A family of finite-dimensional quantum systems with a nondegenerate ground state gives rise to a closed two-form on the parameter space, the curvature of the Berry connection. Its integral over a surface detects the presence of degeneracy points inside the volume enclosed by the surface. We seek generalizations of the Berry curvature to gapped many-body systems in $D$ spatial dimensions which can detect gapless or degenerate points in the phase diagram of a system. Field theory predicts that in spatial dimension $D$ the analog of the Berry curvature is a closed $(D+2)$-form on the parameter space (the Wess-Zumino-Witten form). We construct such closed forms for arbitrary families of gapped interacting lattice systems in all dimensions. We show that whenever the integral of the Wess-Zumino-Witten form over a $(D+2)$-dimensional surface in the parameter space is nonzero, there must be gapless edge modes for at least one value of the parameters. These edge modes arise even when the bulk system is in a trivial phase for all values of the parameters and are protected by the nontrivial topology of the phase diagram.

We propose to use residual anyons of the overscreened Kondo effect for topological quantum computation. A superconducting proximity gap of $\mathrm{\Delta }<T_K$ can be utilized to isolate the anyon from the continuum of excitations and stabilize the nontrivial fixed point. We use weak-coupling renormalization group, dynamical large-N technique, and bosonization to show that the residual entropy of multichannel Kondo impurities survives in a superconductor. We find that while (in agreement with recent numerical studies) the nontrivial fixed point is unstable against intrachannel pairing, it is robust in the presence of a finite interchannel pairing. Based on this observation, we suggest a superconducting charge Kondo setup for isolating and detecting the Majorana fermion in the two-channel Kondo system.

The capability of collective excitations, such as localized surface plasmon resonances, to produce a versatile spectrum of optical phenomena is governed by the interactions within the collective and single-particle responses in the finite system. In many practical instances, plasmonic metallic nanoparticles and arrays are either topologically or chemically heterogeneous, which affects both the constituent transitions and their interactions. Here, the formation of collective excitations in weakly Cu- and Pd-doped Au nanoarrays is described using time-dependent density functional theory. The additional impurity-induced modes in the optical response can be thought to result from intricate interactions between separated excitations or transitions. We investigate the heterogeneity at the impurity level, the symmetry aspects related to the impurity position, and the influence of the impurity position on the confinement phenomena. The chemically rich and symmetry-dependent quantum mechanical effects are analyzed with transition contribution maps demonstrating the possibility to develop nanostructures with more controlled collective properties.

We study spatial noise correlations in a Si/SiGe two-qubit device with integrated micromagnets. Our method relies on the concept of decoherence-free subspaces, whereby we measure the coherence time for two different Bell states, designed to be sensitive only to either correlated or anticorrelated noise, respectively. From these measurements we find weak correlations in low-frequency noise acting on the two qubits, while no correlations could be detected in high-frequency noise. We expect nuclear spin noise to have an uncorrelated nature. A theoretical model and numerical simulations give further insight into the additive effect of multiple independent (anti)correlated noise sources with an asymmetric effect on the two qubits as can result from charge noise. Such a scenario in combination with nuclear spins is plausible given the data and the known decoherence mechanisms. This work is highly relevant for the design of optimized quantum error correction codes for spin qubits in quantum dot arrays, as well as for optimizing the design of future quantum dot arrays.

Coulomb interaction is of central importance in localized energy levels (bound states) or electronic flat bands and can result in many exotic quantum phases. In a graphene monolayer, the relativistic massless Dirac fermion nature of the charge carriers enables us to realize unprecedented quasibound states, which are trapped temporarily via whispering-gallery modes (WGMs), in circular graphene quantum dots (GQDs). Here we show that Coulomb interaction still plays a dominating role in determining the electronic properties of the temporarily confined quasibound states with the lifetime (trapping time) of ∼10 fs. Our scanning tunneling microscopy and spectroscopy measurements demonstrate that the discrete quasibound state in a GQD will split into two peaks when it is partially filled. The energy separation of the two split peaks increases linearly with inverse effective radius of the GQDs, indicating that the splitting arises from the Coulomb interaction. Moreover, we show that the real-space distribution of the two split states separates in different regions of the GQD to reduce the Coulomb interaction, leading to the breaking of the WGM of the quasibound states.

