In the quantum Hall effect (QHE) regime, heat is carried by electrons in the edge states of Landau levels. Here, we study cooling of hot electrons propagating along the edge of graphene at the filling factor $\nu =\pm 2$, mediated by acoustic phonons. We determine the temperature profile extended from a hot spot, where the Hall current is injected into graphene from a metallic contact, taking into account specifics of boundary conditions for lattice displacements in graphene in a van der Waals heterostructure with an insulating substrate. Our calculations, performed using generic boundary conditions for Dirac electrons, show that emission of phonons can explain a short cooling length observed in graphene-based QHE devices by Nahm, Hwang, and Lee [Phys. Rev. Lett. 110, 226801 (2013)].

The Fermi edge singularity (FES) is a prominent manifestation of the Coulomb interaction. It can be observed in a controllable way by studying the transport through a quantum dot (QD), which is electrostatically coupled to the leads. In this paper, we study how FES affects higher-order tunneling processes (cotunneling). To address this problem we use the bosonic description of the electrons in the leads, which naturally accounts for the Coulomb interaction. We report the multiscale dependence of the current through the QD on the energy of the QD level and on the bias between the leads obtained for the limit of large and small bias. The new universal powers are determined by the scattering phases due to the interaction of electrons in the leads with the charge on the QD.

We discuss the possibility of plasmon canalization, collimation, and tunneling over ultrathin metasurfaces, enabled by extreme anisotropy in their complex conductivity dyadic. The interplay between anisotropy, conductivity-near-zero, and loss is exploited here to derive general conditions for plasmon canalization and efficient energy transport. We also demonstrate how the intrinsic in-plane anisotropy of black phosphorus can provide a natural platform to engineer these conditions, exhibiting important advantages over isotropic plasmonic materials. Our findings have implications for plasmonic sensors, planar hyperlenses, and plasmon steering over a surface, and they highlight the potential of two-dimensional materials beyond graphene.

We measured the optical response of ${\mathrm{CuGeO}}_3$ nanorods in order to reveal size effects on the electronic properties. The vibronically activated $d$-to-$d$ color band excitations are activated by the 131 and 478 ${\mathrm{cm}}^{-1}$ phonons, with the relative contribution of the lower frequency O-Cu-O bending mode increasing with decreasing size until it dominates the process. We also uncover trends in the direct band gap, with the charge transfer edge hardening with decreasing size. These findings advance the understanding of size effects in low-dimensional copper oxides.

Topological phase transitions, which have fascinated generations of physicists, are always demarcated by gap closures. In this work, we propose very simple two-dimensional photonic crystal lattices with gap closures, i.e., band degeneracies protected by nonsymmorphic symmetry. Our photonic structures are relatively easy to fabricate, consisting of two inequivalent dielectric cylinders per unit cell. Along high-symmetry directions, they exhibit line degeneracies protected by glide-reflection symmetry and time-reversal symmetry, which we explicitly demonstrate for $pg,pmg,pgg$, and $p4g$ nonsymmorphic groups. They also exhibit point degeneracies (Dirac points) protected by a $Z_2$ topological number associated only with crystalline symmetry. Strikingly, the robust protection of $pg$ symmetry allows a Lifshitz transition to a type-II Dirac cone across a wide range of experimentally accessible parameters, thus providing a convenient route for realizing anomalous refraction. Further potential applications include a stoplight device based on electrically induced strain that dynamically switches the lattice symmetry from $pgg$ to the higher $p4g$ symmetry. This controls the coalescence of Dirac points and hence the group velocity within the crystal.

We introduce a model of $2\pi$-flux loop semimetals which holds nodal loops described by a winding number $\nu =2$. By adding some extra terms, this model can be transformed into a recently discovered Hopf-link semimetal, and the symmetries distinguishing these two types of semimetals are studied. We also propose a simpler physical implementation of $2\pi$-flux loops and of Hopf-link semimetals which only involves nearest-neighbor hoppings, although in the presence of spin-orbit interactions. Finally, we investigate the Floquet properties of the $2\pi$-flux loop, and find that such a loop may be driven into two separated $\pi$-flux loops or four Weyl points by light with circular polarization in certain directions.

