Pezzini et al. reported an unconventional mass enhancement in topological nodal-line semimetal ZrSiS [Nat. Phys. 14, 178 (2018)], whose origin remains puzzling. This material might undergo excitonic instability due to strong short-range interactions. Here we study the renormalization of fermion velocities and find that the mass enhancement in ZrSiS can be well understood if we suppose that ZrSiS is close to the quantum critical point between a semimetal and excitonic insulator. Near this quantum critical point, the fermion velocities are considerably reduced by excitonic quantum fluctuation in proper conditions, leading to sizable mass enhancement. The quasiparticle residue is suppressed as the energy decreases but flows to a small finite value at zero energy. This indicates that ZrSiS is a strongly correlated Fermi liquid, and explains why the mass enhancement is weaker than non-Fermi liquids. Our results suggest that ZrSiS is a rare example of three-dimensional topological semimetal exhibiting unusual quantum criticality.

The ground-state degeneracy of the quantum spin system is a characteristic of nontrivial topology when it is gapped and robust against disordered perturbation. The corresponding quantum phase transition (QPT) is usually driven by a real parameter. We study a non-Hermitian Ising chain with two transverse fields, one real and another imaginary, based on the exact solution and numerical simulation. We show that topological degeneracy still exists and can be obtained by an imaginary transverse field from a topologically trivial phase of a Hermitian system. The topological degeneracy is robust against the random imaginary field and therefore expected to be immune to disordered dissipation from the spontaneous decay in experiment. The underlying mechanism is the nonlocal symmetry, which emerges only in thermodynamic limit and relates two categories of QPTs in the quantum spin system, rooted from topological order and symmetry breaking, respectively.

We investigate the low-energy collective charge excitations (plasmons, holons) in metallic atomic wires deposited on semiconducting substrates. These systems are described by two-dimensional correlated models representing strongly anisotropic lattices or weakly coupled chains. Well-established theoretical approaches and results are used to study their properties: Random phase approximation for anisotropic Fermi liquids and bosonization for coupled Tomonaga-Luttinger liquids as well as Bethe ansatz and density-matrix renormalization group methods for ladder models. We show that the Fermi and Tomonaga-Luttinger liquid theories predict the same qualitative behavior for the dispersion of excitations at long wave lengths. Moreover, their scaling depends on the choice of the effective electron-electron interaction but does not characterize the dimensionality of the metallic state. Our results also suggest that such anisotropic correlated systems can exhibit two-dimensional dispersions due to the coupling between wires but remain quasi-one-dimensional strongly anisotropic conductors or retain typical features of Tomonaga-Luttinger liquids such as the power-law behavior of the density of states at the Fermi energy. Thus it is possible that atomic wire materials such as Au/Ge(100) exhibit a mixture of features associated with one- and two-dimensional metals.

We investigate the problem of intertwined orders in fractional Chern insulators by considering lattice fractional quantum Hall (FQH) states arising from pairing of composite fermions in the square-lattice Hofstadter model. At certain filling fractions, magnetic translation symmetry ensures the composite fermions form Fermi surfaces with multiple pockets, leading to the formation of finite-momentum Cooper pairs in the presence of attractive interactions. We obtain mean-field phase diagrams exhibiting a rich array of striped and topological phases, establishing paired lattice FQH states as an ideal platform to investigate the intertwining of topological and conventional broken symmetry order.

An electron paramagnetic resonance (EPR) study of type-II silicon (Si) clathrate with low sodium (Na) guest concentration provides insights into the properties of this cagelike Si allotrope and into Na as an $n$-type dopant. An EPR line arising from unterminated Si bonds in a disordered Si phase is identified in the system. The presence of this amorphouslike component helps explain the thermally activated, hopping-related conductivity that has long been observed in the system but which has been poorly understood. This study also identifies “superhyperfine” lines surrounding each Na hyperfine line due to interactions of the donor electron with a ${}^{29}\mathrm{Si}$ nucleus on the cage and confirms the identification of hyperfine lines due to interactions of two neighboring Na nuclei. Analysis of the strength of these interactions shows that the electron wave function associated with the Na donor extends onto surrounding cages consistent with identification of Na as a shallow donor, and provides quantitative evidence of Na clustering.

