Many strongly correlated systems exhibit strange metallic behavior in certain parameter regimes characterized by anomalous transport properties that are irreconcilable with a Fermi-liquid-like description in terms of quasiparticles. The Hubbard model is a standard theoretical starting point to examine the properties of such systems and also exhibits non-Fermi-liquid behavior in simulations. Here we analytically study the two-dimensional hole-doped Hubbard model in the large $U$ limit, first identifying a doping-induced percolation transition in the low-energy sector occurring at a definite critical doping $p_c$ depending on lattice structure. Using the critical properties near this transition we rewrite the Hubbard Hamiltonian and motivate a low-energy lattice-independent large-$N$ model with distinct non-Fermi-liquid properties. We show that this model has linear-in-$T$ resistivity with doping-dependent slope maximized at $p=p_c$ and power-law optical conductivity ${\sim \left|\omega \right|}^{-2/3}$. Though the parameters used in developing this theory mean it cannot be directly applied to the cuprate superconductors, we nevertheless reproduce several important phenomena observed in their strange metal phase and also predict these same qualitative behaviors to manifest in other lattices near concrete hole dopings.

The transition metal disulfide ${\mathrm{MnS}}_2$ undergoes a structural phase transition at approximately 14 GPa attributed to a spin state transition. Nonetheless, the electronic characteristics of both the low-pressure and high-pressure phases remain contentious. In this study, we investigate this phase transition and observe that ${\mathrm{MnS}}_2$ experiences an electronic phase transition from a wide band-gap insulator to a narrow band-gap insulator. At near-ambient pressure, pyrite-${\mathrm{MnS}}_2$ exhibits a band gap of approximately 2.0 eV, notably larger than previously calculated values of 1.3 or 1.0 eV. Synchrotron x-ray diffraction confirms the phase transition occurring above 13.2 GPa, appearing to be a first-order phase transition, as indicated by the weakening and broadening of lattice-reconstruction-induced peaks. Contrary to previous proposals, an insulator-metal transition is not observed. The absence of a Raman signal is attributed to the weak response of the high-pressure phase, which possesses a very narrow band gap of approximately 20 meV. Optical absorption and electrical transport measurements exhibit consistent trends in the pressure-dependent electronic behavior. This research offers direct evidence elucidating the intrinsic electronic properties of ${\mathrm{MnS}}_2$. Given its structural similarity to other transition metal disulfide or oxide compounds, these findings contribute to understanding the intricate electronic properties within correlated electron systems featuring $3d$ transition metals.

We study a Cooper pair transistor realized by a mesoscopic superconductor island that couples to a pair of $s$-wave superconducting leads. For a trivial island, the critical supercurrent between the leads exhibits a well-known $2e$ periodicity in the island-gate charge. Here, we show that for an island with spatially separated zero-energy Majorana or Andreev bound states, the periodicity of the magnitude of the critical supercurrent transitions to $1e$ in the island-gate charge. Moreover, for Andreev bound states, the current-phase relation displays a sign reversal when the parity of the charge ground state of the island changes between even and odd. For Majorana bound states the same sign reversal does not occur. Our results highlight that the full current-phase relation of a Cooper pair transistor can depend on the nature of zero-energy bound states in candidate systems for topological superconductors and provide an initial step toward integrating Majorana qubits in superconducting circuits.

The coupling of phonon with spin in two-dimensional (2D) materials provides valuable insights into anomalous phonon behaviors. However, studies on spin-phonon coupling (SPC) in antiferromagnetic (AFM) materials are still in their preliminary stages. In this work, based on the first-principles calculations, we investigate the influence of spin ordering on phonon properties in 2D FeOCl, considering both ferromagnetic and four AFM configurations. Results based on the Raman scattering calculations and second-order SPC analysis demonstrate a strong magnetic dependence in phonon behaviors, which has a maximum frequency shift of $7.40{\mathrm{cm}}^{-1}$. Such a shift is considerably large when compared to other 2D materials such as graphene and ${\mathrm{MoS}}_2$, which are typically around $3–5{\mathrm{cm}}^{-1}$. We also analyze the frequency difference between neighboring $A_g$ Raman peaks for different magnetic configurations, which shows a distinct change of about $18.8{\mathrm{cm}}^{-1}$. Furthermore, we explore the relationship between the frequency of Raman peaks and magnetic exchange interactions under uniaxial strains. Within the strain range from $-3\%$ to $3\%$, an increment in strain leads to a decrement in both phonon frequency and magnetic exchange interactions. Our findings contribute to a deeper understanding of SPC in 2D FeOCl, and establish a solid theoretical foundation for further exploration in spintronics.