We present an impurity solver based on adaptively truncated Hilbert spaces. The solver is particularly suitable for dynamical mean-field theory in circumstances where quantum Monte Carlo approaches are ineffective. It exploits the sparsity structure of quantum impurity models, in which the interactions couple only a small subset of the degrees of freedom. We further introduce an adaptive truncation of the particle or hole excited spaces, which enables computations of Green functions with an accuracy needed to avoid unphysical (sign change of imaginary part) self-energies. The method is benchmarked on the one-dimensional Hubbard model.

We study a novel type of quantum criticality of the Lifshitz ${\phi }^4$ theory below the upper critical dimension $d_u=z+d_c=8$, where the dynamic critical exponent $z=4$ and the spatial upper critical dimension $d_c=4$. Two fixed points, one Gaussian and the other non-Gaussian, are identified with zero and finite interaction strengths, respectively. At zero temperature the particle density exhibits different power-law dependences on the chemical potential in the weak- and strong-interaction regions. At finite temperatures, critical behaviors in the quantum disordered region are mainly controlled by the chemical potential. In contrast, in the quantum critical region critical scalings are determined by temperature. The scaling ansatz remains valid in the strong-interaction limit for the chemical potential, correlation length, and particle density, while it breaks down in the weak-interaction one. Approaching the upper critical dimension, physical quantities develop logarithmic dependence on dimensionality in the strong-interaction region. These results are applied to spin-orbit coupled bosonic systems, leading to predictions testable by future experiments.

We present a detailed study of the low-temperature and high-magnetic-field phases in the chemical substitution series ${\mathrm{URu}}_2{\mathrm{Si}}_{2-x}{\mathrm{P}}_x$ using electrical transport and magnetization in pulsed magnetic fields up to 65 T. Within the hidden-order $x$ regime ($0<x\lesssim 0.035$) the field-induced ordering that was earlier seen for $x$ = 0 is robust, even as the hidden-order temperature is suppressed. Earlier work shows that for $0.035\lesssim x\lesssim 0.26$ there is a Kondo lattice with a no-ordered state that is replaced by antiferromagnetism for $0.26\lesssim x\lesssim 0.5$. We observe a simplified continuation of the field-induced order in the no-order $x$ regime and an enhancement of the field-induced order upon the destruction of the antiferromagnetism with magnetic field. These results closely resemble what is seen for ${\mathrm{URu}}_{2-x}{\mathrm{Rh}}_x{\mathrm{Si}}_2$,1 from which we infer that charge tuning dominantly controls the ground state of ${\mathrm{URu}}_2{\mathrm{Si}}_2$, regardless of whether $s$/$p$ or $d$ electrons are replaced. Contraction of the unit-cell volume may also play a role at large $x$. This provides guidance for determining the specific factors that lead to hidden order versus magnetism in this family of materials and constrains possible models for hidden order.

The concentration in this paper is defined as ${\mathrm{URu}}_{2-x}{\mathrm{Rh}}_x{\mathrm{Si}}_2$ while the chemical formula in the literature is given as $\mathrm{U}({\mathrm{Ru}}_{1-x}{\mathrm{Rh}}_x$)${}_2{\mathrm{Si}}_2$ [28–30].

Nematic order has manifested itself in a variety of materials in the cuprate family. We propose an effective field theory of a layered system with incommensurate, intertwined spin- and charge-density wave (SDW and CDW) orders, each of which consists of two components related by $C_4$ rotations. Using a variational method (which is exact in a large-$N$ limit), we study the development of nematicity from partially melting those density waves by either increasing temperature or adding quenched disorder. As temperature decreases we first find a transition to a single nematic phase, but depending on the range of parameters (e.g., doping concentration) the strongest fluctuations associated with this phase reflect either proximate SDW or CDW order. We also discuss the changes in parameters that can account for the differences in the SDW-CDW interplay between the 214 family and the other hole-doped cuprates.