Strain tuning emerged as an appealing tool for tuning of fundamental optical properties of solid-state quantum emitters. In particular, the wavelength and fine structure of quantum dot states can be tuned using hybrid semiconductor-piezoelectric devices. Here, we show how an applied external stress can directly impact the polarization properties of coupled InAs quantum dot-micropillar cavity systems. In our experiment, we find that we can reversibly tune the anisotropic polarization splitting of the fundamental microcavity mode by approximately 60 $\mu \mathrm{eV}$. We discuss the origin of this tuning mechanism, which arises from an interplay between elastic deformation and the photoelastic effect in our micropillar. Finally, we exploit this effect to tune the quantum dot polarization optomechanically via the polarization-anisotropic Purcell effect. Our work paves the way for optomechanical and reversible tuning of the polarization and spin properties of light-matter-coupled solid-state systems.

We present the theoretical model of the quantum ammeter, a device that is able to measure the full counting statistics of an electron current at quantum timescales. It consists of an Ohmic contact perfectly coupled to chiral quantum Hall channels and of a quantum dot attached to one of the outgoing channels. At energies small compared to its charging energy, the Ohmic contact fractionalizes each incoming electron and redistributes it between outgoing channels. By monitoring the resonant tunneling current through the quantum dot, one gets access to the moment generator of the current in one of the incoming channels at timescales comparable to its correlation time.

We examine the formation of Andreev bound states in a metal-quantum-dot-BCS superconductor system in the Coulomb blockade regime and the Kondo regime. We propose to use the so-called Hubbard-I decoupling scheme in the context of the equation-of-motion technique to provide a proper physical description for the Coulomb blockade regime. Here, we find the appearance of anticrossings in the subgap spectrum due to the prevalence of even parity states independently on the dot gate position. On the contrary, for the Kondo regime the subgap structure exhibits parity crossings between the alternance of singlet and doublet states. Besides, we show how the nature of the ground state in this regime can be distinguished from evolution of the Andreev states in the presence of a magnetic field.

We study the possible valley supercurrent flowing in a valley Josephson junction based on the O-shaped Kekulé graphene superlattice (GS), in which the valley coupling term can make the valley degree of freedom condensate and open an energy gap around the Dirac point. It is found that the GS/G/GS formally resembles a conventional Josephson junction and a nonzero valley supercurrent flows through the junction when the arguments of the complex valley coupling terms in each GS are different. Unlike the conventional Josephson current, a local static potential in the middle G region via gate voltage can manipulate the valley supercurrent direction to form the 0-$\pi$ state transition and, moreover, the valley supercurrent magnitude as well as the current-phase relation can also be modulated by shifting the universal chemical potential of the system. Our findings provide a way to producing the pure valley current in the graphene system.

The Aharonov-Bohm effect allows one to demonstrate the physical meaningfulness of magnetic vector potential bypassing the current in zero magnetic field regions. In the standard (a two-slit-like) setup, a conducting ring is pierced by magnetic flux and the quantum interference for an electron passing simultaneously through the two ring arms is observed. Here we show, by analyzing transport via evanescent waves, that the ballistic Corbino disk in graphene subjected to a solenoid magnetic potential may exhibit conductance oscillations of the Aharonov-Bohm kind although the current flows through a single conducting element only.

We have investigated two-dimensional nanostructures of the topological insulators ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ and ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ by means of temperature and magnetic field dependent Raman spectroscopy. The surface contribution of our samples was increased by using thin films of dropcasted nanoflakes with the aim of enhancing their topological properties. Raman spectroscopy provides a contact-free method to investigate the behavior of topological properties with temperature and magnetic fields at lower dimensions. The temperature dependent Raman study reveals anharmonic phonon behavior for ${\mathrm{Bi}}_2{\mathrm{Te}}_3$ indicative of a two-phonon relaxation mechanism in this material. Contrary to this, ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ shows clear deviations from a two-phonon anharmonic decay model at temperatures below 120 K exhibiting a hardening and broadening, especially of the $A_{1\text{g}}^2$ mode. Similarly, the magnetic field dependent self-energy effects are only observed for the $A_{1\text{g}}^2$ mode of ${\mathrm{Bi}}_2{\mathrm{Se}}_3$, showing a broadening and hardening with increasing field. We interpret our results in terms of corrections to the phonon self-energy for ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at temperatures below 120 K and magnetic fields above 4 T due to electron-hole pair excitations associated with the conducting surface states. The phonon renormalization with increasing magnetic field is explained by a gap opening in the Dirac cone that enables phonon coupling to the changing electric susceptibility.

