Chalcogenide semiconductors and semimetals are a fertile class of efficient thermoelectric materials, which, in most cases, exhibit very low lattice thermal conductivity ${\kappa }_{\mathrm{ph}}$ despite lacking a complex crystal structure such as the tetragonal binary compound InTe. Our measurements of ${\kappa }_{\mathrm{ph}}\left(T\right)$ in single-crystalline InTe along the $c$ axis show that ${\kappa }_{\mathrm{ph}}$ exhibits a smooth temperature dependence upon cooling to about 50 K, the temperature below which a strong rise typical for dielectric compounds is observed. Using a combination of first-principles calculations, inelastic neutron scattering (INS), and low-temperature specific heat and transport properties measurements on single-crystalline InTe, we show that the phonon spectrum exhibits well-defined acoustic modes, the energy dispersions of which are constrained to low energies due to distributions of dispersionless, optical modes, which are responsible for a broad double peak structure in the low-temperature specific heat. The latter are assigned to the dynamics of ${\mathrm{In}}^{+}$ cations in tunnels formed by edge-sharing ${\left({\mathrm{In}}^{3+}{{\mathrm{Te}}_4}^{2-}\right)}^{-}$ tetrahedra chains, the atomic thermal displacement parameters of which, probed as a function of temperature by means of single-crystal x-ray diffraction, suggest the existence of a complex energy potential. Indeed, the ${\mathrm{In}}^{+}$-weighted optical modes are not observed by INS, which is ascribed to the anharmonic broadening of their energy profiles. While the low ${\kappa }_{\mathrm{ph}}$ value of $1.2\mathrm{W}{\mathrm{m}}^{-1}{\mathrm{K}}^{-1}$ at 300 K originates from the limited energy range available for acoustic phonons, we show that the underlying mechanism is specific to InTe and argue that it is likely related to the presence of local disorder induced by the ${\mathrm{In}}^{+}$ site occupancy.

Zirconia $\left({\mathrm{ZrO}}_2\right)$ has been well studied experimentally for decades, but still poses a severe challenge for computational approaches. We present thorough many-electron benchmark calculations within the random-phase approximation framework of the phase stabilities of the most common ${\mathrm{ZrO}}_2$ phases and assess the performance of various density functional theory (DFT) and beyond-DFT methods. We find that the commonly used DFT and hybrid functionals strongly overestimate both the energetic differences of the common phases and the stability of two metastable phases. The many-electron calculations offer a significantly improved description of the predicted bulk properties, especially of the bulk modulus $B_0$. On the DFT level, the van der Waals corrected meta-generalized-gradient approximation (SCAN-rVV10) provides much better agreement with the experimental values than other (semi)local and hybrid approaches.

The reduced dimensionality in two-dimensional materials leads to a wealth of unusual properties, which are currently explored for both fundamental and applied sciences. In order to study the crystal structure, edge states, the formation of defects and grain boundaries, or the impact of adsorbates, high-resolution microscopy techniques are indispensable. Here we report on the development of an electron holography (EH) transmission electron microscopy (TEM) technique, which facilitates high spatial resolution by an automatic correction of geometric aberrations. Distinguished features of EH beyond conventional TEM imaging are gap-free spatial information signal transfer and higher dose efficiency for certain spatial frequency bands as well as direct access to the projected electrostatic potential of the two-dimensional material. We demonstrate these features with the example of $h$-BN, for which we measure the electrostatic potential as a function of layer number down to the monolayer limit and obtain evidence for a systematic increase of the potential at the zig-zag edges.

Quantum fluctuations are ubiquitous in physics. Ranging from conventional examples like the harmonic oscillator to intricate theories on the origin of the universe, they alter virtually all aspects of matter, including superconductivity, phase transitions, and nanoscale processes. As a rule of thumb, the smaller the object, the larger its impact. This poses a serious challenge to modern nanotechnology, which aims at total control via atom-by-atom engineered devices. In magnetic nanostructures, high stability of the magnetic signal is crucial when targeting realistic applications in information technology, e.g., miniaturized bits. Here we show that zero-point spin fluctuations play an important role in determining the fundamental magnetic exchange interactions that dictate the nature and stability of the magnetic state. Based on the fluctuation-dissipation theorem, we show that quantum fluctuations correctly account for the large overestimation of the interactions as obtained from conventional static first-principles frameworks, filling in an important gap between theory and experiment [Zhou et al., Nat. Phys. 6, 187 (2010); Khajetoorians et al., Nat. Phys. 8, 497 (2012)]. Our analysis further reveals that zero-point spin fluctuations tend to promote the noncollinearity and stability of chiral magnetic textures such as skyrmions, a counterintuitive quantum effect that inspires practical guidelines for designing disruptive nanodevices.

