We calculate dynamical spin structure factors for gapped chiral spin liquid states in the spin-1/2 Heisenberg antiferromagnet on the kagome lattice using Schwinger-boson mean-field theory. In contrast to static (equal-time) structure factors, the dynamical structure factor shows clear signatures of time-reversal symmetry breaking for chiral spin liquid states. In particular, momentum inversion $\mathit{k}\to -\mathit{k}$ symmetry as well as the sixfold rotation symmetry around the $\mathrm{\Gamma }$ point are lost. We highlight other interesting features, such as a relatively flat onset of the two-spinon continuum for the cuboc1 state. Our work is based on the projective symmetry group classification of time-reversal symmetry breaking Schwinger-boson mean-field states by Messio, Lhuillier, and Misguich.

The high-pressure superconducting properties and the local structure of the ${\mathrm{Ba}}_8{\mathrm{Si}}_{46}$ clathrate have been studied using electrical resistance and x-ray absorption spectroscopy measurements up to more than 20 GPa. At pressures above 10–13 GPa, corresponding to a well-known volume collapse phase transformation, a sudden increase in the critical temperature leads to a maximum value of the superconducting critical temperature $T_c$ of $\sim 8.5–9$ K at 20 GPa. In the low-pressure clathrate phase, the superconducting critical temperature decreases as pressure is applied with an electron-phonon coupling constant $\lambda =1.1$ derived from the temperature evolution of the electrical resistance at different pressures. A progressive disorder in the Ba-Si correlations in the ${\mathrm{Ba}@\mathrm{Si}}_{24}$ cages of the structure is observed for pressures beyond $\sim 5$ GPa. These observations exclude a structural homothecy at the volume collapse transition. The high-pressure collapsed phase of ${\mathrm{Ba}}_8{\mathrm{Si}}_{46}$ is then associated with local structural changes and shows enhanced superconducting properties.

Motivated by recent developments in the experimental study of superconducting graphene and transition metal dichalcogenides, we investigate superconductivity of the Kane-Mele (KM) model with short-range attractive interactions on the two-dimensional honeycomb lattice. We show that intravalley spin-triplet pairing arises from nearest-neighbor (NN) attractive interaction and the intrinsic spin-orbit coupling. We demonstrate this in two independent approaches: We study superconducting instability driven by condensation of Cooperons, which are in-gap bound states of two conduction electrons, within the $T$-matrix approximation and also study the superconducting ground state within the mean-field theory. We find that Cooperons with antiparallel spins condense at the $K$ and $K^{\prime }$ points. This leads to the emergence of an intravalley spin-triplet pairing state belonging to the irreducible representation ${\mathrm{A}}_1$ of the point group $C_{6v}$. The fact that this pairing state has opposite chirality for $K$ and $K^{\prime }$ identifies this state as a “helical” valley-triplet state, the valley analog to the ${}^3\text{He}$-B phase in two dimensions. Because of the finite center of mass momentum of Cooper pairs, the pair amplitude in NN bonds exhibits spatial modulation on the length scale of lattice constant, such that this pairing state may be viewed as a pair-density wave state. We find that the pair amplitude spontaneously breaks the translational symmetry and exhibits a $p$-Kekulé pattern. We also discuss the selection rule for pairing states focusing the characteristic band structure of the KM model and the Berry phase effects to the emergence of the intravalley pairing state.

We investigate the magnetization processes of a standard Ginzburg-Landau model for chiral $p$-wave superconducting states in an applied magnetic field. We find that the phase diagram is dominated by triangular lattices of doubly quantized vortices. Only in close vicinity to the upper critical field the lattice starts to dissociate into a structure of single-quanta vortices. The degeneracy between states with opposite chirality is broken in a nonzero field. If the magnetization starts with an energetically unfavorable chirality, the process of chirality inversion induced by the external magnetic field results in the formation of a sequence of metastable states with characteristic magnetic signatures that can be probed by standard experimental techniques.

We design a monolayer film of FeSe on ${\mathrm{SrTiO}}_3$(110) substrate [FeSe/STO(110)], with a ${\mathrm{C}}_2$ symmetry induced by epitaxial strain. Compared to FeSe on ${\mathrm{SrTiO}}_3$(001) substrate [FeSe/STO(001)], one of the in-plane lattice constants is reduced by 6%, to $\sim 3.67$ Å. FeSe/STO(110) exhibits a large nearly isotropic superconducting gap of 16 meV filled around 60 K, similar to those obtained on FeSe/STO(001) films. Our results strongly suggest that the lattice ${\mathrm{C}}_4$ symmetry is not essential for reaching high $T_c$'s in Fe-based superconductors and pose a new challenge to theory.