Colloidal quantum dots (QDs) are outstanding quantum materials, which have diverse potential applications in modern optoelectronic technologies. It has been a longstanding challenge to theoretically determine accurately the fundamental gaps of those QDs due to their large sizes and the complex surrounding dielectric environment. Herein, we have presented a first-principles scheme capable of overcoming this challenge. This scheme involves both material-specific Gaussian basis set development and density functional parametrizations. Compared to the prevailing Gaussian basis sets designed for molecules that lack transferability to bulk solid, our developed material-specific basis set designed for bulk solid exhibits excellent transferability to the QD down to ultrasmall sizes $(\sim 1\mathrm{nm})$. This allows the treatment of both bulk solid and the QD on an equal footing. The functional parametrizations are performed by concurrently introducing the QD size dependence in the range-separation parameter of a range-separation hybrid functional and a scalar dielectric constant of surrounding dielectric environment. This enables an efficient yet accurate calculation of the fundamental gap of the QD. While applying this theoretical scheme to 10 types of QDs in groups II-VI and III-V and containing up to 1000 atoms or more, we have explored an unexpected yet unified scaling law, i.e., fundamental gaps scale linearly with the inverse QD diameter in gas phase, irrespective of QD material type. We highlight the importance of the surrounding dielectric environment, which drives the scaling relation significantly apart from linearity, and brings excellent quantitative agreement between theory and experiments. While coupling with linear-response time-dependent density functional theory, this theoretical scheme allows an excellent reproduction of experimentally measured optical gaps of realistic colloidal QDs embedded in a complex dielectric environment. This work provides a cost-effective theoretical scheme for accurately studying the electronic and optical properties in colloidal QDs, and it is broadly applicable for other colloidal QD materials.

The phase diagram of the “gas-magnetoexciton condensate” transition of an excited two-dimensional electron system placed in a quantizing magnetic field has been constructed in the “temperature-laser excitation power density” coordinates. Based on the experimental data, we can conclude that the condensate is a collective, incompressible in real space, excited state of a quantum Hall insulator.

The stacking order of two-dimensional transition metal dichalcogenides (TMDs) is attracting tremendous interest as an essential component of van der Waals heterostructures. A common and fast approach to distinguish between the ${\mathrm{AA}}^{\prime }$ $\left(2H\right)$ and AB $\left(3R\right)$ configurations uses the relative edge orientation of each triangular layer (angle θ) from optical images. Here, we highlight that this method alone is not sufficient to fully identify the stacking order. Instead, we propose a model and methodology to accurately determine the bilayer configuration of ${\mathrm{WSe}}_2$ using second harmonic generation (SHG) and Raman spectroscopy. We demonstrate that the SHG response of the AB phase $(\theta =0^{\circ })$ layers is more intense than the signal from the single-layer structure. However, the SHG totally vanishes in the ${\mathrm{AA}}^{\prime }$ and ${\mathrm{AB}}^{\prime }$ phases ($\theta ={60}^{\circ }$ and $0^{\circ }$, respectively) of homobilayer ${\mathrm{WSe}}_2$. Also, several optical features of homobilayer ${\mathrm{WSe}}_2$ are found to depend on the details of the stacking order, with the difference being the clearest in the low-frequency Raman frequencies, as confirmed by density functional theory simulation. This allows unambiguous, high-throughput, nondestructive identification of stacking order in TMDs, which is not robustly addressed in this emerging research area.

Using the atomistic tight-binding method in combination with symmetry analysis and extended effective mass theory we derive an effective model for the fine structure of the ground electron and hole states in [111]-grown $\mathrm{Pb}X$ $(X$ = S, Se) hexagonal and cylindrical nanowires. Projection of the electron (hole) states calculated using the empirical tight-binding method, onto the basis of valley states enables the explicit parametrization of the valley mixing Hamiltonian. This is essential to analytically obtain the correct combinations of the valley states for electron (hole) ground levels. The effective Hamiltonian of valley mixing allows to convert the intravalley components of the $g$-factors tensors to the Zeeman splittings of electron and hole levels in nanowires. Tensors of the intravalley electron (hole) Landé $g$ factors, parameters of the valley mixing Hamiltonian, as well as the valley and anisotropic splitting energies and velocity matrix elements, are also calculated.