We performed scanning tunneling microscopy (STM) and spectroscopy on a (001) surface of the ferromagnetic semimetal ${\mathrm{EuB}}_6$. Large-amplitude oscillations emanating from the elastic scattering of electrons by the surface impurities are observed in topography and in differential conductance maps. Fourier transform of the conductance maps embracing these regions indicate a holelike dispersion centered around the $\overline{\mathrm{\Gamma }}$ point of the two-dimensional Brillouin zone. Using density functional theory slab calculations, we identify a spin-split surface state, which stems from the dangling $p_z$ orbitals of the apical boron atom. Hybridization with bulk electronic states leads to a resonance enhancement in certain regions around the $\overline{\mathrm{\Gamma }}$ point, contributing to the remarkably strong real-space response around static point defects, which are observed in STM measurements.

The emerging quantum technological applications call for fast and accurate initialization of the corresponding devices to low-entropy quantum states. To this end, we theoretically study a recently demonstrated quantum-circuit refrigerator in the case of nonlinear quantum electric circuits such as superconducting qubits. The maximum refrigeration rate of transmon and flux qubits is observed to be roughly an order of magnitude higher than that of usual linear resonators, increasing flexibility in the design. We find that for typical experimental parameters, the refrigerator is suitable for resetting different qubit types to fidelities above 99.99% in a few or a few tens of nanoseconds depending on the scenario. Thus the refrigerator appears to be a promising tool for quantum technology and for detailed studies of open quantum systems.

Motivated by the recent discovery of superconductivity in infinite-layer $(\text{Sr},\mathrm{Nd}){\mathrm{NiO}}_2$ films with Sr content $x\simeq 0.2$ [D. Li et al., Nature (London) 572, 624 (2019)], we examine the effects of electron correlations and Sr doping on the electronic structure, Fermi-surface topology, and magnetic correlations in $(\text{Nd},\mathrm{Sr}){\mathrm{NiO}}_2$ using a combination of dynamical mean-field theory of correlated electrons and band-structure methods. Our results reveal a remarkable orbital-selective renormalization of the Ni $3d$ bands, with $m^{*}/m\sim 3$ and 1.3 for the $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals, respectively, that suggests orbital-dependent localization of the Ni $3d$ states. We find that upon hole doping, $(\text{Nd},\mathrm{Sr}){\mathrm{NiO}}_2$ undergoes a Lifshitz transition of the Fermi surface which is accompanied by a change of magnetic correlations from three-dimensional (3D) Néel $G$-type (111) to quasi-2D $C$-type (110). We show that magnetic interactions in $(\text{Nd},\mathrm{Sr}){\mathrm{NiO}}_2$ demonstrate an unanticipated frustration, which suppresses magnetic order, implying the importance of in-plane spin fluctuations to explain its superconductivity. Our results suggest that frustration is maximal for Sr doping $x\simeq 0.1–0.2$, which is in agreement with an experimentally observed doping value Sr $x\simeq 0.2$ of superconducting $(\text{Nd},\mathrm{Sr}){\mathrm{NiO}}_2$.

Electron transfer (ET) in biological molecules, such as peptides and proteins, consists of electrons moving between well-defined localized states (donors to acceptors) through a tunneling process. Here, we present an analytical model for ET by tunneling in DNA in the presence of spin-orbit (SO) interaction to produce a strong spin asymmetry with the intrinsic atomic SO strength in the meV range. We obtain a Hamiltonian consistent with charge transport through $\pi$ orbitals on the DNA bases and derive the behavior of ET as a function of the injection state momentum, the spin-orbit coupling, and barrier length and strength. Both tunneling energies, deep below the barrier and close to the barrier height, are considered. A highly consistent scenario arises where two concomitant mechanisms for spin selection arises; spin interference and differential spin amplitude decay. High spin filtering can take place at the cost of reduced amplitude transmission assuming realistic values for the SO coupling. The spin filtering scenario is completed by addressing the spin-dependent torque under the barrier with a consistent conserved definition for the spin current.