The family of hole-doped Pr-based perovskite cobaltites, ${\mathrm{Pr}}_{0.5}{\mathrm{Ca}}_{0.5}{\mathrm{CoO}}_3$ and ${\left({\mathrm{Pr}}_{1-y}{\mathrm{RE}}_y\right)}_{0.3}{\mathrm{Ca}}_{0.7}{\mathrm{CoO}}_3$ (where RE is rare earth), has recently been found to exhibit simultaneous metal-insulator, spin-state, and valence transitions. We have investigated magnetic-field-induced phase transitions of ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ by means of magnetization measurements at 4.2–100 K up to an ultrahigh magnetic field of 140 T with the chemical pressure varied by $y=0.0625$, 0.075, 0.1. The observed magnetic-field-induced transitions were found to occur simultaneously with the metal-insulator transitions up to 100 T. The obtained magnetic-field-temperature ($B-T$) phase diagram and magnetization curves are well analyzed by a spin-crossover model of a single ion with interion interactions. On the other hand, the chemical pressure dependence of the experimentally obtained magnetization change during the phase transition disagrees with the single-ion model when approaching low temperatures. The significant $y$ dependence of the magnetization change at low temperatures may arise from the itinerant magnetism of ${\mathrm{Co}}^{3+}$ in the paramagnetic metallic phase, where the chemical pressure enhances the exchange splitting by promoting the double-exchange interaction. The observed $B-T$ phase diagrams of ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ are quite contrary to that of ${\mathrm{LaCoO}}_3$, indicating that in ${\left({\mathrm{Pr}}_{1-y}{\mathrm{Y}}_y\right)}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{CoO}}_3$ the high-field phase possesses higher entropy than the low-field phase, whereas it is the other way around in ${\mathrm{LaCoO}}_3$.

We study the frequency-dependent structure of electronic self-energy in the pseudogap and superconducting states of the two-dimensional Hubbard model. We present the self-energy calculated with the cellular dynamical mean-field theory systematically in the space of temperature, electron density, and interaction strength. We show that the low-frequency part of the self-energy is well represented by a simple equation, which describes the transitions of an electron to and from a hidden-fermionic state. By fitting the numerical data with this simple equation, we determine the parameters characterizing the hidden fermion and discuss its identity. The simple expression of the self-energy offers a way to organize numerical data of these uncomprehended superconducting and pseudogap states, as well as a useful tool to analyze spectroscopic experimental results. The successful description by the simple two-component fermion model supports the idea of “dark” and “bright” fermions emerging from a bare electron as bistable excitations in doped Mott insulators.

We carry out microphotoluminescence measurements of an acceptor-bound exciton ($A^{0}X$) recombination in the applied magnetic field with a single impurity resolution. In order to describe the obtained spectra we develop a theoretical model taking into account a quantum well (QW) confinement, an electron-hole and hole-hole exchange interaction. By means of fitting the measured data with the model we are able to study the fine structure of individual acceptors inside the QW. The good agreement between our experiments and the model indicates that we observe single acceptors in a pure two-dimensional environment whose states are unstrained in the QW plain.

We studied the population and coherence dynamics of a biexciton in a planar CuCl microcavity. We developed a time-resolved two-photon polarization spectroscopy technique, by which we extracted the population dynamics of the biexciton even when the biexciton was spectrally overlapped with the lower cavity polariton. The observed lifetime of the cavity biexciton in the weak-coupling regime was much shorter than that of a bare CuCl thin film without a cavity, which directly demonstrates cavity enhancement of the radiative decay rate of the biexciton. The cavity enhancement originates from the hybridization of the unbound two-cavity-polariton state to the biexciton wave function by the biexciton-cavity coupling. By comparing the population and coherence decay curves of the biexciton, we found that the coherence of the cavity biexciton was limited by the radiative population decay.

Thermal effects on the total charge density are studied for a one-dimensional correlated quantum dot by means of the path integral Monte Carlo method. The competition between Friedel and Wigner oscillations at zero temperature is driven by the ratio between the interaction of electronic strength and the kinetic energy of electrons. At the onset of the formation of a Wigner molecule, we show that thermal enhancement of Wigner oscillations occurs in a range of temperatures, which can be observed in the electron density. We further show that low-temperature Friedel oscillations may change to Wigner oscillations upon an increase in the temperature.

