We theoretically study quantum size effects in the magnetic response of a spherical metallic nanoparticle (e.g., gold). Using the jellium model in spherical coordinates, we compute the induced magnetic moment and the magnetic susceptibility for a nanoparticle in the presence of a static external magnetic field. Below a critical magnetic field the magnetic response is diamagnetic, whereas above such field the magnetization is characterized by sharp, steplike increases of several tenths of Bohr magnetons, associated with the Zeeman crossing of energy levels above and below the Fermi sea. We quantify the robustness of these regimes against thermal excitations and finite linewidth of the electronic levels. Finally, we propose two methods for experimental detection of the quantum size effects based on the coupling to superconducting quantum interference devices.

Recent advances in quantum engineering have given us the ability to design hybrid systems with novel properties normally not present in the regime they operate in. The coupling of spin ensembles and magnons to microwave resonators has for instance lead to a much richer understanding of collective effects in these systems and their potential quantum applications. We can also hybridize electron and nuclear spin ensembles together in the solid-state regime to investigate collective effects normally only observed in the atomic, molecular, and optical world. Here we explore in the solid state regime the dynamics of a double domain nuclear spin ensemble coupled to the Nambu-Goldstone boson in GaAs semiconductors and show it exhibits both collective and individual relaxation (thermalization) on very different time scales. Further the collective relaxation of the nuclear spin ensemble is what one would expect from superradiant decay. This opens up the possibility for the exploration of novel collective behavior in solid state systems where the natural energies associated with those spins are much less than the thermal energy.

The exchange stiffness is a central material parameter of all ferromagnetic materials. Its value controls the Curie temperature as well as the dynamic properties of spin waves to a large extent. Using ultrashort spin current pulses we excite perpendicular standing spin waves (PSSW) in ultrathin epitaxial iron layers at frequencies of up to 2.4 THz. Our analysis shows that for the PSSWs the observed exchange stiffness of iron is about 20% smaller compared to the established iron bulk value. In addition, we find an interface-related reduction of the effective exchange stiffness for layers with the thickness below 10 nm. To understand and discuss the possible mechanisms of the exchange stiffness reduction we develop an analytical one-dimensional model. In doing so we find that the interface induced reduction of the exchange stiffness is mode dependent.

Within density functional theory, we have studied the interplay between vortex arrays and capillary waves in spinning prolate ${}^4\mathrm{He}$ droplets made of several thousand helium atoms. Surface capillary waves are ubiquitous in prolate superfluid ${}^4\mathrm{He}$ droplets, and depending on the size and angular momentum of the droplet, they may coexist with vortex arrays. We have found that the equilibrium configuration of small prolate droplets is vortex free, evolving towards vortex hosting as the droplet size increases. This result is in agreement with a recent experiment [O'Connell et al., Phys. Rev. Lett. 124, 215301 (2020)] that disclosed that vortex arrays and capillary waves coexist in the equilibrium configuration of very large drops. In contrast to viscous droplets executing rigid-body rotation, the stability phase diagram of spinning ${}^4\mathrm{He}$ droplets cannot be universally described in terms of dimensionless angular momentum and angular velocity variables: Instead, the rotational properties of superfluid helium droplets display a clear dependence on the droplet size and the number of vortices they host.

In this study, we numerically investigate the internal structure of localized quantum turbulence in superfluid ${}^4\mathrm{He}$ at zero temperature with the expectation of self-similarity in the real space. In our previous study, we collected the statistics of vortex rings emitted from a localized vortex tangle. As a result, the power law between the minimum size of detectable vortex rings and the emission frequency is obtained, which suggests that the vortex tangle has self-similarity in the real space [Nakagawa et al., Phys. Rev. B 101, 184515 (2020)]. In this work, we study the fractal dimension and vortex length distribution of localized vortex tangles, which can show their self-similar structure. We generate statistically steady and localized vortex tangles by injecting vortex rings with a fixed size. We used two types of injection methods that produce anisotropic or isotropic tangles. The injected vortex rings develop into a localized vortex tangle consisting of vortex rings of various sizes through reconnections (fusions and splitting of vortices). The fractal dimension is an increasing function of the vortex line density and becomes saturated to a value of approximately 1.8, as the density increases sufficiently. The behavior of the fractal dimension was independent of the anisotropy of the vortex tangles. The vortex length distribution indicates the number of vortex rings of each size that are distributed in a tangle. The distribution of the anisotropic vortex tangle shows the power law in the range above the injected vortex size, although the distribution of the isotropic vortex does not.