The temperature dependence of the perpendicular upper critical field of superconducting granular aluminum films has been measured for samples approaching the metal-to-insulator transition. Analysis of the results shows a shift from an orbital to a Pauli-limited critical field, which we propose is made possible by electron mass renormalization. In that regime, the critical field transition becomes of the first order, as predicted by Fulde, Ferrel, Larkin, and Ovchinikov. The phase-coherence length ${\xi }_{\mathrm{phase}}$ and the superconducting gap $\mathrm{\Delta }$ obtained from the analysis are consistent with a Bardeen-Cooper-Schrieffer–Bose-Einstein condensate crossover region, which we propose is triggered by the Mott transition of the granular films.

${\mathrm{Ta}}_2\mathrm{Ni}{\mathrm{Se}}_5$ became one of the most investigated candidate materials for hosting an excitonic insulator ground state. Many studies describe the corresponding phase transition as a condensation of excitons breaking a continuous symmetry. This view got challenged recently pointing out the importance of the loss of two mirror symmetries at a structural phase transition that occurs together with the semiconductor—excitonic insulator transition. For such a scenario an unstable optical zone-center phonon at low energy is proposed to drive the transition. Here we report on the experimental observation of such a soft mode behavior using Raman spectroscopy. In addition we find a novel spectral feature, likely of electronic or joint electronic and phononic origin, that is clearly distinct from the lattice dynamics and that becomes dominant at $T_c$. This suggests a picture of joint structural and electronic orders driving the phase transition.

In this paper, we suggest an extension of the Rosenzweig-Porter (RP) model, the LN-RP model, in which the off-diagonal matrix elements have a wide, log-normal distribution. We argue that this model is more suitable to describe a generic many-body localization problem. In contrast to RP model, in LN-RP model, a fragile weakly ergodic phase appears that is characterized by broken basis-rotation symmetry which the fully ergodic phase, also present in this model, strictly respects in the thermodynamic limit. Therefore, in addition to the localization and ergodic transitions in LN-RP model, there exists also the transition between the two ergodic phases (FWE transition). We suggest new criteria of stability of the nonergodic phases that give the points of localization and ergodic transitions and prove that the Anderson localization transition in LN-RP model involves a jump in the fractal dimension of the egenfunction support set. We also formulate the criterion of FWE transition and obtain the full phase diagram of the model. We show that truncation of the log-normal tail shrinks the region of weakly ergodic phase and restores the multifractal and the fully ergodic phases.

We introduce a framework to distinguish long-range quantum entanglement from long-range classical correlations close to a finite temperature critical point in a quantum system. In particular, we employ “tripartite entanglement negativity,” a mixed-state entanglement measure, to cancel out critical correlations that are purely classical in origin. As an application, we study an exactly solvable model, and find that the tripartite negativity does not exhibit any singularity in the thermodynamic limit across the transition. This indicates that the long-distance critical fluctuations are completely classical, and it allows one to define a “quantum correlation length” that remains finite at the transition despite a divergent physical correlation length. Motivated by our model, we also study mixed-state entanglement in tight-binding models of bosons with $\text{U}\left(1\right)$ and time-reversal symmetries. By employing Glauber-Sudarshan representation, we find a surprising result that such states have zero entanglement.

The evolution of magnetic polarons in Sr doped $\mathrm{La}\mathrm{Co}{\mathrm{O}}_3$ (${\mathrm{La}}_{1\text{−}x}{\mathrm{Sr}}_x\mathrm{Co}{\mathrm{O}}_3$) single crystal and polycrystalline samples are investigated by employing dc and ac magnetic measurement and small angle neutron scattering (SANS) techniques. The effect of magnetic field and temperature on magnetic polarons is experimentally studied for ${\mathrm{La}}_{0.875}{\mathrm{Sr}}_{0.125}\mathrm{Co}{\mathrm{O}}_3$ and ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}\mathrm{Co}{\mathrm{O}}_3$ compounds that belong to the spin glass insulating regime of the broader compositional phase diagram of this system. Langevin analyses of the isothermal magnetization curves in the notional paramagnetic regime prove the existence of magnetic polarons with large moments. The dc field superimposed ac susceptibility data and the analysis of the glassy dynamics prove that the size of polarons in 15% Sr doped crystal increase as the field is increased while the field effect is not visible in the 12.5% Sr doped crystal. A polycrystalline sample of ${\mathrm{La}}_{0.85}{\mathrm{Sr}}_{0.15}\mathrm{Co}{\mathrm{O}}_3$ is analyzed by SANS experiments, which confirm nonzero correlation length at temperatures far above the macroscopic ordering temperature and hence the presence of magnetic polarons.