Here we resolve the real-space nature of the high-temperature, short-range charge correlations in the kagome metal ${\mathrm{ScV}}_6{\mathrm{Sn}}_6$. Diffuse scattering appears along a frustrated wave vector ${\mathbf{q}}_H=(\frac{1}{3},\frac{1}{3},\frac{1}{2})$ at temperatures far exceeding the charge order transition $T_{CO}=92$ K, preempting long-range charge order with wave vectors along ${\mathbf{q}}_{\overline{K}}=(\frac{1}{3},\frac{1}{3},\frac{1}{3})$. Using a combination of real space and reciprocal space analysis, we resolve the nature of the interactions between the primary out-of-plane Sc-Sn chain instability and the secondary strain-mediated distortion of the in-plane V kagome network. A minimal model of the diffuse scattering data reveals a high-temperature, short-ranged “zig-zag” phase of in-plane correlations that maps to a frustrated triangular lattice Ising model with antiferromagnetic interactions and provides a real-space understanding of the origin of frustrated charge order in this material.

Locally noncentrosymmetric structures in crystals are attracting much attention owing to emergent phenomena associated with the sublattice degree of freedom. The newly discovered heavy fermion superconductor ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ is considered to be an excellent realization of this class. Angle-resolved photoemission spectroscopy experiments recently observed low-energy spectra of electron and hole bands and characteristic Van Hove singularities, stimulating us to explore the electronic correlation effect on the band structure. In this Letter, we theoretically study the electronic state and topological superconductivity from first principles. Owing to the Coulomb repulsion $U$ of Ce $4f$ electrons, the low-energy band structure is modified in accordance with the experimental result. We show that Fermi surfaces change significantly from a complicated three-dimensional structure to a simple two-dimensional one. Fermi surface formulas for one-dimensional ${\mathbb{Z}}_2$ invariants in class D indicate topological crystalline superconductivity protected by the glide symmetry in a broad region for $U$. The classification of superconducting gap structure reveals the topologically protected excitation gap and node. Our findings of the correlation-induced evolution of electronic structure provide a basis to clarify the unusual phase diagram of ${\mathrm{CeRh}}_2{\mathrm{As}}_2$ including superconductivity, magnetic order, and quadrupole density wave, and accelerate the search for topological superconductivity in strongly correlated electron systems.

In recent years there has been great interest towards optical cavities as a tool to manipulate the properties and phases of embedded quantum materials. Due to the Purcell effect, a cavity changes the photon phase space and thus the rate of electromagnetic transitions within the material, modifying the exchange rate of heat radiation with the photon environment. This work derives a simple expression for the radiative heat absorbed by the material, investigates how it changes in the presence of a cavity, and shows that it is enhanced dramatically for appropriate cavity geometries. This effect is compared with typical energy dissipation processes, providing a criterion to estimate its impact on the temperature of a material in the cavity and applying it to 1T-${\mathrm{T}aS}_2$.

We show that spin-orbit splitting (SOS) in monolayers of semiconducting transition metal dichalcogenides (TMDs) is substantially enhanced by an electron-electron interaction. This effect, similar to the exchange enhancement of the electron $g$ factor, is most pronounced for conduction band electrons (in particular, in ${\mathrm{MoS}}_2$), and it has a nonmonotonic dependence on the carrier sheet density $n$. That is, the SOS enhancement is peaked at the onset of filling of the higher-energy spin-split band by electrons, $n_{*}$, which also separates the regimes of slow (at $n<n_{*}$) and fast (for $n>n_{*}$) spin and valley relaxation of charge carriers. Moreover, this density itself is determined by the enhanced SOS value, making the account of exchange renormalization important for the analysis of the spintronic performance of field-effect transistors based on two-dimensional TMDs.

We study the Hanle and spin polarization recovery effects on resident electrons in a monolayer ${\mathrm{MoSe}}_2$ on EuS. We demonstrate that localized electrons provide the main contribution to the spin dynamics signal at low temperatures below 15 K for small magnetic fields of only a few mT. The spin relaxation of these electrons is determined by random effective magnetic fields due to a contact spin interaction, namely the hyperfine interaction with the nuclei in ${\mathrm{MoSe}}_2$ or the exchange interaction with the magnetic ions of the EuS film. From the magnetic field angular dependence of the spin polarization we evaluate the anisotropy of the intervalley electron $g$ factor and the spin relaxation time. The nonzero in-plane $g$ factor $|g_x|\approx 0.1$, the value of which is comparable to its dispersion, is attributed to randomly localized electrons in the ${\mathrm{MoSe}}_2$ layer.