The induced spin polarization of Ir atoms on a ferromagnetic Fe double layer on W(110) has been investigated with spin-polarized scanning tunneling microscopy. An unoccupied state is observed with a spin polarization exceeding $60\%$ that is inverted with respect to the Fe layer. This inversion is due to the tunneling gap acting as an orbital and spin filter. Distance dependent measurements show that the spin polarization remains approximately constant over the entire experimentally accessible range, from far in the tunneling regime to 1 Å from the point of contact formation. This is corroborated by density functional theory calculations which show that the inversion of spin polarization occurs within 0.5 Å of the adatom.

Semimetallic tungsten ditelluride displays an extremely large nonsaturating magnetoresistance, which is thought to arise from the perfect $n-p$ charge compensation with low carrier densities in $\mathrm{WT}{\mathrm{e}}_2$. We find a strong Rashba spin-orbit effect in density functional calculations due to the noncentrosymmetric structure. This lifts twofold spin degeneracy of the bands. A prominent umklapp interference pattern is observed by our scanning tunneling microscopic measurements at 4.2 K, which differs distinctly from the surface atomic structure demonstrated at 77 K. The energy dependence of umklapp interference shows a strong correspondence with densities of states integrated from ARPES measurement, manifesting a fact that the bands are spin-split on the opposite sides of Γ. Spectroscopic survey reveals the electron/hole asymmetry changes alternately with lateral locations along the $b$ axis, providing a microscopic picture for double-carrier transport of semimetallic $\mathrm{WT}{\mathrm{e}}_2$. The conclusion is further supported by our ARPES results and Shubnikov–de Haas (SdH) oscillations measurements.

Using density-matrix renormalization-group calculations for infinite cylinders, we elucidate the properties of the spin-liquid phase of the spin-$\frac{1}{2}{J}_1\text{−}{J}_2$ Heisenberg model on the triangular lattice. We find four distinct ground states characteristic of a nonchiral, $Z_2$ topologically ordered state with vison and spinon excitations. We shed light on the interplay of topological ordering and global symmetries in the model by detecting fractionalization of time-reversal and space-group dihedral symmetries in the anyonic sectors, which leads to the coexistence of symmetry protected and intrinsic topological order. The anyonic sectors, and information on the particle statistics, can be characterized by degeneracy patterns and symmetries of the entanglement spectrum. We demonstrate the ground states on finite-width cylinders are short-range correlated and gapped; however, some features in the entanglement spectrum suggest that the system develops gapless spinonlike edge excitations in the large-width limit.

The strong mechanical anisotropy of phosphorene combined with the atomic-scale thickness challenges the commonly employed elastic continuum idealizations. Using objective boundary conditions and a density functional based potential, we directly uncover the flexibility of individual $\alpha$, $\beta$, and $\gamma$ phosphorene allotrope layers along an arbitrary bending direction. A correlation analysis with the in-plane elasticity finds that although a monolayer thickness cannot be defined in the classical continuum sense, an unusual orthotropic plate with a directional-dependent thickness can unambiguously describe the out-of-plane deformation of $\alpha$ and $\gamma$ allotropes. Such decoupling of the in-plane and out-of-plane nanomechanics might be generic for two-dimensional materials beyond graphene.

We study the half-filled Hubbard model on a triangular lattice with spin-dependent Kitaev-like hopping. Using the variational cluster approach, we identify five phases: a metallic phase, a non-coplanar chiral magnetic order, a ${120}^{\circ }$ magnetic order, a nonmagnetic insulator (NMI), and an interacting Chern insulator (CI) with a nonzero Chern number. The transition from CI to NMI is characterized by the change of the charge gap from an indirect band gap to a direct Mott gap. Based on the slave-rotor mean-field theory, the NMI phase is further suggested to be a gapless Mott insulator with a spinon Fermi surface or a fractionalized CI with nontrivial spinon topology, depending on the strength of the Kitaev-like hopping. Our work highlights the rising field in which interesting phases emerge from the interplay between band topology and Mott physics.

