Transition metal dichalcogenides attract considerable attention due to a variety of interesting properties, including long-range magnetism in nanocrystals. Here we investigate the magnetic, thermal, and electrical properties of an ${\mathrm{FeTe}}_2$ single crystal with iron vacancy defects. Magnetic measurements show a paramagnetic state and the absence of magnetic order with low anisotropy in the magnetic susceptibility. Fe $3d$ orbitals are well hybridized, contributing to the bad metal electrical resistivity. Observed thermal conductivity values below room temperature are rather low and comparable to those of high-performance thermoelectric materials. Our results indicate that ${\mathrm{FeTe}}_2$ can form in a highly defective marcasite crystal structure which can be exploited in future materials design.

Distinguishing the dynamics of an Anderson insulator from a many-body localized (MBL) phase is an experimentally challenging task. In this work we propose a method based on machine learning techniques to analyze experimental snapshot data to separate the two phases. We show how to train three-dimensional convolutional neural networks (CNNs) using space-time Fock-state snapshots, allowing us to obtain dynamic information about the system. We benchmark our method on a paradigmatic model showing MBL ($t\text{−}V$ model with quenched disorder), where we obtain a classification accuracy of $\approx 80\%$ between an Anderson insulator and an MBL phase. We underline the importance of providing temporal information to the CNNs and we show that CNNs learn the crucial difference between an Anderson localized and an MBL phase, namely the difference in the propagation of quantum correlations. Particularly, we show that the misclassified MBL samples are characterized by an unusually slow propagation of quantum correlations, and thus the CNNs label them wrongly as Anderson localized. Finally, we apply our method to the case with quasiperiodic potential, known as the Aubry-André model (AA model). We find that the CNNs have more difficulties in separating the two phases. We show that these difficulties are due to the fact that the MBL phase of the AA model is characterized by a slower information propagation for numerically accessible system sizes.

Dimerized magnets forming alternating Heisenberg chains exhibit quantum coherence and entanglement and thus can find potential applications in quantum information and computation. However, magnetic systems typically undergo thermal decoherence at finite temperatures. Here, we show inelastic neutron scattering results on an alternating antiferromagnetic-ferromagnetic chain compound ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$ that the excited quasiparticles can counter thermal decoherence and maintain strong correlations at elevated temperatures. At low temperatures, we observe clear dispersive singlet-triplet excitations arising from the dimers formed along the crystalline $b$ axis. The excitation gap is of $\sim 18\mathrm{meV}$ and the bandwidth is about half of the gap. The band top energy has a weak modulation along the [100] direction, indicative of a small interchain coupling. The gap increases while the bandwidth decreases with increasing temperature, leading to a strong reduction in the available phase space for the triplons. As a result, the Lorentzian-type energy broadening becomes highly asymmetric as the temperature is raised. These results are associated with a strongly correlated state resulting from hard-core constraint and quasiparticle interactions. We consider these results to be not only evidence for strong correlations at finite temperatures in ${\mathrm{Na}}_2{\mathrm{Cu}}_2{\mathrm{TeO}}_6$, but also for the universality of the strongly correlated state in a broad range of quantum magnetic systems.

We report on an experimentally observed doubly split spectrum and its split-width fluctuation due to charge fluctuation in an ultrastrongly coupled superconducting qubit and resonator. From an analysis of the circuit model Hamiltonian, we found that the doubly split spectrum and split-width fluctuation are caused by discrete charge hops due to quasiparticle tunneling and a continuous background charge fluctuation in islands of a flux qubit. During 70 h of spectrum measurement, the split width fluctuated but the middle frequency of the split was constant. This observation indicates that the quasiparticles in our device mainly tunnel in one particular junction, as expected from the energy difference between quasiparticle states, during this 70 h. The background offset charge obtained from the split width has the $1/f$ noise characteristic.

