Quantum criticality emerges from the collective behavior of many interacting quantum particles, often at the transition between different phases of matter. It is one of the cornerstones of condensed matter physics, which we access on noisy intermediate-scale (NISQ) quantum devices by leveraging a dynamically driven phenomenon. We probe the critical properties of the one-dimensional quantum Ising model on a programmable superconducting quantum chip via a Kibble-Zurek process, obtain scaling laws, and estimate critical exponents despite inherent sources of errors on the hardware. In addition, we investigate how the improvement of NISQ computers (more qubits, less noise) will consolidate the computation of those universal physical properties. A one-parameter noise model captures the effect of imperfections and reproduces the experimental data. Its systematic study reveals that the noise, analogously to temperature, induces a new length scale in the system. We introduce and successfully verify modified scaling laws, directly accounting for the noise without any prior knowledge. It makes data analyses for extracting physical properties transparent to noise. By understanding how imperfect quantum hardware modifies the genuine properties of quantum states of matter, we enhance the power of NISQ processors considerably for addressing quantum criticality and potentially other phenomena and algorithms.

Several studies have reported the possible growth of germanene on various metal surfaces. However, the exact structure of the layers formed upon evaporation at or above room temperature remains controversial. In particular, the layers obtained after Ge deposition on Al(111) have been either considered as honeycomb organization of Ge atoms on top of a nonreconstructed substrate, or as alloyed layers. Using quantitative measurements by surface x-ray diffraction compared to density functional theory calculations, we show that the $\left(\sqrt{7}\times \sqrt{7}\right)R\pm 19.1^{\circ }$ reconstruction obtained after room temperature deposition is a mixed Ge-Al honeycomb layer, on top of an Al(111) plane, and not a pure germanene layer.

Magnetic susceptibility data obtained above the magnetic critical temperatures of the intercalated transition metal dichalcogenides ${\mathrm{Cr}}_x{\mathrm{NbSe}}_2$ and ${\mathrm{Fe}}_x{\mathrm{TiS}}_2$ are indicative of the presence of short-range magnetic correlations (clusters) in a wide temperature range in the nominally paramagnetic state. Bearing in mind that the Cr and Fe atoms located between Se-Nb-Se and S-Ti-S tri-layers, respectively, are diluted with vacancies, these compounds can be considered as appropriate systems for the appearance of the so-called Griffiths phase. To identify the Griffiths behavior among the intercalated materials, the consistency of the magnetization behavior of these compounds, with that expected for the Griffiths phase, was tested using the scaling law proposed by Chan et al. [Phys. Rev. Lett. 97, 137201 (2006)]. As a result, the compounds ${\mathrm{Cr}}_x{\mathrm{NbSe}}_2$ with $x=0.33$ and $x=0.45$ and ${\mathrm{Fe}}_{0.25}{\mathrm{TiS}}_2$ are found to exhibit the Griffiths phase above magnetic ordering temperatures while other compounds demonstrate non-Griffiths behavior.

Crystal phase quantum dots are formed during the nanowire growth by vertically stacking distinct crystal phases of the same chemical compound. In this Letter we show, using an atomistic many-body approach, that InP crystal phase quantum dots may exhibit a peculiar and rare antibonding hole ground state. Interestingly, even small strains due to a wurtzite–zinc-blende lattice mismatch—which is often neglected—can strongly affect the properties of the lowest hole states, resulting in unusual double-peak features in the excitonic optical spectra.