Fracton topological order hosts fractionalized pointlike excitations (e.g., fractons) that have restricted mobility. In this paper, we explore even more bizarre realization of fracton phases that admit spatially extended excitations with restriction on both mobility and deformability. First, we present exactly solvable lattice quantum frustrated spin models and study their ground states and excited states analytically. We construct a family tree in which parent models and descendent models share excitation DNA. Second, with the help of solvability and novel excitation spectrum of these models, we initiate the first step of general discussions on quantitative and qualitative properties of spatially extended excitations whose mobility and deformability are restricted to some extent. Especially, as a useful viewpoint for understanding such fracton physics, all excitations are divided into four mutually distinct sectors, namely, simple excitations, complex excitations, intrinsically disconnected excitations, and trivial excitations. Several implications in, e.g., condensed matter physics and gravity are briefly discussed.

According to time-dependent density functional theory, the exact exchange-correlation kernel $f_{xc}(n,q,\omega )$ for wave vector q and frequency $\omega$ determines not only the ground-state energy but also the excited-state energies/lifetimes and time-dependent linear density response of an electron gas of uniform density $n=3/\left(4\pi r_s^3\right)$. Here we propose a parametrization of this function based upon the satisfaction of exact constraints. For the static $(\omega =0)$ limit, we modify the model of Constantin and Pitarke to recover at small q the known second-order gradient expansion, and to correct its approach to the large q limit. For all $\omega$ at $q=0$, we use the model of Gross, Kohn, and Iwamoto. A Cauchy integral extends this model to complex $\omega$. Scaling relations are identified. We then combine these ingredients, damping out the $\omega$ dependence at large q. Away from $q=0$ and $\omega =0$, the correlation contribution to the kernel becomes dominant over exchange, even at $r_s=4$. The resulting correlation energies for $1\le r_s\le 10$ from integration over imaginary $\omega$ are essentially exact. The plasmon pole of the density response function is found by analytic continuation of $f_{xc}$ to $\omega$ just below the real axis, and the resulting plasmon lifetime at $r_s=$ 4 is found for $q<k_F$. A static charge-density wave is found for $r_s>69$, and shown to be associated with softening of the plasmon mode.

Rocksalt $\mathrm{Z}{\mathrm{n}}_x\mathrm{M}{\mathrm{g}}_{1-x}\mathrm{O}$ alloys are theoretically and experimentally investigated for near- and deep-UV optoelectronics with a tunable band gap of 4.2–7.8 eV. Regarding the key question about the composition $x$, at which there is a transition between the direct and indirect gaps, we performed ab initio calculations for various Zn concentrations and all possible atomic arrangements in eight- to 64-atom supercells. We show that, depending on the detailed Zn distribution (clustered, random, or uniform distribution), the alloy band gap can vary by as much as 1.27 eV. The band gap is indirect for clustered and random Zn arrangements in the supercell. For uniform Zn arrangements, the gap is also indirect, except for $x<0.5$ and atom uniform arrangements excluding Zn-O-Zn nearest neighbor bridges, for which the direct gap can be lowered below the indirect gap by about 0.1 eV. The mechanisms of band-gap fluctuation, Zn clustering, and direct-indirect band-gap transitions are analyzed and explained in terms of atomic contributions to band structures by projecting Bloch functions onto localized Wannier functions. Simultaneously, cathodoluminescence measurements were performed on a set of $\mathrm{Z}{\mathrm{n}}_x\mathrm{M}{\mathrm{g}}_{1-x}\mathrm{O}$ multiquantum wells grown by molecular beam epitaxy on MgO substrates. We observed strong and broad emission bands, redshifting with increasing Zn concentration but featuring no clear-cut evidence for any direct to indirect band-gap crossover. We argue that these alloys are well suited for deep-UV optoelectronics, thanks to the rare combination of strong exciton binding energy, coupling to phonons, and carrier localization, which is favored by the marked flattening of the top valence bands by both short-range and long-range Zn-Zn interactions.