CeRhSn with the Ce atoms forming a quasikagome lattice in the hexagonal plane has recently been discussed in the context of quantum criticality driven by magnetic frustration. Furthermore, it has been reported that the successive substitution of Rh by Pd leads to magnetic order. Here we have investigated the change of the $4f$ occupation in the substitution series ${\mathrm{CeRh}}_{1-x}{\mathrm{Pd}}_x\mathrm{Sn}$ for for $x=0$, 0.1, 0.3, 0.5, 0.75 by means of photoelectron spectroscopy with hard x rays (HAXPES). The quantitative analysis of the core level spectra with a combined full multiplet and configuration interaction analysis shows a smooth decrease of the $4f^0$ contribution with rising $x$ due to an increase of the effective $4f$ binding energy ${\varepsilon }_{4f}$ and the reduction of the effective hybridization $V_{\text{eff}}$. We further compare valence band data with the calculated partial density of states and find that the Pd $4d$ states are about 1 eV further away from the Ce $4f$ states at the Fermi energy than the Rh $4d$ states. In fact, the effective binding energy ${\varepsilon }_{4f}$ of the $4f$ states in the configuration interaction analysis of the core level spectra decreases by the same amount.

We construct a tensor network representation of the three-dimensional (3D) toric code ground state that is stable to a generating set of uniform local tensor perturbations, including those that do not map to local operators on the physical Hilbert space. The stability is established by mapping the phase diagram of the perturbed tensor network to that of the 3D Ising gauge theory, which has a nonzero finite temperature transition. More generally, we find that the stability of a topological tensor network state is determined by the form of its virtual symmetries and the topological excitations created by virtual operators that break those symmetries. In particular, a dual representation of the 3D toric code ground state, as well as representations of the $X$-cube and cubic code ground states, for which pointlike excitations are created by such operators, are found to be unstable.

Using field-theoretic techniques, we study the $\mathrm{SU}\left(3\right)$ analog of antiferromagnetic Heisenberg spin model on the triangular lattice putting the $p$-box symmetric representation on each site. Taking the large-$p$ limit, we show that the low-energy effective theory is described by a $(2+1)$-dimensional relativistic $\mathrm{SU}\left(3\right)/\mathrm{U}{\left(1\right)}^2$ nonlinear sigma model. Since the target space has a nontrivial homotopy ${\pi }_2(\mathrm{SU}\left(3\right)/\mathrm{U}{\left(1\right)}^2)\simeq {\mathbb{Z}}^2$, this model has two kinds of magnetic skyrmions, which can be created and annihilated by monopole instantons. By careful analysis of the Wess-Zumino term in the spin coherent path integral, we compute the Berry phase for these monopoles and it produces the destructive interference. This restricts possible perturbations of the effective Lagrangian by monopole operators, and we see that the valence-bond-solid (VBS) phase should have degenerate ground states when $p\notin 3\mathbb{Z}$. We also compute 't Hooft anomalies to constrain possible phases of this system, and a direct phase transition between Néel and VBS phases is supported from the anomaly matching.

We investigate the role of anharmonic phonon renormalization and four-phonon scattering in the thermal transport of cubic ${\mathrm{SrTiO}}_3$ from 200 to 800 K by constructing an accurate first-principles machine-learning potential with active learning. The anharmonic phonon dispersion relations show obvious low-frequency phonon hardening as temperature increases, implying potential importance of fourth order lattice anharmonicity in cubic ${\mathrm{SrTiO}}_3$. Under the perturbation theory and Boltzmann transport equation scheme, we find that phonon frequency renormalization can make a significant improvement in the calculations of thermal conductivity and its temperature dependence of cubic ${\mathrm{SrTiO}}_3$ compared to experimental data. Four-phonon scattering is usually important in strongly anharmonic materials with ultralow thermal conductivity and simple crystals featured by large acoustic-optical phonon band gaps. Nonetheless, we find that four-phonon scattering can make a considerable contribution to suppressing phonon transport in cubic ${\mathrm{SrTiO}}_3$ which has a moderate thermal conductivity. This study deepens the understanding of anharmonic phonon renormalization and four-phonon scattering in oxide perovskites.

Inorganic-organic hybrid lead halide perovskites have shown great prospects in solar cells in recent years, and their power conversion efficiency (PCE) has reached up to $25.5\%$. However, Pb toxicity and poor ambient stability have remained two unsolved urgent problems. To address these problems, more and more researchers have begun to focus on the application of inorganic lead-free perovskites in the photovoltaic field. Herein, several cerium-based lead-free inorganic chalcogenide perovskites $A{\mathrm{CeTe}}_3$ $(A=\mathrm{Ca},\mathrm{Sr},\mathrm{Ba})$ are proposed as suitable optical absorbers for thin-film solar cells based on first-principles calculations. The distorted-phase $A{\mathrm{CeTe}}_3$ perovskites are predicted as direct-band-gap semiconductors with a gap value of 1.02–1.13 eV. Moreover, the distorted $A{\mathrm{CeTe}}_3$ not only possess a strong optical absorption coefficient $({10}^5–{10}^6{\mathrm{cm}}^{-1})$, superior to ${\mathrm{MAPbI}}_3 ({\mathrm{MA}}^{+}={\mathrm{CH}}_3{\mathrm{NH}}_3^{+})$, but also have excellent PCE, especially for ${\mathrm{BaCeTe}}_3$, whose PCE can reach $\sim 29\%$ even in a thin film $\left(1\mu \mathrm{m}\right)$. All these characteristics render distorted $A{\mathrm{CeTe}}_3$ perovskites promising photovoltaic materials.

