A metal nanoparticle (MNP) coupled to a quantum emitter (QE) is a versatile composite nanostructure with unique chemical and physical properties which has been studied intensively, owing to its vast range of promising applications in nanoscience and nanotechnology. When pumped into a higher gain level, an MNP-QE composite nanostructure functions as a nanoplasmonic counterpart of a conventional laser, which is capable of operating at subwavelengths. The theory of plasmonic lasers is hitherto developed on the local optical response of the MNP, disregarding the nanoscale effects of its free electrons. In this paper, we perform a comprehensive quantum mechanical analysis of a complex MNP-QE composite nanostructure, capturing the size-dependent nonclassical effects through the nonlocal optical response of the MNP. Our study reveals that the nonlocal correction introduces significant deviations to the plasmon statistics of the hybrid particle suggested by the local calculations, becoming more prominent when the number of QEs coupled to the MNP increases. Furthermore, for the typical material parameter values used in the literature, we observed the initiation of quenching effects at lower pumping rates than suggested by the local response formalism. In essence, nonlocally assessed plasmon statistics of MNP-QE composite nanostructures demand the concerted coupling of an even higher number of QEs to compensate the deterioration in coherence and to sustain lasing.

Understanding the robustness of topological phases of matter in the presence of interactions poses a difficult challenge in modern condensed matter, showing interesting connections to high-energy physics. In this work, we leverage these connections to present a complete analysis of the continuum long-wavelength description of a generic class of correlated topological insulators: Wilson-Hubbard topological matter. We show that a Wilsonian renormalization group (RG) approach, combined with the so-called topological Hamiltonian, provide a quantitative route to understand interaction-induced topological phase transitions that occur in Wilson-Hubbard matter. We benchmark two-loop RG predictions for a quasi-1D Wilson-Hubbard model by means of exhaustive numerical simulations based on matrix product states (MPS). The agreement of the RG predictions with MPS simulations motivates the extension of the RG calculations to higher-dimensional topological insulators.

We present a continuum theory of graphene, treating on an equal footing both the homogeneous Cauchy-Born (CB) deformation and the microscopic degrees of freedom associated with the two sublattices. While our theory recovers all extant results from homogeneous continuum theory, the Dirac-Weyl equation is found to be augmented by new pseudogauge and chiral fields fundamentally different from those that result from homogeneous deformation. We elucidate three striking electronic consequences: (i) non-CB deformations allow for the transport of valley-polarized charge over arbitrarily long distances, e.g., along a designed ridge; (ii) the triaxial deformations required to generate an approximately uniform magnetic field are unnecessary with non-CB deformation; and finally (iii) the vanishing of the effects of a one-dimensional corrugation seen in ab initio calculation upon lattice relaxation is explained as a compensation of CB and non-CB deformation.

We conduct a theoretical and numerical study on the second-harmonic (SH) optical response of a nanostructured metamaterial composed of a periodic array of inclusions. Both the inclusions and their surrounding matrix are made of centrosymmetrical materials, for which SH is strongly suppressed, but, by appropriately choosing the shape of the inclusions, we may produce a geometrically noncentrosymmetric system which does allow efficient SH generation. Variations in the geometrical configuration allow tuning the linear and quadratic spectra of the optical response of the system. We develop a theory that allows the calculation of the nonlinear polarization from the geometry of the system and its linear dielectric function at the fundamental and second-harmonic frequencies, and we implement an efficient scheme for its numerical computation, extending a formalism for the calculation of the macroscopic dielectric function using Haydock's recursion method. We apply our formalism to an array of holes within an Ag matrix, but it can be readily applied to any metamaterial made of arbitrary materials and for inclusions of any geometry within the long-wavelength regime.

Topological transitions of isofrequency surfaces are studied in the long-wavelength approximation for a biaxial gyroelectromagnetic medium influenced by an external static magnetic field. For the lossless case, the topological transitions from a closed ellipsoid to open type-I and type-II hyperboloids as well as a bihyperboloid are demonstrated. Conditions for critical points where the topological transitions occur are found out. It is revealed that material losses in the constituents of the composite medium strongly influence the dispersion of the extraordinary waves. In particular, the loss-induced topological transitions take place for these waves. It is shown that the loss-induced topological transitions from a type-I hyperboloid to a bihyperboloid appear in the frequency band where the real part of at least one principal component of the anisotropic constitutive parameter (permittivity or permeability tensor) is close to zero, whereas its imaginary part is high.