Using the Ginzburg-Landau approach, we show that the strain of the nematic superconductor can generate a specific (nematic) vorticity. In the case of doped topological insulators that vorticity forms a spin vortex. We find two types of topologically different spin vortices that either enhance (type I) or suppress (type II) superconductivity far from the vortex core. We apply Bogoliubov–de Gennes equations to study electronic states in the nematic superconductor with spin vortices. We find that in the case of the type I vortex, zero-energy states are localized near the vortex core. These states can be identified as Majorana Kramers pairs. In the case of the type II vortex, there are no localized zero-energy states. Thus, we establish a nontrivial connection between the strain and Majorana fermions in the doped topological insulators with nematic superconductivity.

The crystal structure of iron, the major component of the Earth's inner core (IC), is unknown under the IC high pressure (P) (3.3–3.6 Mbar) and temperature (T) (5000–7000 K). Experimental and theoretical data on the phase diagram of iron at these extreme PT conditions are contradictory. Applying quasi-ab initio and ab initio molecular dynamics we computed free energies of the body-centered cubic (bcc), hexagonal close-packed (hcp), and liquid phases. The ionic free energies, computed for the embedded-atom model, were corrected for electronic entropy. Such correction brings the melting temperatures of the hcp iron in very good agreement with previous ab initio data. This validates the calculation of the bcc phase, where fully ab initio treatment is not technically possible due to large sizes required for convergence. The resulting phase diagram shows stabilization of the bcc phase prior to melting in the pressure range of the IC. The melting temperature of the bcc phase is equal to 7190 K at the pressure 360 GPa.

Precision in laser material structuring is critically defined by the energy flow during the irradiation process, particularly in ultrafast regimes. Alternative to thermal evolutions, nonequilibrium electronic excitation can exercise a direct influence on the atomic bonding delivering a potentially rapid destructuring process. In this context, the atomic disordering of transition metals (Cr, Ni, and Ti) induced by nonequilibrium electronic excitation typical for ultrafast laser processing is studied with an emphasis on the role of the $d$-band filling and crystalline structure. The density functional theory is used to obtain, from first principles, structural stability criteria and nonthermal disordering pathways on timescales shorter than picosecond electron-phonon dynamics. We show that the hot electrons distort charge distribution in the cold crystalline arrangement and increase the entropy of the system, thus driving the mechanical expansion. The calculated nonequilibrium free-energy potentials indicate that a solid destabilization is possible when electron temperature reaches a universal value of around 2 eV for all considered metals. Under a uniaxial lattice relaxation expected in the laser ablation of surface layers, the charge redistribution and loss of lattice stability is shown to be oriented along specific crystal directions. Moreover, we show that the interatomic potential destabilization is energetically more favorable for Ni than for Cr and Ti. Lattice dynamics is affected by space-charge redistribution induced by Fermi smearing associated with anisotropic expansion due to electron pressure gradients. This ultrafast destructuring mechanism is shown to be a general feature of partially filled $d$-band transition metals.

