We investigate the ground-state phase diagram of frustrated mixed-spin (1, 1/2) chains with three-site four-spin interactions employing different approaches, such as cluster mean field theory, cluster variational method, and spin-wave theory. The interplay of next-nearest-neighbor and three-site interactions leads to a plethora of magnetic and nonmagnetic phases in the ground-state phase diagram. We show that aside from the ferromagnetic and Neel orders, the ground state possesses a magnetic phase in which magnon excitations are dispersionless. The emergence of flat-band magnons is a consequence of the inhomogeneity of our frustrated mixed-spin chains and does not occur in similar uniform spin chains. We also demonstrate that the presence of three-site interaction gives rise to various nonmagnetic phases, such as antiferroquadrupole order and quantum spin-liquid phase.

We report results of the density functional theory and density functional perturbation theory calculations on thermodynamic stability, magnetic structure, and magnetocrystalline anisotropy (MCA) of a ${\mathrm{MnPt}}_3\left(001\right)$ thin film. It is predicted that the magnetic ground state of the Pt-terminated ${\mathrm{MnPt}}_3$(001) thin films is the ferromagnetic as found in its bulk structure, while the MnPt termination favors the A-type antiferromagnetic order at the surface. Even more drastic effect of the surface termination on MCA is revealed; in contrast to an in-plane magnetization preferred for the MnPt termination, the Pt-terminated ${\mathrm{MnPt}}_3$(001) thin films exhibit an extremely large perpendicular MCA (PMCA) up to an order of 10 erg/${\mathrm{cm}}^2$ regardless of its film thickness. Moreover, from detailed single-particle energy spectra analyses, this magnetization reversal is mainly determined by an interplay between two in-plane orbital states, $d_{\text{xy}}$ and $d_{{\text{x}}^2-{\text{y}}^2}$ states, of the Pt atom with induced magnetism at the surface, as a result of their energy level changes associated with the Mn $3d-\mathrm{Pt}5d$ hybridization. This reported thin-film system can act as a prototype for the in-depth study of the importance of the surface termination with large perpendicular magnetocrystalline anisotropy in spintronic applications.

Gate voltage tunable ultrathin high-$T_c$ cuprates supply a unique platform to investigate the electronic phase diagram and superconductor-insulator transition. One of the challenges in this field is the precise determination of the doping content in the underdoped nonsuperconducting region. Here we report the discovery of a universal relation between the doping content $p$ and the normal-state resistance at a fixed temperature $R\left(T_f\right)$, $p=\alpha +\beta \ln [1/R\left(T_f\right)]$, in the ultrathin ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+x}$ flakes. The in-depth analysis shows that the evolution of carrier scattering probability with doping content and the change of effective mass caused by superconductor-insulator transition are two key factors leading to this logarithmic relation. Based on our finding, the more precise electronic phase diagram can be established. In addition, the superconductor-insulator transition is verified to be a quantum phase transition using a finite size scaling analysis. The scaling exponent $z\nu$ is found to have a close correlation with the disorder levels. The present result provides an important foundation to investigate the fascinating electronic states in the ultra-thin cuprates.

Arrays of Rydberg atoms constitute a highly tunable, strongly interacting venue for the pursuit of exotic states of matter. We develop a strategy for accessing a family of fractionalized phases known as quantum spin liquids in two-dimensional Rydberg arrays. We specifically use effective field theory methods to study arrays assembled from Rydberg chains tuned to an Ising phase transition that famously hosts emergent fermions propagating within each chain. This highly entangled starting point allows us to naturally access spin liquids familiar from Kitaev's honeycomb model—albeit from an entirely different framework. In particular, we argue that finite-range repulsive Rydberg interactions, which frustrate nearby symmetry-breaking orders, can enable coherent propagation of emergent fermions between the chains in which they were born. Delocalization of emergent fermions across the full two-dimensional Rydberg array yields a gapless ${\mathbb{Z}}_2$ spin liquid with a single massless Dirac cone. Here, the Rydberg occupation numbers exhibit universal power-law correlations that provide a straightforward experimental diagnostic of this phase. We further show that explicitly breaking symmetries perturbs the gapless spin liquid into gapped, topologically ordered descendants: Breaking lattice symmetries generates toric-code topological order, whereas introducing Floquet-mediated chirality generates non-Abelian Ising topological order. In the toric-code phase, we analytically construct microscopic incarnations of non-Abelian defects, which can be created and transported by dynamically controlling the atom positions in the array. Our work suggests that appropriately tuned Rydberg arrays provide a cold-atoms counterpart of solid-state “Kitaev materials” and, more generally, it spotlights a different angle for pursuing experimental platforms for Abelian and non-Abelian fractionalization.

The first and second excited states of 7,7,8,8-tetracyanoquinodimethane (TCNQ) anions in acetonitrile were studied employing femtosecond time-resolved photoluminescence and transient absorption spectroscopy. It was found that the second excited state, which is a superexcited state, undergoes ultrafast decay due to spontaneous electron emission with a time constant of 0.16 ps. The rationale for the fast electron emission in acetonitrile is discussed in terms of the interactions with the surrounding polar solvent molecules.