“Pumping” of phonons by a dynamic magnetization promises to extend the range and functionality of magnonic devices. We explore the impact of phonon pumping on room-temperature ferromagnetic resonance (FMR) spectra of bilayers of thin yttrium iron garnet films on thick gadolinium gallium garnet substrates over a wide frequency range. At low frequencies the Kittel mode hybridizes with standing ultrasound waves across the layer stack that acts as a bulk acoustic resonator to form magnon polarons with rapid oscillations in the magnetic susceptibility, as reported before. At higher frequencies, the individual phonon resonances overlap due to their increasing acoustic attenuation, leading to an additional slowly oscillating phonon pumping contribution to the FMR line shape. The broadband frequency dependence of the magnetoelastic coupling strength follows the predictions from phonon pumping theory in the thick substrate limit. In addition, we find substantial magnon-phonon coupling of a perpendicular standing spin wave mode. This evidences the importance of the mode overlap between the acoustic and magnetic modes and provides a route towards engineering the magnetoelastic mode coupling.

Piezoresistance, the change of the electrical resistance ($R$) of a material in response to an applied mechanical stress ($\sigma$), is the driving principle of electromechanical devices such as strain gauges, accelerometers, and cantilever force sensors. Enhanced piezoresistance has been traditionally observed in two classes of uncorrelated materials: nonmagnetic semiconductors and composite structures. We report the discovery of a remarkably large piezoresistance in ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$ single crystals, wherein anisotropic metallic clusters naturally form within a semiconducting matrix due to electronic interactions. ${\mathrm{Eu}}_5{\mathrm{In}}_2{\mathrm{Sb}}_6$ shows a highly anisotropic piezoresistance, and uniaxial pressure along [001] of only 0.4 GPa leads to a resistivity drop of >99.95%, which results in a colossal piezoresistance factor of $5000\times {10}^{-11}{\mathrm{Pa}}^{-1}$. Our result not only reveals the role of interactions and phase separation in the realization of colossal piezoresistance, but it also highlights a route to multifunctional devices with large responses to both pressure and magnetic field.

Structure prediction has become a key task of the modern atomistic sciences and depends on the rapid and reliable computation of energy landscapes. First-principles density functional based calculations are highly reliable, faithfully describing entire energy landscapes. They are, however, computationally intensive and slow compared to interatomic potentials. Great progress has been made in the development of machine learning, or data derived, potentials, which promise to describe entire energy landscapes at first-principles quality. Compared to first-principles approaches, their preparation can be time consuming and delay searching. Ab initio random structure searching (AIRSS) is a straightforward and powerful approach to structure prediction, based on the stochastic generation of sensible initial structures and their repeated local optimization. Here, a scheme, compatible with AIRSS, for the rapid construction of disposable, or ephemeral, data derived potentials (EDDPs) is described. These potentials are constructed using a homogeneous, separable many-body environment vector and iterative neural network fits, sparsely combined through non-negative least squares. The approach is first tested on methane, boron nitride, elemental boron, and urea. In the case of boron, an EDDP generated using data from small unit cells is used to rediscover the complex $\gamma$-boron structure without recourse to symmetry or fragments. Finally, an EDDP generated for silane ($\mathrm{Si}{\mathrm{H}}_4$) at 500 GPa enables the discovery of an extremely complex, dense structure which significantly modifies silane's high pressure phase diagram. This has implications for the theoretical exploration for high temperature superconductivity in dense hydrides, which have so far largely depended on searches in smaller unit cells.

We synthesize homogeneous compositions of $\mathrm{Na}{\mathrm{Lu}}_x{\mathrm{Yb}}_{1\text{−}x}{\mathrm{Se}}_2$, connecting nonmagnetic $\mathrm{Na}\mathrm{Lu}{\mathrm{Se}}_2$ to the triangular lattice spin liquid candidate $\mathrm{Na}\mathrm{Yb}{\mathrm{Se}}_2$. Thermal and magnetic properties are studied as the system evolves from one with dilute magnetic defects to one of a dense magnetic lattice. The field- and temperature-dependent heat capacity show the carriers of entropy cross over from isolated magnetic ions to a correlated lattice born from spin dimers. For the dilute system we estimate the single-ion anisotropy $(g_{\perp }/g_{\parallel }=3.13)$ and also the dimer exchange couplings $J_{\parallel }(=5.4 \mathrm{K})$ and $J_{\perp }(=9.6 \mathrm{K})$, in order to draw comparison to the half-doped and full magnetic compounds.