Thin films of topological insulators (TI) usually exhibit multiple parallel conduction channels for the transport of electrical current. Aside from the topologically protected surface states (TSS), parallel channels may exist, namely, the interior of the not-ideally insulating TI film, the interface layer to the substrate, and the substrate itself. To be able to take advantage of the auspicious transport properties of the TSS, the influence of the parasitic parallel channels on the total current transport has to be minimized. Because the conductivity of the interior (bulk) of the thin TI film is difficult to access by measurements, we propose here an approach for calculating the mobile charge carrier concentration in the TI film. To this end, we calculate the near-surface band bending using parameters obtained experimentally from surface-sensitive measurements, namely, (gate-dependent) four-point resistance measurements and angle-resolved photoelectron spectroscopy. While in most cases another parameter in the calculations, i.e., the concentration of unintentional dopants inside the thin TI film, is unknown, it turns out that in the thin-film limit the band bending is largely independent of the dopant concentration in the film. Thus, a well-founded estimate of the total mobile charge carrier concentration and the conductivity of the interior of the thin TI film proves possible. Since the interface and substrate conductivities can be measured by a four-probe conductance measurement prior to the deposition of the TI film, the total contribution of all parasitic channels, and therefore also the contribution of the vitally important TSS, can be determined reliably.

The asymmetric and achiral character of caffeine (${\mathrm{C}}_8{\mathrm{H}}_{10}{\mathrm{N}}_4{\mathrm{O}}_2$) leads to two on-surface chiralities which has an impact on its on-surface formation. An analysis of its on-surface behavior reveals new insights of its crystallite growth. In this study the structural formation of caffeine monolayers on a Au(111) surface was analyzed by scanning tunneling microscopy (STM), low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) calculations. The monolayers were prepared by molecular beam epitaxy (MBE) and analyzed at room temperature. Caffeine molecules self-assemble in a quasihexagonal phase on Au(111) similar to the high-temperature $\alpha$ phase. Two mirrored hexagonal domains are present with respect to the surface. Within the XPS measurements, no strong surface interaction was found. Therefore, a theoretical analysis of a hypothetical free-standing caffeine monolayer structure was performed by ab initio simulations. We found that a caffeine monolayer with three molecules per unit cell is preferable to one with just a single molecule, as could be expected from the LEED pattern.

Two approximations used by Sengupta [Phys. Rev. B 100, 075429 (2019)] in numerically computing the adsorption rate of cold hydrogen atoms on suspended graphene are critically examined. The independent boson model approximation (IBMA) was used to compute the atom self-energy, and the single-pole approximation (SPA) was used to obtain the adsorption rate from the self-energy. It is shown explicitly that there are additional contributions to the self-energy appearing at the same order of the atom-phonon coupling as the IBMA terms that alter the value of the real part of the self-energy at low energies by several orders of magnitude in the regime of interest. This shift in the self-energy, consequently, renders the use of SPA invalid.

Based on a new self-energy for atom-phonon interaction, the preceding Comment [Phys. Rev. B 101, 247401 (2020)] argues about the insufficiency of the mathematical techniques within the independent boson model (IBM) to study physisorption in graphene membranes. In this Reply, I show that the new self-energy reported in the Comment is a perturbative expansion approximated for a two-phonon process, severely divergent for membrane sizes larger than 100 nm and within its current mathematical form, ill suited for investigating the physics of physisorption in graphene micromembranes. Additionally, I provide further evidence of the adsorption rate within the IBM that reinforces the physical soundness of the mathematical techniques reported in Phys. Rev. B 100, 075429 (2019).