Epitaxial $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films have been grown on $\mathrm{SrTi}{\mathrm{O}}_3$ (001) substrates via pulsed laser deposition. In a 22-nm-thick LSMO film with a low residual resistivity of ${\rho }_0\approx 59\mu \mathrm{\Omega }\mathrm{cm}$, we found a zero-field dip in the magnetoresistance (MR) below 10 K, manifesting the weak antilocalization (WAL) effect due to strong spin-orbit coupling (SOC). We have analyzed the MR data by including the D’yakonov-Perel’ spin-relaxation mechanism in the WAL theory. We explain that the delocalized spin-down electron sub-band states play a crucial role for facilitating marked SOC in clean LSMO. Moreover, we find that the SOC strength and gate voltage tunability are similar to those in a two-dimensional electron gas at the $\mathrm{LaAl}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface, indicating the presence of an internal electric field near the LSMO/$\mathrm{SrTi}{\mathrm{O}}_3$ interface. In a control measurement on a 5-nm-thick high resistivity $\left({\rho }_0\approx 280\mu \mathrm{\Omega }\mathrm{cm}\right)$ LSMO film, we observe only a small zero-field peak in MR from weak localization effect, indicating negligible SOC.

We study surface temperature effects on the adsorption and relaxation of N atoms on Ag(111). To this aim, we perform ab initio molecular dynamics simulations with electronic friction, in which the surface is coupled to a thermostat that fixes the desired surface temperature. Simulations performed at 80 and 700 K show that the surface temperature has minor effects on magnitudes such as the initial adsorption probability, the relaxation rate of the adsorbing N, and the energy lost in electronic excitations. Slight differences are identified in the adsorption paths with the appearance of subsurface absorption events at 700 K that are not observed at 80 K. Furthermore, we perform additional simulations without a thermostat in order to examine the validity of commonly used ab initio molecular dynamics simulations in which no heat dissipation from the simulation cell is allowed. Our results show that such a methodology may not suffice to simulate the low-temperature regime since the surface becomes unphysically heated within a few picoseconds upon adsorption of the N atom. However, neither in this unfavorable case are the magnitudes defining the dynamics of the adsorbates at the same time scale significantly modified from those obtained at constant surface temperature.

For impenetrable electromagnetic surfaces, a metasurface design approach for perfect control of the reflection phenomena using gradient anisotropic tensor surface impedance is presented. It utilizes a set of orthogonally polarized auxiliary surface waves to create pointwise reactive impedance characteristics by channeling power along the tangential direction of the surface in the near zone in a carefully designed manner. The propagating incident and reflected fields do not interfere with the surface waves due to the polarization orthogonality. Design examples of an anomalous reflector and a power splitter for an incident plane wave are presented and numerically verified. Realization possibilities using an array of rotated metallic resonators on a thin grounded dielectric substrate are discussed.

Metal nitride has robust bonding between metal and nitrogen atoms. We have synthesized a monoatomic layer of a hexagonal-type iron nitride with strong Fe-N bond on the Cu(001) substrate with a fourfold lattice symmetry. From atomic-scale observation using scanning tunneling microscopy, we have found that the Fe-N film exhibits a strip structure consisting of regular and deformed hexagonal lattices. The lattices are deformed along a hypothetical Moiré pattern made by a perfect hexagonal lattice on the square Cu(001) lattice. With increasing the lattice deformation, the strain energy increases, whereas the periodic deformation of the Fe-N lattice can minimize the total lattice energy including the strain energy of the Fe-N in-plane bonding and interfacial bonding energy between the Fe-N layer and the Cu substrate. In $\mathrm{d}I/\mathrm{d}V$ spectra, periodic spatial change of the unoccupied density of states due to the lattice strain was observed. This is attributed to an energy shift of the antibonding states due to compressive strain.