Activating transitions between a set of atomic internal states has emerged as an elegant scheme by which lattice models can be designed in ultracold atomic gases. In this approach, the internal states can be viewed as fictitious lattice sites defined along a synthetic dimension, hence offering a powerful method by which the spatial dimensionality of the system can be extended. Interparticle collisions generically lead to infinite-range interactions along the synthetic dimensions, which a priori precludes the design of Bose-Hubbard-type models featuring on-site interactions. In this paper, we solve this obstacle by introducing a protocol that realizes strong and tunable “on-site” interactions along an atomic synthetic dimension. Our scheme is based on pulsing strong intraspin interactions in a fast and periodic manner, hence realizing the desired on-site interactions in a digital (Trotterized) manner. We explore the viability of this protocol by means of numerical calculations, which we perform on various examples that are relevant to ultracold-atom experiments. This general method, which could be applied to various atomic species by means of fast-response protocols based on Fano-Feshbach resonances, opens the route for exploration of strongly correlated matter in synthetic dimensions.

Topological phases and edge-state topological protection are explicitly related by the bulk-edge correspondence which assumes the existence a real space boundary in one-dimensional (1D), 2D, or 3D systems. Extensions of this picture to systems where the boundary is present in the domain of generalized coordinates may generate unusual topological behavior in real space. In this paper, we discuss topological radial states in molecules with rotational symmetry and staggered hopping amplitudes in the radial direction. In the absence of the central site, the model is characterized as a weak topological insulator supporting radial edge states in the topological phase. When the central site is considered, we show that it behaves as an effective perturbation coupled in different ways to the radial inner edge, depending on the type of orbital considered at this central site. Tuning the parameters of the central orbital allows for a precise control over the radial inner edge physics of the model. We further break rotational symmetry by studying sliced molecules, which creates open boundaries in the azimuthal direction also, and show that higher-order boundary (corner) modes are present and how they can be driven into an energy gap through coupling with the central orbital. The results are corroborated by numerical conductance studies.

We derive generalized Kronig identities expressing quadratic fermionic terms including momentum transfer to bosonic operators and use them to obtain the exact solution for one-dimensional fermionic models with linear dispersion in the presence of position-dependent local interactions and scattering potential. In these Luttinger droplets, which correspond to Luttinger liquids with spatial variations or constraints, the position dependencies of the couplings break the translational invariance of correlation functions and modify the Luttinger-liquid interrelations between excitation velocities.

We analyze the disorder-driven localization of the two-dimensional Bose-Hubbard model by evaluating the full low-energy quasiparticle spectrum via a recently developed fluctuation operator expansion method. For any strength of the local interaction we find a mobility edge that displays an approximately exponential decay with increasing disorder strength. We determine the finite-size scaling collapse and exponents at this critical line finding that the localization of excitations is characterized by weak multifractality and a thermal-like critical gap ratio. A direct comparison to a recent experiment yields an excellent match of the predicted finite-size transition point and scaling of single particle correlations.

Cross-correlation between magnetism and dielectric is expected to offer distinct emergent phenomena. Here, magnetic excitations in the organic donor-acceptor spin-chain system, tetrathiafulvalene-bromanil (TTF-BA), with a ferroelectric ground state is investigated by ${}^1\mathrm{H}$-NMR spectroscopy. A nonmagnetic transition with a ferroelectric order is marked by sharp drops in the NMR shift and the nuclear spin relaxation rate $T_1^{-1}$ at 53 K. Remarkably, the analyses of the NMR shift and $T_1^{-1}$ dictate that the paramagnetic spin susceptibility in TTF-BA is substantially suppressed from that expected for the one-dimensional Heisenberg spins. We propose that the spin-lattice coupling and the ferroelectric instability cooperate to promote precursory polar singlet formation in the ionic spin system with a nonmagnetic ferroelectric instability.