We perform a $\mathbf{k}·\mathbf{p}$ theory analysis of the spectra of the lowest energy and excited states of the excitons in few-layer atomically thin films of InSe taking into account in plane electric polarizability of the film and the influence of the encapsulation environment. For the thinner films, the lowest-energy state of the exciton is weakly indirect in momentum space, with its dispersion showing minima at a layer-number-dependent wave number, due to an inverted edge of a relatively flat topmost valence band branch of the InSe film spectrum, and we compute the activation energy from the momentum dark exciton ground state into the bright state. For the films with more than seven ${\mathrm{In}}_2{\mathrm{Se}}_2$ layers, the exciton dispersion minimum shifts to $\mathrm{\Gamma }$ point.

Optical spectra of two-dimensional transition-metal dichalcogenides (TMDC) are influenced by complex multiparticle excitonic states. Their theoretical analysis requires solving the many-body problem, which in most cases, is prohibitively complicated. In this work, we calculate the optical spectra by exact diagonalization of the three-particle Hamiltonian within the Tamm-Dancoff approximation where the doping effects are accounted for via the Pauli blocking mechanism, modelled by a discretized mesh in the momentum space. The single-particle basis is extracted from the ab initio calculations. Obtained three-particle eigenstates and the corresponding transition dipole matrix elements are used to calculate the linear absorption spectra as a function of the doping level. Results for negatively doped ${\mathrm{MoS}}_2$ monolayer (ML) are in excellent quantitative agreement with the available experimental data, validating our approach. The results predict additional spectral features due to the intervalley exciton that is optically dark in an undoped ML but is brightened by the doping. Our approach can be applied to a plethora of other atomically thin semiconductors, where the doping induced brightening of the many-particle states is also anticipated.

We study a stochastic lattice gas of particles in one dimension with strictly finite-range interactions that respect the fractonlike conservation laws of total charge and dipole moment. As the charge density is varied, the connectivity of the system's charge configurations under the dynamics changes qualitatively. We find two distinct phases: Near half filling the system thermalizes subdiffusively, with almost all configurations belonging to a single dynamically connected sector. As the charge density is tuned away from half filling there is a phase transition to a frozen phase, where locally active finite bubbles cannot exchange particles and the system fails to thermalize. The two phases exemplify what has recently been referred to as weak and strong Hilbert space fragmentation, respectively. We study the static and dynamic scaling properties of this weak-to-strong fragmentation phase transition in a kinetically constrained classical Markov circuit model, obtaining some conjectured exact critical exponents.

When magnetic skyrmions move under spin-orbit torque in magnetic nanowires, they experience a skyrmion Hall effect, which pushes them towards the nanowire edge where they risk being annihilated; this puts an upper limit on how fast they can be driven. However, the same magnetic multilayer harboring skyrmions can sustain a Néel-type strip domain wall along the nanowire length, potentially keeping the skyrmions separated from the edge. Here we study the interplay between current driven skyrmions and domain walls and find that they increase the annihilation current and allow the skyrmions to move faster. Based on the Thiele formalism, we confirm that the emergent longitudinal repulsive force and the modified energy landscape linked to the domain wall are responsible for the enhanced skyrmion motion. Furthermore, we identify that the longitudinal repulsive force emerges because of the broken axisymmetry in the local magnetization in front of the skyrmion. Our study uncovers key aspects in the interplay between two topological magnetic textures from different homotopy groups and may inspire new device concepts.