We study geometric energy transport in a slowly driven single-level quantum dot weakly coupled to electronic contacts and with strong on-site interaction, which can be either repulsive or attractive. Exploiting a recently discovered fermionic duality for the evolution operator of the master equation, we provide compact and insightful analytic expressions of energy pumping curvatures for any pair of driving parameters. This enables us to systematically identify and explain the pumping mechanisms for different driving schemes, thereby also comparing energy and charge pumping. We determine the concrete impact of many-body interactions and show how particle-hole symmetry and fermionic duality manifest, both individually and in combination, as system-parameter symmetries of the energy pumping curvatures. Building on this transport analysis, we study the driven dot acting as a heat pump or refrigerator, where we find that the sign of the on-site interaction plays a crucial role in the performance of these thermal machines.

Neutron diffraction and nuclear magnetic resonance (NMR) measurements have been used to analyze the magnetic structure in the $S=\frac{1}{2}$ kagome antiferromagnet ${\mathrm{CdCu}}_3{\left(\mathrm{OH}\right)}_6{\left({\mathrm{NO}}_3\right)}_2·{\mathrm{H}}_2\mathrm{O}$. Below the magnetic ordering temperature $T_{\text{N}}\simeq 4\mathrm{K}$, magnetic Bragg reflections at (101) and (100) were found in the neutron diffraction pattern, which suggests a $\mathbf{q}=0$ magnetic structure. Furthermore, the vector spin chirality for the $\mathbf{q}=0$ structure was successfully identified from the internal field direction obtained by the ${}^{14}\mathrm{N}$-NMR measurement, previous magnetization measurements, and magnetic symmetry analysis. Our findings point to a chirality-ordered magnetic structure with negative vector chirality and $\langle 100\rangle$ anisotropy.

We photoexcite high-energy holon-doublon pairs as a way to alter the magnetic free energy landscape and resulting phase diagram of the frustrated honeycomb magnet $\alpha \text{−}{\mathrm{RuCl}}_3$. The pair recombination through multimagnon emission is tracked through the time evolution of the magnetooptical response originating from $\alpha \text{−}{\mathrm{RuCl}}_3$'s competing zigzag spin-ordered ground state. A small holon-doublon density suffices to reach a spin-disordered state. The phase transition is described within a dynamical Ginzburg-Landau framework, corroborating the quasistationary nature of the transient phase. Our work suggests a new route to reach a nontrivial spin-disordered state in Kitaev-like magnets.

We provide a classification of invertible topological phases of interacting fermions with symmetry in two spatial dimensions for general fermionic symmetry groups $G_f$ and general values of the chiral central charge $c_{-}$. Here $G_f$ is a central extension of a bosonic symmetry group $G_b$ by fermion parity, ${(-1)}^F$, specified by a second cohomology class $\left[{\omega }_2\right]\in {\mathcal{H}}^2(G_b,{\mathbb{Z}}_2)$. Our approach proceeds by gauging fermion parity and classifying the resulting $G_b$ symmetry-enriched topological orders while keeping track of certain additional data and constraints. We perform this analysis through two perspectives, using $G$-crossed braided tensor categories and $\mathrm{Spin}{\left(2c_{-}\right)}_1$ Chern-Simons theory coupled to a background $G$ gauge field. These results give a way to characterize and classify invertible fermionic topological phases in terms of a concrete set of data and consistency equations, which is more physically transparent and computationally simpler than the more abstract methods using cobordism theory and spectral sequences. Our results also generalize and provide a different approach to the recent classification of fermionic symmetry-protected topological phases by Wang and Gu, which have chiral central charge $c_{-}=0$. We show how the tenfold way classification of topological insulators and superconductors fits into our scheme, along with general nonperturbative constraints due to certain choices of $c_{-}$ and $G_f$. Mathematically, our results also suggest an explicit general parametrization of deformation classes of $(2+1)$D invertible topological quantum field theories with $G_f$ symmetry.