We propose to characterize Lévy-distributed stochastic fluctuations through the measurement of the average voltage drop across a current-biased Josephson junction. We show that the noise induced switching process in the Josephson washboard potential can be exploited to reveal and characterize Lévy fluctuations, also if embedded in a thermal noisy background. The measurement of the average voltage drop as a function of the noise intensity allows to infer the value of the stability index that characterizes Lévy-distributed fluctuations. An analytical estimate of the average velocity in the case of a Lévy-driven escape process from a metastable state well agrees with the numerical calculation of the average voltage drop across the junction. The best performances are reached at small bias currents and low temperatures, i.e., when both thermally activated and quantum tunneling switching processes can be neglected. The effects discussed in this work pave the way toward an effective and reliable method to characterize Lévy components eventually present in an unknown noisy signal.

Wearable thermoelectric conversion devices are one of the key technologies for future sustainable society, and flexible conducting polymer films are strong candidates for these applications. However, the thermoelectric transport mechanism of these materials is still unclear due to their unique disordered nature, in particular the poor interconnectivity between crystalline domains. Here, to overcome this limit, donor-acceptor (D-A) copolymer films are selected because of the efficient connection between domains via rigid tie molecules. We investigated their thermoelectric and carrier transport properties using the electrolyte-gating technique and perfectly described both of them based on the two-dimensional variable range hopping model with a linear density of states around the Fermi level energy. The present results provide a fundamental understanding of the thermoelectric physics in D-A copolymer films towards application in wearable devices.

The spatial fluctuations of a superfluid flowing in a weak random potential are investigated. We employ classical field theory to demonstrate that the disorder-averaged nonequilibrium second-order correlation of the order parameter at zero temperature is identical to the thermally averaged equilibrium counterpart of a uniform superfluid at an effective temperature. The physics behind this equivalence is that scattering of a moving condensate by disorder has the same effect on the correlation function as equilibrium thermal excitations. The correlation function exhibits an exponential decay in one dimension and a power-law decay in two dimensions. We show that the effective temperature can be measured in an interference experiment of ultracold atomic gases.

Scanning transmission electron microscopy (STEM) and scanning electron microscopy (SEM) are prominent techniques for the structural characterization of materials. STEM in particular provides high spatial resolution down to the sub-ångström range. The spatial resolution in STEM and SEM is ultimately limited by the electron-beam diameter provided by the microscope's electron optical system. However, the resolution is frequently degraded by the interaction between electron and matter leading to beam broadening, which depends on the thickness of the analyzed sample. Numerous models are available to calculate beam broadening. However, most of them neglect the energy loss of the electrons and large-angle scattering. These restrictions severely limit the applicability of the approaches for large sample thicknesses in STEM and SEM. In this work, we address beam broadening in a more general way. We numerically solve the electron transport equation without any simplifications, and take into account energy loss along the electron path. For this purpose, we developed the software package CeTE (Computation of electron Transport Equation). We determine beam broadening, energy deposition, and the interaction volume of the scattered electrons in homogeneous matter. The calculated spatial and angular distributions of electrons are not limited to forward scattering and small sample thicknesses. We focus on low electron energies of 30 keV and below, where beam broadening is particularly pronounced. These electron energies are typical for SEM and STEM in scanning electron microscopes.

Lattice deformations act on the low-energy excitations of Dirac materials as effective axial vector fields. This allows one to directly detect quantum anomalies of Dirac materials via the response to axial gauge fields. We investigate the parity anomaly in Dirac nodal line semimetals induced by lattice vibrations and establish a topological piezoelectric effect; that is, periodic lattice deformations generate topological Hall currents that are transverse to the deformation field. The currents induced by this piezoelectric effect are dissipationless and their magnitude is completely determined by the length of the nodal ring, leading to a semiquantized transport coefficient. Our theoretical proposal can be experimentally realized in various nodal line semimetals, such as CaAgP and ${\mathrm{Ca}}_{{}_3}\mathrm{P}{}_2$.

Persistent homology (PH) is a relatively new field in applied mathematics that studies the components and shapes of discrete data. In this paper, we demonstrate that PH can be used as a universal framework to identify phases of classical spins on a lattice. This demonstration includes hidden order such as spin-nematic ordering and spin liquids. By converting a small number of spin configurations to barcodes we obtain a descriptive picture of configuration space. Using dimensionality reduction to reduce the barcode space to color space leads to a visualization of the phase diagram.