${}^7\mathrm{Li}$ nuclear magnetic resonance and terahertz (THz) spectroscopies are used to probe magnetic excitations and their field dependence in the hyperhoneycomb Kitaev magnet $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3$. Spin-lattice relaxation rate ($1/T_1$) measured down to 100 mK indicates the gapless nature of the excitations at low fields (below $H_c\simeq 2.8$ T), in contrast to the gapped magnon excitations found in the honeycomb Kitaev magnet $\alpha -{\mathrm{RuCl}}_3$ at zero applied magnetic field. At higher temperatures in $\beta -{\mathrm{Li}}_2{\mathrm{IrO}}_3, 1/T_1$ passes through a broad maximum without any clear anomaly at the Néel temperature $T_N\simeq 38$ K, suggesting the abundance of low-energy excitations that are indeed observed as two peaks in the THz spectra; both correspond to zone-center magnon excitations. At higher fields (above $H_c$), an excitation gap opens, and a redistribution of the THz spectral weight is observed without any indication of an excitation continuum, in contrast to $\alpha -{\mathrm{RuCl}}_3$ where an excitation continuum was reported.

We studied the infrared response of polycrystalline samples of the iron arsenide superconductor $(\mathrm{Rb},\mathrm{Cs}){\mathrm{Ca}}_2{\mathrm{Fe}}_4{\mathrm{As}}_4{\mathrm{F}}_2$ (Rb,Cs-12442), which has a bilayer structure similar to the high-$T_c$ cuprates ${\mathrm{YBa}}_2{\mathrm{Cu}}_3{\mathrm{O}}_7$ (YBCO) and ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_8$. The $c$-axis reflectivity spectra $R_c$ have been derived from the reflectivity spectra of the polycrystalline samples $R_{\mathrm{poly}}$ and the in-plane spectrum of a corresponding Cs-12442 crystal $R_{ab}$ using a geometrical averaging approach with $R_c=3R_{\mathrm{poly}}-2R_{ab}$. In analogy to the $c$-axis response of the cuprates, we observe a superconductivity-induced transverse plasma mode and a phonon anomaly that are both signatures of local electric field effects that arise from a large difference between the local conductivities in the intra- and interbilayer regions. Using a multilayer model developed for the cuprates, we obtain a good description of the $c$-axis response and derive the local conductivities at $T\simeq T_c$ of ${\sigma }_1^{\mathrm{bl}}(\omega \to 0)\simeq 1000{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$ and ${\sigma }_1^{\mathrm{int}}(\omega \to 0)\simeq 15{\mathrm{\Omega }}^{-1}{\mathrm{cm}}^{-1}$, respectively, that are similar to the ones previously found in underdoped YBCO. Different from the cuprates, we find no evidence of a normal-state pseudogap in terms of a partial suppression of the low-energy electronic states that sets in already well above $T_c$. There is also no clear sign of an onset of precursor superconducting pairing correlations well above $T_c\simeq$ 30 K. This highlights that the pseudogap and the precursor superconducting pairing well above $T_c$ are unique features of the cuprates with their strong electronic correlations and, for example, not just the result of a strongly anisotropic electronic response due to the layered crystal structure.

Insight into why superconductivity in pristine and doped monolayer graphene seems strongly suppressed has been central for the recent years' various creative approaches to realize superconductivity in graphene and graphene-like systems. We provide further insight by studying electron-phonon coupling and superconductivity in doped monolayer graphene and hexagonal boron nitride based on intrinsic phonon modes. Solving the graphene gap equation using a detailed model for the effective attraction based on electron tight binding and phonon force constant models, the various system parameters can be tuned at will. Consistent with results in the literature, we find slight gap modulations along the Fermi surface, and the high-energy phonon modes are shown to be the most significant for the superconductivity instability. The Coulomb interaction plays a major role in suppressing superconductivity at realistic dopings. Motivated by the direct onset of a large density of states at the Fermi surface for small charge dopings in hexagonal boron nitride, we also calculate the dimensionless electron-phonon coupling strength there, but the comparatively large density of states cannot immediately be capitalized on, and the charge doping necessary to obtain significant electron-phonon coupling is similar to the value in graphene.