We study speed and skew deflection-angle dependence on skyrmion deformations of a tightly bound two-skyrmion state in a synthetic ferrimagnet. We consider here, an in-plane uniaxial magnetocrystalline anisotropy-term in order to induce lateral shape distortions and an overall size modulation of the skyrmions due to a reduction of the effective out-of-plane anisotropy, thus affecting the skyrmion speed, skew-deflection and inducing anisotropy in these quantities with respect to the driving current-angle. Because of frustrated dipolar interactions in a synthetic ferrimagnet with perpendicular anisotropy, sizable skyrmion deformations can be induced with relatively small induced anisotropy constants and thus a wide range of tuneability can be achieved. We also show analytically, that a consequence of the skyrmion deformation can, under certain conditions cause a skyrmion deflection with respect to driving-current angles, unrelated to the topological charge. Results are analyzed by a combination of micromagnetic simulations and a compound particle description within the Thiele formalism from which an overall mobility tensor is constructed. This work offers an additional path towards in situ tuning of skyrmion dynamics with the potential of enhancing the operational speed of skyrmionic racetrack memory devices using only small induced uniaxial anisotropies.

Pair distribution function analysis of neutron-scattering data and of ab initio molecular dynamics results have been employed to study short-range structural correlations and their temperature dependence in a heavily disordered dielectric material ${\mathrm{Sr}}_x{\mathrm{Ba}}_{1-x}{\mathrm{Nb}}_2{\mathrm{O}}_6(x=0.35,0.5$, and 0.61). Intrinsic disorder caused by a partial occupation of the cationic sites by differently sized Sr and Ba atoms and their vacancies introduces important local strains to the structure and directly influences the Nb-${\mathrm{O}}_6$ octahedra tilting. The resulting complex system of tilts is found to be both temperature and Sr-doping sensitive with the biggest tilt magnitudes reached at low temperatures and high strontium contents, where ferroelectric relaxor behavior appears. We find evidence for two Nb-${\mathrm{O}}_6$ subsystems with different variations of niobium-oxygen bond lengths, distinct dynamics, and disparate levels of deviation from macroscopic polarization direction. These findings establish a detailed picture of the local structure of ${\mathrm{Sr}}_x{\mathrm{Ba}}_{1-x}{\mathrm{Nb}}_2{\mathrm{O}}_6$ and provide a deeper insight into the origins of the materials dielectric properties.

Clear anomalies in the lattice thermal expansion (deviation from linear variation) and elastic properties (softening of the sound velocity) at the antiferromagnetic-to-paramagnetic transition are observed in the prototypical multiferroic ${\mathrm{BiFeO}}_3$ using a combination of picosecond acoustic pump-probe and high-temperature x-ray-diffraction experiments. Similar anomalies are also evidenced using second-principles calculations supporting our experimental findings. Those calculations, in addition to a simple Landau-like model we also developed, allow us to understand the elastic softening and lattice change at $T_N$ as a result of magnetostriction combined with electrostrictive and magnetoelectric couplings, which renormalize the elastic constants of the high-temperature reference phase when the critical $T_N$ temperature is reached.

The honeycomb $K-\mathrm{\Gamma }$ model is known to have both deconfined spin liquid and confined phases. We study here a confinement transition between the Kitaev spin liquid and a dimerized phase in the limit of strong bond anisotropy. By partially projecting out Majorana states we are able to map the model onto a model of weakly coupled Ising chains in a transverse field. Within this mapping the ordered Ising phase corresponds to the condensation of $Z_2$ fluxes, or confinement. Our results may improve our understanding of the extensively studied spin liquid candidate material $\alpha -{\mathrm{RuCl}}_3$, where $K-\mathrm{\Gamma }$ interactions are dominant.