In an artificial neural network composed of neuromorphic computing units, the resistance of the memristor can be modulated dynamically and repeatedly under external stimuli, such as electric fields, magnetic fields, and light illumination, leading to variations in local conductivity and memory effects. Here we show, using an all-photonic memristor, that the memristance arises naturally in an optical system in which solid-state ionic transport is realized under low-power external incident light. These results show an extremely stable multilevel storage weight and a high signal-to-noise ratio. In all-photonic memristors, the light signal is employed as the extra stimuli of the memristor devices to ensure a large memory window and a variation margin of multiple storage levels.

Polaritons have a demonstrated impact on nanophotonic applications in the midinfrared through visible spectral range. Surface phonon-polaritons (SPhPs) offer a way to bring the potential of polaritons to the longer infrared wavelengths. Strontium titanate (STO) is a perovskite polar dielectric with diverse technologically advantageous properties and it can support SPhPs in a uniquely broad spectral range of the far infrared. Despite these advantages, STO has mostly been overlooked as a nanophotonic material. In this work we investigate SPhP propagation in STO in the far-infrared through midinfrared spectral range using broadband, near-field nanospectroscopy. We developed a tabletop, laser sustained plasma light source that enabled us to obtain amplitude and phase resolved hyperspectral line scan maps of SPhPs across the surface of the STO sample. Analytical modeling of experimental data reveals the dispersion characteristics of SPhPs in STO. This work establishes STO as a platform for perovskite-based broadband far-infrared and terahertz nanophotonics.

The ion-induced nanoscale pattern formation on a crystalline Ge(001) surface is observed in situ by means of grazing incidence small angle x-ray scattering (GISAXS). Analysis of the GISAXS intensity maps yields the temporal development of geometric parameters characterizing the changing pattern morphology. In comparison with theoretical predictions and with simulations of the patterning process based on a continuum equation we find good agreement for the temporal evolution of the polar facet angle, characteristic length, and surface roughness in the nonlinear regime. To achieve this agreement, we included an additional term in the continuum equation which adjusts the pattern anisotropy.

Single molecular magnets (SMMs) have become promising paradigms to develop novel spintronics for futuristic information technologies such as high-density information and quantum computing. The efficiency and characteristics of SMM devices are determined by the intrinsic nature of the molecular magnets placed in the spin transport pathway. In this work, to understand the role of the central magnetic ion on the performance of SMM devices, we screen the spin-conductance properties of whole $3d$ and $4d$ metalloporphyrins using the nonequilibrium Green's function formalism in conjunction with density functional theory. Our results show that the investigated SMMs according to spintronic conductance behavior can be categorized into three groups: type I non-spin-polarized, type ${\mathrm{II}}^{\prime }$ minor spin-polarized current, and type ${\mathrm{II}}^{″}$ major spin-polarized current devices. Type-${\mathrm{II}}^{\prime }$ and type-${\mathrm{II}}^{″}$ molecular systems show perfect spin filtering and spin-dependent negative differential resistance. The optimal energy alignment of spin-polarized molecular orbitals with gold electrodes results in one-channel spin transport (minor for type ${\mathrm{II}}^{\prime }$ and major for type ${\mathrm{II}}^{″}$). Thus type-II junctions are half-metal. The type-${\mathrm{II}}^{″}$ junctions also show a voltage-induced spin switchability at low bias voltages. In this regard, type-II molecular systems are promising candidates for a low-power consumption spin filter, spin switch, memory, to name just a few. Our results highlight the practical applications of metalloporphyrin for the development of multipurpose miniature spintronic devices.