The topological phase transition in ${\mathrm{ZrTe}}_5$ remains a challenge for its sensitivity to the structural parameters and carrier densities. One important issue is that the temperature dependence of the band shifts has been reported controversially. Using angle-resolved photoemission spectroscopy, we study the temperature evolution of the band structures of ${\mathrm{ZrTe}}_5$ with samples prepared by chemical vapor transport and flux methods. We find two distinct types of band shift with temperature, one monotonically moving downward to high binding energies with the increase of temperature and another one moving downward first and then reversely upward. By assuming ${\mathrm{ZrTe}}_5$ is a semiconductor with a limited amount of impurities, we analyze the temperature-induced ionization of the impurities and chemical potential shift to interpret the discrepant experimental observations. In this scenario, the discrepancies in band structures and the controversial results on temperature-dependent band shifts can be reconciled in a unified picture.

We have studied the microscopic magnetic properties, the nature of the 130 K phase transition, and the ground state in the recently synthesized compound ${\mathrm{Ce}}_2{\mathrm{Rh}}_2\mathrm{Ga}$ by use of ${}^{69,71}\mathrm{Ga}$ nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t\sim 130$ K, yielding a splitting of the high-temperature single NQR line into two well-resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate $1/T_1$ shows three distinct regimes, with onset temperatures at $T_t$ and 2 K. The temperature-independent $1/T_1$, observed between $T_t$ and 2 K, crosses over to a Korringa process, $1/T_1\propto T$, below $\sim$2 K, which evidences a rare two-ion Kondo scenario: The system evolves into a dense Kondo coherent state below 2.0 and 0.8 K probed by the two different Ga sites.

Employing a realistic tight-binding model describing the Fermi surface in the normal state of ${\text{Sr}}_2{\text{RuO}}_4$, we map out magnetic field versus temperature phase diagrams for $d_{x^2-y^2}\left(B_{1g}\right)$ and $d_{xz}+i{d}_{yz}\left(E_g\right)$ pairing types. Both produce (i) a similar Knight shift suppression of $\sim 80\%$ and (ii) a bicritical point at $T=0.88\mathrm{K}$ separating low field second-order phase transitions from high-field Pauli limiting first-order transitions. We find, however, strikingly different phase behavior within the high-field Pauli limiting region. For $d_{x^2-y^2}$ pairing symmetry, an additional lower critical line of first-order transitions is found (terminating in a critical point at $T=0.09\text{−}0.22\mathrm{K}$ depending on the choice of Hubbard $U$ parameters), while for $d_{xz}+i{d}_{yz}$ no such additional high-field phase transitions are found for any choice of Hubbard $U$. In conjunction with our earlier finding [Phys. Rev. B 102, 235203 (2020)] for $p$-wave helical pairing of a still different high-field phase structure (a lower critical field line meeting the upper critical field line exactly at the bicritical point), we suggest high-field Pauli limiting phase structure as a possible route to distinguish pairing symmetries in this material.

The term $N$-photon bundles has been coined for a specific type of photon emission, where light quanta are released from a cavity only in groups of $N$ particles. This emission leaves a characteristic number distribution of the cavity photons that may be taken as one of their fingerprints. We study this characteristic $N$-photon bundle statistics considering two solid-state cavity quantum electrodynamics (cQED) systems. As one example, we consider a semiconductor quantum-dot–microcavity system coupled to longitudinal acoustic phonons. There, we find the environmental influence to be detrimental to the bundle statistics. The other example is a superconducting qubit inside a microwave resonator. In these systems, pure dephasing is not important and an experimentally feasible parameter regime is found, where the bundle statistics prevails.

Spin-helical states, which arise in quasi-one-dimensional (1D) channels with spin-orbital (SO) coupling, underpin efforts to realize topologically protected quantum bits based on Majorana modes in semiconductor nanowires. Detecting helical states is challenging due to non-idealities present in real devices. Past efforts have focused on devices with normal contacts. In contrast, here we show by means of tight-binding calculations that by using ferromagnetic contacts it is possible to detect helical modes with high sensitivity even in the presence of realistic device effects, such as quantum interference. We show that by taking advantage of the spin-selective transmission properties of helical states, new transport signatures become possible. In addition, we show that spin-polarized contacts provide a unique path to investigate the spin-texture and spin-momentum locking properties of helical states. Our results are of interest not only for the ongoing development of Majorana qubits, but also for realizing possible spin-based quantum devices, such as quantum spin modulators and interconnects based on spin-helical channels.