We introduce lattice gauge theories which describe three-dimensional, gapped quantum phases exhibiting the phenomenology of both conventional three-dimensional topological orders and fracton orders, starting from a finite group $G$, a choice of an Abelian normal subgroup $N$, and a choice of foliation structure. These hybrid fracton orders, examples of which were introduced in a previous paper [Tantivasadakarn et al., Phys. Rev. B 103, 245136 (2021)], can also host immobile, pointlike excitations that are non-Abelian, and therefore give rise to a protected degeneracy. We construct solvable lattice models for these orders which interpolate between a conventional, three-dimensional $G$ gauge theory and a pure fracton order, by varying the choice of normal subgroup $N$. We demonstrate that certain universal data of the topological excitations and their mobilities are directly related to the choice of $G$ and $N$, and also present complementary perspectives on these orders: certain orders may be obtained by gauging a global symmetry which enriches a particular fracton order, by either fractionalizing on or permuting the excitations with restricted mobility, while certain hybrid orders can be obtained by condensing excitations in a stack of initially decoupled, two-dimensional topological orders.

We present a tree tensor-network impurity solver suited for general multiorbital systems. The network is constructed to efficiently capture the entanglement structure and symmetry of an impurity problem. The solver works directly on the real-time/frequency axis and generates spectral functions with energy-independent resolution of the order of one percent of the correlated bandwidth. Combined with an optimized representation of the impurity bath, it efficiently solves self-consistent dynamical mean-field equations and calculates various dynamical correlation functions for systems with off-diagonal Green's functions. For the archetypal correlated Hund's metal ${\mathrm{Sr}}_2{\mathrm{RuO}}_4$, we show that both the low-energy quasiparticle spectra related to the van Hove singularity and the high-energy atomic multiplet excitations can be faithfully resolved. In particular, we show that while the spin-orbit coupling has only minor effects on the orbital-diagonal one-particle spectral functions, it has a more profound impact on the low-energy spin and orbital response functions.

The optical response functions and band structures of ${\mathrm{LiCoO}}_2$ are studied at different levels of approximation, from density functional theory (DFT) in the generalized gradient approximation (GGA) to quasiparticle self-consistent $\mathrm{QS}GW$ (with $G$ for Green's function and $W$ for screened Coulomb interaction) without and with ladder diagrams $\left(\mathrm{QS}G\widehat{W}\right)$ and the Bethe Salpeter Equation (BSE) approach. The $\mathrm{QS}GW$ method is found to strongly overestimate the band gap and electron-hole or excitonic effects are found to be important. They lower the quasiparticle gap by only about 11% but the lowest energy peaks in absorption are found to be excitonic in nature. The contributions from different band to band transitions and the relation of excitons to band-to-band transitions are analyzed. The excitons are found to be strongly localized. A comparison to experimental data is presented.

The GaAs/AlGaAs quantum well (QW) system is utilized to investigate the electron spin relaxation in the satellite X valley of indirect band gap ${\mathrm{Al}}_{0.63}{\mathrm{Ga}}_{0.37}\mathrm{As}$ epitaxial layers through polarization-resolved photoluminescence (PL) excitation spectroscopy. Solving the rate equations, steady-state electronic distribution in various valleys of ${\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}\mathrm{As}$ is estimated against continuous photocarrier generation, and an expression for the degree of circular polarization (DCP) of PL coming from the adjacent QW is derived. Amalgamating the experimental results with analytical expressions, the X-valley electron spin relaxation time $\left({\tau }_S^X\right)$ is determined to be 2.7 ± 0.1 ps at 10 K. To crosscheck its validity, theoretical calculations are performed based on density functional theory within the framework of the projector augmented wave method which support the experimental result quite well. Further, temperature dependence of ${\tau }_S^X$ is studied over 10–80 K range, which is explained by considering the intravalley scattering of carriers and Dyakonov-Perel spin relaxation in the X valley of the indirect band gap AlGaAs barrier layer. It is learned that the strain-induced modification of band structure lifts the degeneracy in the X valley, which dominates the electron spin relaxation $<50\mathrm{K}$. Furthermore, the DCP spectra of hot electrons in indirect band gap AlGaAs layers are found to be significantly different than that of direct bandgap AlGaAs. It is understood as a signature of linear $k$-dependent Dresselhaus spin splitting and the faster energy relaxation procedure in the X valley.