We extend the scope of the Klein bottle entropy, originally introduced by Tu [Phys. Rev. Lett. 119, 261603 (2017)] in the rational conformal field theory (CFT), to the compactified boson CFT, which are relevant to the studies of Luttinger liquids. We first review the Klein bottle entropy in rational CFT and discuss details of how to extract the Klein bottle entropy from lattice models using the example of the transverse field Ising model. We then go beyond the scope of rational CFT and study the Klein bottle entropy $lng$ in the compactified boson CFT, which turns out to have a straightforward relation to the compactification radius $R$, $lng=lnR$. This relation indicates a convenient and efficient method to extract the Luttinger parameter from lattice model calculations. Our numerical results on the Klein bottle entropy in the spin-$1/2$ XXZ chain show excellent agreement with the CFT predictions, up to some small deviations near the isotropic point, which we attribute to the marginally irrelevant terms. For the $S=1$ XXZ chain that cannot be exactly solved, our numerical results provide an accurate numerical determination of the Luttinger parameter in this model.

The functional-renormalization-group aided density functional theory (FRG-DFT) is applied to the two-dimensional homogeneous electron gas (2DHEG). The correlation energy of the 2DHEG is derived as a function of the Wigner-Seitz radius $r_{\mathrm{s}}$ directly. We find that our correlation energy completely reproduces the exact behavior at the high-density limit. For finite density, the result of FRG-DFT shows good agreement with the Monte Carlo (MC) results in the high-density region, although the discrepancy between the FRG-DFT and MC results becomes larger as the system becomes more dilute. Our study is the first example in which the FRG-DFT is applied to more-than-one-dimensional models, and shows that the FRG-DFT is a feasible and promising method even for the analysis of realistic models for quantum many-body systems.

We first analyze the recent experimental data on the nuclear spin-lattice relaxation rate of the Weyl semimetal TaP. We argue that its nonmonotonic temperature dependence is explained by the temperature-dependent chemical potential of Weyl fermions. We also develop the theory of the Knight shift in Weyl semimetals, which contains two counteracting terms. The diamagnetic term follows $-ln[W/max(\left|\mu \right|,k_{B}T\left)\right]$ with $W,\mu$, and $T$ being the high-energy cutoff, chemical potential, and temperature, respectively, and is always negative. The paramagnetic term scales with $\mu$ and changes sign depending on the doping level. Altogether, the Knight shift is predicted to vanish or even change sign upon changing the doping or the temperature, making it a sensitive tool to identify Weyl points. We also calculate the Korringa relation for Weyl semimetals which shows an unusual energy dependence rather than being constant as expected for a noninteracting Fermi system.

We demonstrate a design principle for unidirectionally invisible non-Hermitian structures that are invisible not only for one specific wavelength but rather for a broad frequency range. Our idea is based on the concept of constant-intensity waves, which can propagate even through highly disordered media without backscattering or intensity variations. In contrast to already existing invisibility studies, our design principle requires neither a specific symmetry (like $\mathcal{PT}$ symmetry) nor periodicity and can thus be applied in a much wider context. This generality combined with broadband frequency stability allows a pulse to propagate through a disordered medium as if the medium were entirely uniform.

Control and manipulation of electric current and, especially, its degree of spin polarization (spin filtering) across single molecules are currently of great interest in the field of molecular spintronics. We explore one possible strategy based on the modification of nanojunction symmetry which can be realized, for example, by a mechanical strain. Such modification can activate new molecular orbitals which were inactive before due to their orbital mismatch with the electrode's conduction states. This can result in several important consequences such as (i) quantum interference effects appearing as Fano-like features in electron transmission and (ii) the change in molecular level hybridization with the electrode's states. We argue that the symmetry change can affect very differently two majority- and minority-spin conductances and thus alter significantly the resulting spin-filtering ratio as the junction symmetry is modified. We illustrate the idea for two basic molecular junctions: Ni/benzene/Ni (perpendicular vs tilted orientations) and Ni/Si chain/Ni (zigzag vs linear chains). In both cases, one highest occupied molecular orbital (HOMO) and one lowest unoccupied molecular orbital (LUMO) (out of HOMO and LUMO doublets) are important. In particular, their destructive interference with other orbitals leads to dramatic suppression of majority-spin conductance in low-symmetry configurations. For a minority-spin channel, on the contrary, the conductance is strongly enhanced when the symmetry is lowered due to an increase in hybridization strength. We believe that our results may offer a potential route for creating molecular devices with a large on-off ratio of spin polarization via quantum interference effects.

