning tunneling microscopy. Oxygen and pentacene adsorbates are utilized as strongly and weakly interacting impurities, respectively, on the well-known CDW state of the In atomic wire array on the Si(111) surface. Distinct CDW pinning configurations are identified for oxygen impurities with different atomic structures, indicating the strong pinning. The governing role of local strain field for the strong pinning is elucidated. In contrast, a few different pinning configurations occur for a unique adsorption structure of pentacene indicating a weak pinning. Pentacene molecules commonly induce characteristic phase shifts, which readily couple with other phase defects, in particular, solitons in order to avoid interwire phase misfits. This work provides the mechanism and methodology for the atomic scale control over phases, solitons, and domain boundaries of CDW.

We calculate the nonsymmetrized current noise in a quantum dot connected to two reservoirs by using the nonequilibrium Green function technique. We show that both the current autocorrelator (inside a single reservoir) and the current cross-correlator (between the two reservoirs) are expressed in terms of transmission amplitude and coefficient through the barriers. We identify the different energy-transfer processes involved in each contribution to the autocorrelator, and we highlight the fact that when there are several physical processes, the contribution results from a coherent superposition of scattering paths. Varying the gate and bias voltages, we discuss the profile of the differential Fano factor in light of recent experiments, and we identify the conditions for having a distinct value for the autocorrelator in the left and right reservoirs.

We study a single-mode cavity weakly coupled to a voltage-biased quantum point contact. In a perturbative analysis, the lowest order predicts a thermal state for the cavity photons, driven by the emission noise of the conductor. The cavity is thus emptied as all transmission probabilities of the quantum point contact approach one or zero. Two-photon processes are identified at higher coupling, and pair absorption dominates over pair emission for all bias voltages. As a result, the number of cavity photons, the cavity damping rate, and the second-order coherence $g^{\left(2\right)}$ are all reduced and exhibit less bunching than the thermal state. These results are obtained with a Keldysh path-integral formulation and reproduced with rate equations. They can be seen as a backaction of the cavity measuring the electronic noise. Extending the standard $P\left(E\right)$ theory to a steady-state situation, we compute the modified noise properties of the conductor and find quantitative agreement with the perturbative calculation.

Two-dimensional (2D) topological insulators (TIs) are recently recognized states of quantum matter that are highly interesting for lower-power-consuming electronic devices owing to their nondissipative transport properties protected from backscattering. So far, only few 2D TIs, suffering from small bulk band gap ($<10\mathrm{meV}$), have been experimentally confirmed. Here, through first-principles calculations, we propose a family of 2D TIs in group-11 chalcogenide 2D crystals, $M_2\mathrm{Te}\left(M=\mathrm{Cu},\mathrm{Ag}\right)$. The nontrivial topological states in $\mathrm{C}{\mathrm{u}}_2\mathrm{Te}$ and $\mathrm{A}{\mathrm{g}}_2\mathrm{Te}$ 2D crystals, identified by topological invariant and edge state calculations, exhibit sizeable bulk gaps of 78 and 150 meV, respectively, suggesting that they are candidates for room-temperature applications. Moreover, strain engineering leads to effective control of the nontrivial gaps of $\mathrm{C}{\mathrm{u}}_2\mathrm{Te}$ and $\mathrm{A}{\mathrm{g}}_2\mathrm{Te}$, and a topological phase transition can be realized in $\mathrm{C}{\mathrm{u}}_2\mathrm{Te}$, while the nontrivial phase in $\mathrm{A}{\mathrm{g}}_2\mathrm{Te}$ is stable against strain. Their dynamic and thermal stabilities are further confirmed by employing phonon calculations and ab initio molecular dynamic simulations.

Diffusion Monte Carlo (DMC) simulations for fermions are becoming the standard for providing high-quality reference data in systems that are too large to be investigated via quantum chemical approaches. DMC with the fixed-node approximation relies on modifications of the Green's function to avoid singularities near the nodal surface of the trial wave function. Here we show that these modifications affect the DMC energies in a way that is not size consistent, resulting in large time-step errors. Building on the modifications of Umrigar et al. and DePasquale et al. we propose a simple Green's function modification that restores size consistency to large values of the time step, which substantially reduces time-step errors. This algorithm also yields remarkable speedups of up to two orders of magnitude in the calculation of molecule-molecule binding energies and crystal cohesive energies, thus extending the horizons of what is possible with DMC.

By means of first-principles calculations, we predict two new types of partially hydrogenated graphene systems: ${\mathrm{C}}_6{\mathrm{H}}_1$ and ${\mathrm{C}}_6{\mathrm{H}}_5$, which are shown to be a ferromagnetic (FM) semimetal and a FM narrow-gap semiconductor, respectively. When properly doped, the Fermi surface of the two systems consists of an electron pocket or six hole patches in the first Brillouin zone with completely spin-polarized charge carriers. If superconductivity exists in these systems, the stable pairing symmetries are shown to be $p+ip$ for both electron- and hole-doped cases. The predicted systems may provide fascinating platforms for studying the novel properties of the coexistence of ferromagnetism and triplet-pairing superconductivity.

Calculations of the electronic and magnetic properties of the nonmagnetic metallic compound ${\mathrm{YFe}}_2{\mathrm{Si}}_2$ are reported. These predict at the density functional level a magnetic state involving ordering along the $c$ axis. This predicted ground state contrasts with experiment, which does not show magnetic order. The electronic structure is three dimensional, and is similar to that of the unconventional superconductor ${\mathrm{YFe}}_2{\mathrm{Ge}}_2$ as well as that of the high-pressure collapsed tetragonal phase of ${\mathrm{KFe}}_2{\mathrm{As}}_2$, which is also a superconductor. Based on the results in relation to experiment, we infer that properties of ${\mathrm{YFe}}_2{\mathrm{Si}}_2$ are strongly influenced by a nearby antiferromagnetic quantum critical point.

We provide a full derivation of Laughlin's Jastrow-type wave functions for quantized Hall states subject to periodic boundary conditions using an operator formalism. The construction includes the quasihole and the technically more challenging quasielectron excitation, which was left as an open problem in the classic paper by Haldane and Rezayi [Phys. Rev. B 31, 2529 (1985)].

The interplay between charge and spin degrees of freedom in strongly correlated fermionic systems, in particular of Dirac fermions, is a long-standing problem in condensed matter physics. We investigate the competing orders in the half-filled $\mathrm{SU}\left(2N\right)$ Hubbard model on a honeycomb lattice, which can be accurately realized in optical lattices with ultracold large-spin alkaline-earth fermions. Employing large-scale projector determinant quantum Monte Carlo simulations, we have explored quantum phase transitions from the gapless Dirac semimetals to the gapped Mott insulating phases in the SU(4) and SU(6) cases. Both of these Mott insulating states are found to be columnar valence bond solid (cVBS) and to be absent of the antiferromagnetic Néel ordering and the loop current ordering. Inside the cVBS phases, the dimer ordering is enhanced by increasing fermion components and behaves nonmonotonically as the interaction strength increases. Although the transitions generally should be of first order due to a cubic invariance possessed by the cVBS order, the coupling to gapless Dirac fermions can soften the transitions to second order through a nonanalytic term in the free energy. Our simulations provide important guidance for the experimental explorations of novel states of matter with ultracold alkaline-earth fermions.