In the large family of two-dimensional (2D) layered materials including graphene, its honeycomb analogs, and transition-metal dichalcogenides, the interlayer coupling plays a rather intriguing role. On the one hand, the weak van der Waals interaction that holds the layers together endows these compounds with quasi-2D properties, which might imply small interlayer effects on the electronically active bands. On the other hand, the oft-witnessed differences in electronic, optical, and magnetic behaviors of monolayers, bilayers, and multilayers of the same compound must have as their microscopic origin the detailed interlayer hopping parameters. Given the few experimental reports that have attempted to explicitly extract these parameters, we employ soft x-ray angle-resolved photoemission spectroscopy (SX-ARPES) to probe the interlayer coupling in superconducting $2H\text{−}{\mathrm{NbS}}_2$. We visualize the S $3p_z$ bands that disperse with respect to the out-of-plane momentum and introduce a simple tight-binding model to extract the interlayer hopping parameters. From first-principles calculations, we clarify how atomic distances and the proper accounting for screening via hybrid functionals influence these bands. The knowledge of interlayer hopping parameters is particularly pertinent in ${\mathrm{NbS}}_2$, where recent experiments have uncovered fingerprints of finite-momentum superconductivity in the bulk material and heterostructures.

The renormalization group equations describing a finite 2DXY (two-dimensional XY) system with fourfold anisotropy are solved in two steps, in order to study the magnetic transition to paramagnetism in an ultrathin film. First, the equations are linearized near the critical coupling $K=J/k_{B}T=2/\pi$. This allows integration constants to be evaluated at the fixed point, and the tuning of the constants to represent a ferromagnetic ultrathin film. An exact solution of the linearized equations confirms that a finite-size Kosterlitz-Thouless (KT) transition occurs in the presence of weak fourfold anisotropy, and that an Ising transition occurs for strong anisotropy. The behavior of a given system, and the crossover region between these two types of transitions, is determined by the system parameters through the product of an anisotropy parameter and the logarithm of the system size. The linearized RG equations are not quantitatively reliable across the extended temperature range of the finite-size transition, but they do define the parameter space where a second approach, where the fourfold anisotropy is treated as a perturbation, is valid. This perturbative treatment provides a quantitative determination of the renormalized exchange coupling, vortex density, and anisotropy throughout the transition. In particular, the coupling has a universal point of inflection where vortex-antivortex pairs unbind (as opposed to a “universal jump”), and goes to zero asymptotically in the paramagnetic state, as is expected for a finite system. These results are used to calculate the magnetic susceptibility as the system moves from one dominated by spin waves to one dominated by a free vortex gas. The presence of anisotropy makes it necessary to include both the susceptibility ${\chi }_{\left|\right|}$ due to fluctuations of the magnitude of the magnetization, and ${\chi }_{\perp }$ due to angular fluctuations of the magnetization about a fourfold easy axis. Comparison to recent measurements of the magnetic susceptibility of ultrathin Fe/W(001) films suggests that a detailed quantitative analysis of the experimental results can provide information on vortex formation, disappearance of anisotropy, and dissipative processes in the finite-size KT transition of a real system.

We put forward a phenomenological theory for entanglement dynamics in monitored quantum many-body systems with well-defined quasiparticles. Within this theory entanglement is carried by ballistically propagating non-Hermitian quasiparticles which are stochastically reset by the measurement protocol with a rate given by their finite inverse lifetime. We write down a renewal equation for the statistics of the entanglement entropy and show that, depending on the spectrum of quasiparticle decay rates, different entanglement scalings can arise and even sharp entanglement phase transitions. When applied to a quantum Ising chain where the transverse magnetization is measured by quantum jumps, our theory predicts a critical phase with logarithmic scaling of the entanglement, an area-law phase and a continuous phase transition between them, with an effective central charge vanishing as a square root at the transition point. We compare these predictions with exact numerical calculations on the same model and find an excellent agreement.