The discrepancy between the theoretically calculated and experimentally measured activation gaps in quantum Hall effect has long been a puzzle. We revisit this issue in the context of the $\nu =1$ quantum Hall state, while also incorporating the skyrmion physics. We find that the finite width and the Landau level mixing effects are not sufficient to explain the observed activation gap. We further show that the presence of charged impurities located adjacent to the quantum well can cause a significant reduction in the activation gap, while also causing a suppression of the skyrmion size.

Using terahertz spectroscopy, we investigate resonant transmission through a dielectric slab with a conducting layer of varied mobility on one side. We observe the well-known Fabry-Pérot resonances in the transmission. Two fundamentally different regimes of the electrodynamic response of the system are established. In the clean limit, when the radiation frequency exceeds the carrier relaxation rate, $\omega \tau \gg 1$, the system exhibits a plasmonic response. As a result, there is a continuous shift in the Fabry-Pérot resonances with respect to the system parameters. By contrast, in the dirty regime, when $\omega \tau \ll 1$, there is an abrupt $\pi$ shift in the resonance frequency, as the renormalized film resistance matches the impedance of free space. We find that under this condition, Fabry-Pérot oscillations disappear entirely over a broad frequency range. Such a matching effect might be of interest in various terahertz applications, for example, involving broadband filters or absorbers.

Friedel oscillations are ubiquitous around localized defects in Hermitian quantum mechanics. Here we study their fate around a non-Hermitian, imaginary impurity and evaluate the corresponding non-Hermitian Lindhard function in one dimension, where quantum effects are traditionally enhanced due to spatial confinement. Most importantly, we find that the static limit of the non-Hermitian Lindhard function has no divergence at twice the Fermi wave number and vanishes identically for all other wave numbers at zero temperature. Consequently, no Friedel oscillations are induced by a non-Hermitian, imaginary impurity to lowest order in the impurity potential at zero temperature. Our findings are corroborated numerically on a tight-binding ring by switching on a weak real or imaginary potential. We identify conventional Friedel oscillations or a heavily suppressed density response, respectively.

We show that two-dimensional non-Hermitian photonic crystals made of lossy material can exhibit nontrivial point-gap topology in terms of topological winding in its complex frequency band structure. Such crystals can be made of either lossy isotropic media which is reciprocal or lossy magneto-optical media which is nonreciprocal. We discuss the effects of reciprocity on the properties of the point-gap topology. We also show that such a nontrivial point-gap topology leads to the non-Hermitian skin effect when the photonic crystal is truncated. In contrast to most previous studies on point-gap topology which used tight-binding models, our work indicates that such non-Hermitian topology can studied in photonic crystals, which represent a more realistic system that has technological significance.

Twisted two-dimensional bilayers exhibit many intriguing physical phenomena through interlayer twisting and coupling. Manipulating the “twisted angle” between the two evanescently coupled layers enables the hybridization of polaritons, and the dispersion engineering of polaritons in these structures can be well controlled. In this paper, we study the near-field radiative heat transfer (NFRHT) between two nanoparticles in the presence of a bilayered hyperbolic metasurface, which is modeled as two arrays of graphene strips (GSs) in parallel. We prove that the topological transition of the surface state under different twisted angles [from open (hyperbolic) to closed (elliptical) contours] has significant effect on NFRHT between nanoparticles. When both the integral and interlayer twisted angles are adjusted to proper values, the bilayered GS can canalize the energy transmission channel of the nanoparticles, hence significantly amplifying the NFRHT. A modulation factor beyond five orders of magnitude is achieved. We also demonstrate that, when the nanoparticles are arranged in different directions along the twisted bilayered system, NFRHT between nanoparticles can be strongly enhanced or inhibited. By changing the chemical potential and filling factor of the twisted bilayered GS, NFRHT can be effectively modulated. This paper reveals a hybridization effect of polaritons on NFRHT between nanoparticles, giving a degree of freedom (twisted angle) for controlling the heat transfer at the nanoscale with potential for effective energy management.