The light-matter interaction associated with a two-dimensional excitonic transition coupled to a zero-dimensional photonic cavity is fundamentally different from cavity-coupled localized excitations in quantum dots or color centers, which have negligible spatial extent compared to the cavity-confined mode profile. We provide a succinct expression for calculating the light-matter interaction of a two-dimensional optical transition coupled to a zero-dimensional confined cavity mode. From this expression, we found there is an optimal spatial extent of the excitonic transition that maximizes such an interaction strength due to the competition between minimizing the excitonic envelope function area and maximizing the total integrated field. We also found that at near zero exciton-cavity detuning, the direct transmission efficiency of a waveguide-integrated cavity can be severely suppressed, which suggests performing experiments using a side-coupled cavity.

An ideal half-semimetal is characterized by a single spin-polarized electronic relativistic band across the Fermi energy. Realizing such state in the correlated electron systems depends not only on the way the time reversal symmetry breaks, but also on the electron filling and electron correlations. Here we investigate a honeycomb Anderson lattice model in the strong $f$ electron Coulomb interaction limit and with an additional $f$ electron ferromagnetic coupling. A generic phase diagram is presented showing the successive first-order phase transitions from the decoupled local ferromagnetic phase to the heavy fermion ferromagnetic and paramagnetic phases. In particular, the magnetic heavy fermion phase emerges in the intermediate region but with two magnetic states, identified as the ground state or metastable state with small or large spin magnetization, respectively. When the total electron filling is fixed at 3/8, the band crossing of the spin-down polarized channel in the small spin magnetization state locates at the Fermi energy, without the existence of other Fermi surfaces. Such ideal half-semimetal feature is protected by the Kondo gap in the spin-up polarized channel.

Unlike a conventional two-dimensional electron gas system, which has parabolic band structure, the nonparabolic band dispersion of mono- to few-layer graphene violates Kohn's theorem. Thus, Landau levels (LLs) in graphene are sensitive to many-body interactions. This modifies the LL spacing, depending on the location of the Fermi energy ($E_{\mathrm{F}}$). Such effects have been extensively studied in $h$-BN/monolayer graphene/$h$-BN through observation of inter-LL optical transitions known as cyclotron resonances (CRs). However, thus far, the influence of many-body interactions on the CR of bilayer graphene (BLG) has been rarely studied, even though BLG also possesses nonparabolic band dispersion. Here, we investigate CR in the $h$-BN/BLG/$h$-BN structure via magneto-photothermoelectric measurements under infrared laser irradiation. This method enables sensitive detection of cyclotron resonances while tuning $E_{\mathrm{F}}$ of BLG. The CR magnetic field value shifted significantly when $E_{\mathrm{F}}$ of BLG approached the charge-neutrality point (the Dirac point, DP). We attribute this to a change in the Fermi velocity of BLG near the DP, which occurs as a result of many-body interactions.

Due to an increase in the variety of candidate compounds, kagomé metals have recently gained significant attention. Among other features, kagomé metals host Dirac cones as a key band structure feature away from half filling, and potentially yield an exceptionally large fine structure, beyond values found in other 2D Dirac materials such as graphene. We investigate the possibility of chiral symmetry breaking in kagomé metals. Based on a heuristic lattice model, we determine the critical coupling strength and the ordering pattern by means of a Schwinger-Dyson mean-field analysis. As the leading instability we identify a dimerization pattern which spontaneously opens an excitation gap at the Dirac point and breaks the chiral symmetry.

We investigate magnetic, charge, and transport properties of hexagonal graphene nanoflakes (GNFs) connected to two metallic leads by using the functional renormalization group method. The interplay between the on-site and long-range interactions leads to a competition of semimetal (SM), spin-density-wave (SDW), and charge-density-wave (CDW) phases. The ground-state phase diagrams are presented for the GNF systems with screened realistic long-range electron interaction [T. O. Wehling et al., Phys. Rev. Lett. 106, 236805 (2011)], as well as uniformly screened long-range Coulomb potential $\propto 1/r$. We demonstrate that the realistic screening of Coulomb interaction by $\sigma$ bands causes moderate (strong) enhancement of critical long-range interaction strength, needed for the SDW (CDW) instability, compared to the results for the uniformly screened Coulomb potential. This enhancement gives rise to a wide region of stability of the SM phase for realistic interaction, such that freely suspended GNFs are far from both SM-SDW and SM-CDW phase-transition boundaries and correspond to the SM phase. Close relation between the linear conductance and the magnetic or charge states of the systems is discussed. A comparison of the results with those of other studies on GNF systems and infinite graphene sheets is presented.