Crystal-field (CF) effects on the rare-earth (RE) ions in ferrimagnetic intermetallics ${\mathrm{NdCo}}_5$ and ${\mathrm{TbCo}}_5$ are evaluated using an ab initio density functional $+$ dynamical mean-field theory approach in conjunction with a quasiatomic approximation for on-site electronic correlations on the localized $4f$ shell. The study reveals an important role of the high-order sectoral harmonic component of the CF in the magnetism of ${\mathrm{RECo}}_5$ intermetallics. An unexpectedly large value is computed in both systems for the corresponding crystal-field parameter (CFP) $A_6^6\langle r^6\rangle$, far beyond what one would expect from only electrostatic contributions. It allows solving the enigma of the nonsaturation of zero-temperature Nd magnetic moments in ${\mathrm{NdCo}}_5$ along its easy axis in the Co exchange field. This unsaturated state had been previously found out from magnetization distribution probed by polarised neutron elastic scattering but had so far remained theoretically unexplained. The easy plane magnetic anisotropy of Nd in ${\mathrm{NdCo}}_5$ is strongly enhanced by the large value of $A_6^6\langle r^6\rangle$. Counterintuitively, the polar dependence of anisotropy energy within the easy plane remains rather small. The easy plane magnetic anisotropy of Nd is reinforced up to high temperatures, which is explained through $J$-mixing effects. The calculated ab initio anisotropy constants of ${\mathrm{NdCo}}_5$ and their temperature dependence are in quantitative agreement with experiment. Unlike ${\mathrm{NdCo}}_5$, the $A_6^6\langle r^6\rangle$ CFP has negligible effects on the Tb magnetism in ${\mathrm{TbCo}}_5$ suggesting that its impact on the RE magnetism is ion-specific across the ${\mathrm{RECo}}_5$ series. The origin of its large value is the hybridization of RE and Co states in a hexagonally coordinated local environment of the RE ion in ${\mathrm{RECo}}_5$ intermetallics.

Using resonant x-ray diffraction and Raman spectroscopy, we study charge correlations and lattice dynamics in two model cuprates, ${\mathrm{HgBa}}_2{\mathrm{CuO}}_{4+\delta }$ and ${\mathrm{HgBa}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{6+\delta }$. We observe a maximum of the characteristic charge order temperature around the same hole concentration ($p\approx 0.09$) in both compounds, and concomitant pronounced anomalies in the lattice dynamics that involve the motion of atoms in and/or adjacent to the ${\mathrm{CuO}}_2$ layers. These anomalies are already present at room temperature, and therefore precede the formation of the static charge correlations, and we attribute them to an instability of the ${\mathrm{CuO}}_2$ layers. Our finding implies that the charge order in the cuprates is an emergent phenomenon, driven by a fundamental variation in both lattice and electronic properties as a function of doping.

We consider a finite-size scaling function across a topological phase transition in one-dimensional models. For models of noninteracting fermions it was shown to be universal for all topological symmetry classes and markedly asymmetric between trivial and topological sides of the transition [T. Gulden, M. Janas, Y. Wang, and A. Kamenev, Phys. Rev. Lett. 116, 026402 (2016)]. Here we verify its universality for the topological transition between dimerized and Haldane phases in bilinear-biquadratic spin-1 chain. To this end we perform high-accuracy variational matrix product state simulations. We show that the scaling function, expressed in terms of $L/\xi$, where $L$ is the chain length and $\xi$ is the correlation length, coincides with that of three species of noninteracting massive Majorana fermions. The latter is known to be a proper description of the conformal critical theory with central charge $c=3/2$. We have shown that it still holds away from the conformal point, including the finite-size corrections. We have also observed peculiar differences between even- and odd-size chains, which may be fully accounted for by residual interactions of the edge states.

We explore the effect of mechanical strain on the electronic spectrum of patterned graphene based heterostructures. We focus on the competition of Kekulé-O type distortion favoring a trivial phase and commensurate Kane-Mele type spin-orbit coupling generating a topological phase. We derive a simple low-energy Dirac Hamiltonian incorporating the two gap promoting mechanisms and include terms corresponding to uniaxial strain. The derived effective model explains previous ab initio results through a simple physical picture. We show that while the trivial gap is sensitive to mechanical distortions, the topological gap stays resilient.

GeS and its analog compounds exhibit unique properties that combine some of the most desired features of other two-dimensional compounds, such as transition-metal dichalcogenides and graphene. These include high electron mobilities or valley physics that result in strong optical and electronic anisotropy. Here, we present an experimental and theoretical study of the electronic band structure of GeS at high hydrostatic pressures. Polarization-resolved high-pressure photoreflectance measurements allow us to extract the energies, optical dichroic ratios, and pressure coefficients of the direct optical transitions. These findings are discussed in view of first-principles calculations, which predict that nondegenerate states in different valleys can be individually selected through linearly polarized light. Based on this, an assignation of the direct optical transitions to the electronic band structure is provided. Finally, the effect of pressure on the electronic band structure is discussed in terms of orbital composition. These results provide evidence that GeS is a strong candidate for valleytronic applications in nondegenerate systems.