We discover novel topological effects in the one-dimensional Kitaev chain modified by long-range Hamiltonian deformations in the hopping and pairing terms. This class of models display symmetry-protected topological order measured by the Berry/Zak phase of the lower-band eigenvector and the winding number of the Hamiltonians. For exponentially decaying hopping amplitudes, the topological sector can be significantly augmented as the penetration length increases, something experimentally achievable. For power-law decaying superconducting pairings, the massless Majorana modes at the edges get paired together into a massive nonlocal Dirac fermion localized at both edges of the chain: a new topological quasiparticle that we call topological massive Dirac fermion. This topological phase has fractional topological numbers as a consequence of the long-range couplings. Possible applications to current experimental setups and topological quantum computation are also discussed.

We study the fractionalization of space group symmetries in two-dimensional topologically ordered phases. Specifically, we focus on $Z_2$-fractionalized phases in two dimensions whose deconfined topological excitations transform trivially under translational symmetries but projectively under glide reflections, whose quantum numbers are hence fractionalized. We accomplish this by generalizing the dichotomy between even and odd gauge theories to incorporate additional symmetries inherent to nonsymmorphic crystals. We show that the resulting fractionalization of point group quantum numbers can be detected in numerical studies of ground state wave functions. We illustrate these ideas using a microscopic model of a system of bosons at integer unit cell filling on a lattice with space group $p4g$ that can be mapped to a half-magnetization plateau for an $S=1/2$ spin system on the Shastry-Sutherland lattice.

On the one hand, electromagnetic dual particles preserve the helicity of light upon interaction. On the other hand, chiral particles respond differently to light of opposite helicity. These two properties on their own constitute a source of fascination. Their combined action, however, is less explored. Here, we study on analytical grounds the force and torque as well as the optical cross sections of dual chiral particles in the dipolar approximation exerted by a particular wave of well-defined helicity: A circularly polarized plane wave. We put emphasis on particles that possess a maximally electromagnetic chiral and hence dual response. Besides the analytical insights, we also investigate the exerted optical force and torque on a real particle using the example of a metallic helix that is designed to approach the maximal electromagnetic chirality condition. Various applications in the context of optical sorting but also nanorobotics can be foreseen considering the particles studied in this contribution.

We consider a general class of approximations which guarantees the conservation of particle number in many-body perturbation theory. To do this we extend the concept of $\mathrm{\Phi }$ derivability for the self-energy $\mathrm{\Sigma }$ to a larger class of diagrammatic terms in which only some of the Green's function lines contain the fully dressed Green's function $G$. We call the corresponding approximations for $\mathrm{\Sigma }$ partially $\mathrm{\Phi }$ derivable. A special subclass of such approximations, which are gauge invariant, is obtained by dressing loops in the diagrammatic expansion of $\mathrm{\Phi }$ consistently with $G$. These approximations are number conserving but do not have to fulfill other conservation laws, such as the conservation of energy and momentum. From our formalism we can easily deduce whether commonly used approximations will fulfill the continuity equation, which implies particle number conservation. We further show how the concept of partial $\mathrm{\Phi }$ derivability plays an important role in the derivation of a generalized sum rule for the particle number, which reduces to the Luttinger-Ward theorem in the case of a homogeneous electron gas, and the Friedel sum rule in the case of the Anderson model. To do this we need to ensure that the Green's function has certain complex analytic properties, which can be guaranteed if the spectral function is positive-semidefinite. The latter property can be ensured for a subset of partially $\mathrm{\Phi }$-derivable approximations for the self-energy, namely those that can be constructed from squares of so-called half diagrams. For the case in which the analytic requirements are not fulfilled we highlight a number of subtle issues related to branch cuts, pole structure, and multivaluedness. We also show that various schemes of computing the particle number are consistent for particle number conserving approximations.

The strong coupling between electronic transport in a single-level quantum dot and a capacitively coupled nanomechanical oscillator may lead to a transition towards a mechanically bistable and blocked-current state. Its observation is at reach in carbon-nanotube state-of-art experiments. In a recent publication [Phys. Rev. Lett. 115, 206802 (2015)] we have shown that this transition is characterized by pronounced signatures on the oscillator mechanical properties: the susceptibility, the displacement fluctuation spectrum, and the ring-down time. These properties are extracted from transport measurements, however the relation between the mechanical quantities and the electronic signal is not always straightforward. Moreover the dependence of the same quantities on temperature, bias or gate voltage, and external dissipation has not been studied. The purpose of this paper is to fill this gap and provide a detailed description of the transition. Specifically we find (i) the relation between the current-noise and the displacement spectrum; (ii) the peculiar behavior of the gate-voltage dependence of these spectra at the transition; (iii) the robustness of the transition towards the effect of external fluctuations and dissipation.