Minimally twisted bilayer graphene with interlayer potential asymmetry hosts one-dimensional topological helical states at domain walls between AB/BA-stacking regions. However, the nature of topological helical states' propagation remains elusive. Although it is widely believed that they form a two-dimensional triangular network, a few argue that they self-organize into one-dimensional topological zigzag modes that propagate independently. In this Letter, we propose a protocol based on a two-terminal twisted bilayer graphene nanoflake transport device and resolve this issue. Through rigorous calculations on the differential conductance and the nonequilibrium local density of states, we show that these topological helical states indeed self-construct the one-dimensional distorted topological zigzag modes, each bypassing the AA-stacking spots and propagating independently. By considering a long twisted bilayer graphene nanoflake, we obtain a nearly quantized conductance plateau with its value close to 1, 2, and 3 (in units of $2e^2/h$), which serves as a strong experimental sign for the existence of topological zigzag modes. Our work not only clarifies the propagation nature of the topological helical states, but also realizes an array of one-dimensional conducting channels on a two-dimensional platform. This work provides a way of engineering topological states intrinsic in twisted bilayer graphene.

We identify a time crystal phase characterized by a frequency half of the driving frequency in disordered superconductors by employing the time-dependent Bogoliubov–de Gennes formalism at zero temperature with a periodically driven coupling constant. After a period of exponential increase of spatial inhomogeneities and exponential suppression of the order parameter amplitude, the time crystal develops islands of different sizes. Each of these islands is a time crystal with the same frequency albeit with a phase shift $\pi$ with respect to the homogeneous time crystal. After its emergence, the island gradually becomes smaller, though the phase shift persists, until it is abruptly synchronized at a time that it depends on its initial size. We find a critical disorder strength, still deep in the metallic phase, at which the time crystal phase terminates. For even stronger disorder, the order parameter oscillates with the driving frequency in regions where localization effects are not important.

In ergodic quantum spin chains, locally conserved quantities such as energy or particle number generically evolve according to hydrodynamic equations as they relax to equilibrium. We investigate the complexity of simulating hydrodynamics at infinite temperature with multiple methods: time evolving block decimation (TEBD), TEBD with density matrix truncation (DMT), the recursion method with a universal operator growth hypothesis, and operator-size truncated (OST) dynamics. Density matrix truncation and the OST dynamics give consistent dynamical correlations to $t=60/J$ and diffusion coefficients agreeing within $1\%$. TEBD only converges for $t\lesssim 20$, but still produces diffusion coefficients accurate within 1%. The universal operator growth hypothesis fails to converge and only matches other methods on short times. We see no evidence of long-time tails in either DMT or OST calculations of the current-current correlator, although we cannot rule out that they appear at longer times. We do observe power-law corrections to the energy density correlator. At finite wavelength, we observe a crossover from purely diffusive, overdamped decay of the energy density to underdamped oscillatory behavior similar to that of cold-atom experiments. We dub this behavior “hot band second sound” and offer a microscopically motivated toy model.

Physical ageing of a strained metallic glass has caused its nonexponential decay to shift from compressed to stretched, yet the reason is unknown. Compared to structural relaxation taking place at temperatures close to the glass-transition temperature, where most ageing data were made, straining a metallic glass at room temperature causes rejuvenation. We discovered that the varying stretching-factor values are due to the changing rate of free volume, as calculated from a theoretical model using shear stress and temperature as inputs. Furthermore, independent of relaxation, equilibrium, or rejuvenation, the rate magnitude rather than the rate sign is what matters for stretching factor. The correlation of the stretching factor and free-volume changing rate is confirmed by a tensile creep test that promotes viscoplastic flow inside the metallic glass. Stretched exponential decay is interpreted by a modified diffusive-trapping model that allows a new band to develop through straining.

In contrast to standard Bardeen-Cooper-Schrieffer superconductivity, that in flat bands (FBs) possesses an interesting degree of freedom that enables the control of the superfluid weight (SFW), referred to as the quantum metric. In the present work, we consider the stub lattice and study the impact of the dilution of FB eigenstates on superconductivity. Among the most remarkable results, it is revealed that the SFW can be boosted by the decimation of the FB eigenstates. In addition, it is shown that the widely used uniform pairing hypothesis systematically predicts the suppression of the SFW, appears misleading and qualitatively incorrect. With the great progress in nanotechnologies, we believe that our findings could be realized and tested experimentally in covalent organic frameworks or in decorated structures in which defects/vacancies/ad-atoms are created/deposited in a controlled manner and even in multilayered structures with intercalated atoms.