We consider a two-dimensional disordered conductor in the regime when the superconducting phase is destroyed by the magnetic field. We analyze a combination of fluctuations of different origin which results in an effective interaction amplitude suitable for a spontaneous $s$-wave odd-frequency pairing instability. We observe that the end point of the superconductivity is a quantum critical point separating the conventional superconducting phase from a state with the odd-frequency spin-triplet pairing instability. We speculate that this could shed light on a rather mysterious insulating state observed in strongly disordered superconducting films in a broad region of the magnetic fields.

Spin-orbit interaction in low-dimensional systems, namely, Rashba systems and the edge states of topological materials, has been extensively studied in this decade as a promising source to realize various fascinating spintronic phenomena, such as the source of the spin current and spin-mediated energy conversion. Here, we show the odd fluctuation in the spin-orbital texture in a surface Rashba system on Bi/InAs(110)-($2\times 1$) by spin- and angle-resolved photoelectron spectroscopy and a numerical simulation based on a density-functional theory (DFT) calculation. The surface state shows a paired parabolic dispersion with the spin degeneracy lifted by the Rashba effect. Although its spin polarization should be fixed in a particular direction based on the Rashba model, the observed spin polarization varies greatly and even reverses its sign depending on the wave number. DFT calculations also reveal that the spin directions of two inequivalent Bi chains on the surface change from nearly parallel (canted parallel) to antiparallel in real space in the corresponding wave vector region. These results point out an oversimplification of the nature of spin in Rashba and Dirac systems and provide more freedom than expected for spin manipulation of photoelectrons.

We present conditional probability (CP) density functional theory (DFT) as a formally exact theory. In essence, CP-DFT determines the ground-state energy of a system by finding the CP density from a series of independent Kohn-Sham (KS) DFT calculations. By directly calculating CP densities, we bypass the need for an approximate XC energy functional. In this paper we discuss and derive several key properties of the CP density and corresponding CP-KS potential. Illustrative examples are used throughout to help guide the reader through the various concepts and theory presented. We explore a suitable CP-DFT approximation and discuss exact conditions, limitations, and results for selected examples.

It is known that a Bloch domain wall in an anisotropic ferromagnet is transparent to spin waves. This result is derived by approximating the dipolar interaction between magnetic moments by an effective anisotropy interaction. In this paper we study the scattering of spin waves by a domain wall, taking into account the full complexity of the dipolar interaction, treating it perturbatively in the distorted wave Born approximation. Due to the peculiarities of the dipolar interaction, the implementation of this approximation is not straightforward. The difficulties are circumvented here by realizing that the contribution of the dipolar interaction to the spin wave operator can be split into two terms: (i) an operator that commutes with the spin wave operator in the absence of dipolar interaction and (ii) a local operator suitable to be treated as a perturbation in the distorted wave Born approximation. We analyze the scattering parameters obtained within this approach. It turns out that the reflection coefficient does not vanish in general, and that the transmitted waves suffer a lateral shift which is of order one (not infinitesimal) even at nearly normal incidence. This lateral shift can be greatly enhanced by making the spin wave go through an array of well-separated domain walls. The outgoing spin wave will not be appreciably attenuated by the scattering at the domain walls since the transmission coefficient is very close to 1 at nearly normal incidence. This effect may be very useful to control the spin waves in magnonic devices.

We use two-photon excitation to create biexcitonic complexes with two electron-hole pairs in different orbital levels of a GaAs/AlAs quantum dot. In addition to $p$-shell emission of the biexcitonic triplet states generated by two-photon excitation, we observe additional higher-energy resonances which are a signature of the radiative cascade of two-photon excited singlet states. The detection of these signals obtained in a high excitation regime is made possible by the use of a waveguiding structure in which the quantum dots are inserted, allowing an orthogonal excitation and detection geometry with an excellent laser rejection.