The dynamic equations for the fluxes through the superconducting quantum interference devices (SQUIDs) that form a two-dimensional metamaterial on a Lieb lattice are derived and then linearized around zero flux to obtain the linear frequency spectrum according to the standard procedure. That spectrum due to the Lieb lattice geometry possesses a frequency band structure exhibiting two characteristic features: two dispersive bands, which form a Dirac cone at the corners of the first Brillouin zone and a flat band crossing the Dirac points. It is demonstrated numerically that localized states can be excited in the system when it is initialized with single-site excitations; depending on the amplitude of those initial states, the localization is either due to the flat-band or to nonlinear effects. Flat-band localized states are formed in the nearly linear regime, whereas localized excitations of the discrete breather type are formed in the nonlinear regime. These two regimes are separated by an intermediate turbulent regime for which no localization is observed. Notably, initial single-site excitations of only edge SQUIDs of a unit cell may end up in flat-band localized states; no such states are formed for initial single-site excitations of a corner SQUID of a unit cell. The degree of localization of the resulting states is in any case quantified using well-established measures, such as the energetic participation ratio and the second moment.

We study the temperature-dependent static and dynamic magnetic properties of polycrystalline bilayers of permalloy ($\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$, or Py) and gadolinium (Gd) bilayers using DC magnetometry and broadband ferromagnetic resonance. Magnetometry measurements reveal that the 3-nm-thick Gd layers undergo a magnetic ordering transition below 100 K, consistent with finite size suppression of their Curie temperature. Upon cooling below this Gd ordering temperature, ferromagnetic resonance spectroscopy reveals a sharp increase in both the gyromagnetic ratio (γ) and effective Gilbert damping parameter (${\alpha }_{\mathrm{eff}}$) of the neighboring Py layers. The increase of γ is attributed to the onset of strong antiferromagnetic coupling between the Gd and Py layers as the Gd orders magnetically. We argue that the increase of ${\alpha }_{\mathrm{eff}}$, on the other hand, can be explained by spin pumping into the rare-earth layer when taking into account the increase of γ, the decrease of the Gd spin diffusion length as it orders, and, most significantly, the corresponding increase of the Py/Gd interfacial spin mixing conductance in the vicinity of the magnetic ordering transition. We propose that these observations constitute a qualitative confirmation of a recent theoretical prediction of spin sinking enhancement in this situation.

The influence of a strain-induced uniaxial magnetoelastic anisotropy on the magnetic vortex core dynamics in microstructured magnetostrictive ${\mathrm{Co}}_{40}{\mathrm{Fe}}_{40}{\mathrm{B}}_{20}$ elements was investigated with time-resolved scanning transmission x-ray microscopy. The measurements revealed a monotonically decreasing eigenfrequency of the vortex core gyration with the increasing magnetoelastic anisotropy, which follows closely the predictions from micromagnetic modeling.

The comprehensive magnetic field–temperature ($H\text{−}T$) phase diagram of solid oxygen including the $\theta$ phase is discussed in the context of the ultrahigh-field measurement and the magnetocaloric effect (MCE) measurement. The problems originating from the short duration of the pulse field, nonequilibrium conditions and MCEs, are pointed out and dealt with. The obtained phase diagram manifests the entropy relation between the phases as $S_{\theta }\sim S_{\alpha }<S_{\beta }\ll S_{\gamma }$.

Many-body localization (MBL) is an emergent phase in correlated quantum systems with promising applications, particularly in quantum information. Here, we unveil the existence and analyze this phase in a chiral multiferroic model system. Conventionally, MBL occurrence is traced via level statistics by implementing a standard finite-size scaling procedure. Here, we present an approach based on the full distribution of the ratio of adjacent energy spacings. We find a strong broadening of the histograms of counts of these level spacings directly at the transition point from MBL to the ergodic phase. The broadening signals reliably the transition point without relying on an averaging procedure. The fast convergence of the histograms even for relatively small systems allows monitoring the MBL dynamics with much less computational effort. Numerical results are presented for a chiral spin chain with a dynamical Dzyaloshinskii-Moriya interaction, an established model to describe the spin excitations in a single-phase spin-driven multiferroic system. The multiferroic MBL phase is uncovered and it is shown how to steer it via electric fields.