Emergence of exceptional points in two dimensions is one of the remarkable phenomena in non-Hermitian systems. We here elucidate the impacts of symmetry on the non-Hermitian physics. Specifically, we analyze chiral symmetric correlated systems in equilibrium where the non-Hermitian phenomena are induced by the finite lifetime of quasiparticles. Intriguingly, our analysis reveals that the combination of symmetry and non-Hermiticity results in topological degeneracies of energy bands which we call symmetry-protected exceptional rings (SPERs). We observe the emergence of SPERs by analyzing a non-Hermitian Dirac Hamiltonian. Furthermore, by employing the dynamical mean-field theory, we demonstrate the emergence of SPERs in a correlated honeycomb lattice model whose single-particle spectrum is described by a non-Hermitian Dirac Hamiltonian. We uncover that the SPERs survive even beyond the non-Hermitian Dirac Hamiltonian, which is related to a zeroth Chern number. The argument of symmetry protection also holds for three dimensions, elucidating the presence of a symmetry-protected exceptional torus.

The structure and dynamics of an ice surface upon melting are of paramount importance in a variety of phenomena on earth and in the universe because undercoordinated water molecules at an ice surface have peculiar physical and chemical properties distinct from fully coordinated molecules in bulk. Using surface-specific sum-frequency generation spectroscopy and molecular dynamics simulations, we demonstrate that a topologically disordered hydrogen-bond network emerges at the topmost surface layer of crystalline ice Ih(0001) at $\sim 120\mathrm{K}$ that is much lower than the conventionally believed premelting temperature of $\sim 200\mathrm{K}$. The ice surface undergoes a cascade of structural transitions from a low-temperature solid phase to high-temperature quasiliquid phase via the topologically disordered mesophase between 120 and 200 K. Because the lower limit of temperature of the earth's atmosphere is $\sim 120\mathrm{K}$ around the mesopause, the topmost surface layers of crystalline ice on earth are unlikely to be the perfectly ordered solid.

We investigate on the same footing the time-dependent electronic transport properties and vibrational dynamics of a molecular junction. We show that fluctuations of both the molecular vibron displacement and the electronic current across the junction undergo damped oscillations toward the steady state. We assign the former to the onset of electron tunneling events assisted by vibron emission. The time-dependent buildup of electron-hole correlations is revealed as a departure of the charge-transfer statistics from the generalized-binomial one after a critical time $t_c$. The phonon backaction on the tunneling electrons is shown to amplify and accelerate this buildup mechanism.

Bulk $1T\text{−}\mathrm{TiT}{\mathrm{e}}_2$ (hexagonal, space group P-$3m1$) exhibits semimetallic conductivity with weak Anderson localization characteristic at ambient pressure, but superconductivity under high pressure of $\sim 5\mathrm{GPa}$. Here the detailed evolutions of structural and transport properties in $\mathrm{TiT}{\mathrm{e}}_2$ during compression are investigated by extending pressure up to 50.2 GPa. The combined high-pressure electrical transport and synchrotron x-ray diffraction experiments unravel two critical pressure points. At $P_{c1}\sim 5.4\mathrm{GPa}$, the superconductivity emerges, which is accompanied by a structural transition from $P\text{−}3m1$ to a mixture of $P\text{−}3m1$ and monoclinic $C2/m$ phase. The localization effect maintains until the structural transition is over at $P_{c2}\sim 19.1\mathrm{GPa}$, meanwhile the Hall coefficient $R_{\mathrm{H}}$ changes from positive to negative. Our results demonstrate that the superconductivity in the pressurized $\mathrm{TiT}{\mathrm{e}}_2$ can be assigned to the high-pressure $C2/m$ phase which is electron dominated.