The recent demonstrations of viscous hydrodynamic electron flow in two-dimensional electron systems poses serious questions to the validity of existing transport theories, including the ballistic model, the collision-induced and collisionless hydrodynamics. While the theories of transport at hydrodynamic-to-ballistic crossover for free 2D electrons are well established, the same is not true for electrons in magnetic fields. In this paper, we develop an analytically solvable model describing the transition from ballistic to hydrodynamic transport with changing the strength of electron-electron collisions in magnetic fields. Within this model, we find an expression for the high-frequency nonlocal conductivity tensor of 2D electrons. It is valid at arbitrary relation between frequency of external field $\omega$, the cyclotron frequency ${\omega }_c$, and the frequency of e-e collisions ${\tau }_{ee}^{-1}$. We use the obtained expression to study the transformation of 2d magnetoplasmon modes at hydrodynamic-to-ballistic crossover. In the true hydrodynamic regime, $\omega {\tau }_{ee}\ll 1$, the 2DES supports a single magnetoplasmon mode that is not split at cyclotron harmonics. In the true ballistic regime, $\omega {\tau }_{ee}\gg 1$, the plasmon dispersion develops splittings at cyclotron harmonics, forming the Bernstein modes. A formal long-wavelength expansion of kinetic equations (“collisionless hydrodynamics”) predicts the first splitting of plasmon dispersion at $\omega \approx 2{\omega }_c$. Still, such expansion fails to predict the zero and negative group velocity sections of true magnetoplasmon dispersion, for which the full kinetic model is required.

Metasurfaces, together with graphene plasmonics, have become prominent for the emissivity control in thermal engineering, both passively, through changing the geometric parameters and packing density of the metasurfaces, and actively, through graphene gating or doping. We demonstrate a graphene-nanowire coupled plasmonic metasurface utilizing the hybrid localized surface plasmon modes of the nanowire array and graphene. The nanowire array makes the hybrid surface plasmon mode localized, allowing a free-space excitation. The single-layer graphene, via the gating between the underneath mirror and a top electrode, can actively tune the spectral emissivity by almost 90%. In addition, the graphene surface plasmon modes remove the strict polarization dependence of nanowire array emission, resulting in a fivefold enhancement of the P-polarized emissivity, especially for large emission angles.

Two-dimensional Janus monolayers with specular asymmetry exhibit excellent physicochemical properties and are a good candidate for optoelectronic, valley electronic, and nanoelectronics devices. Based on the density functional theory, the Janus $2H$-VTeSe monolayer is an intrinsic ferromagnetic semiconductor with an indirect band gap of 0.324 eV and exhibits a good thermodynamic and kinetic stability, in-plane magnetocrystal anisotropy, large spontaneous valley polarization of 155 meV, and high Curie temperature ($T_c$) of 380 K. The biaxial strain ($-6\%<\varepsilon <6\%$) can effectively tune the Janus $2H$-VTeSe monolayer from the bipolar magnetic semiconductor phase to half-semiconductor, spin gapless semiconductor, and half-metallic phases. Moreover, the magnetocrystal anisotropy energy (MAE) is modulated by the strain from 0.54 meV ($\varepsilon =-6\%$) to 1.32 meV ($\varepsilon =-2\%$), and the easy [100] and hard [001] magnetic axes could be switched from each other by charge carrier doping. The calculated valley optical response of the Janus $2H$-VTeSe monolayer exhibits a valley-selective circular dichroism. Due to the broken inversion and time-reversal symmetry, the valley polarization and Berry curvature can be continuously tuned by applying biaxial strains (modulation range 24.2%), external electric field (modulation range 2%), and varying the magnetization angle (0 to 180 degrees). The Janus $2H$-VTeSe monolayer possesses intrinsic ferromagnetic ordering, large spontaneous valley polarization, high $T_c$ of 380 K, and considerable MAE of 1.15 meV, giving it a potential application in the two-dimensional spintronic devices.

In $f$-electron heavy fermion systems, a wide variety of fascinating magnetic and electronic properties arise, and they are often thought to be related to a crossover behavior between localized and itinerant regimes for $f$ electrons. Uranium-based compounds with partially occupied $5f$ orbitals provide an ideal platform to elucidate the outstanding issue of the competition between magnetic order and Kondo entanglement. Using angle-resolved photoemission spectroscopy, complemented by density functional theory simulations, we probe the temperature-dependent evolution of the low-energy electronic structure of ${\mathrm{UAs}}_2$ and observe two nearly flat bands of different origin in the antiferromagnetic (AFM) state. A nearly flat band which behaves with quasi-two-dimensional character has been observed and contributed to the heavy spectral weight at the Г point around $E_{\mathrm{F}}$. This flat band reveals typical Kondo hybridization. Another flat band around the $M$ point is closely related to paramagnetic (PM)-AFM phase transition, which opens an energy gap below the transition temperature ∼273 K. Our results provide direct spectral demonstration of the electronic structure evolution across the $f\text{−}c$ hybridization and PM-AFM transition in ${\mathrm{UAs}}_2$.

We show that the gapless boundary signatures—namely, chiral/helical hinge modes or localized zero modes—of three-dimensional higher-order topological insulators and superconductors with inversion symmetry can be gapped without symmetry breaking upon the introduction of non-Abelian surface topological order. In each case, the fractionalization pattern that appears on the surface is “anomalous” in the sense that it can be made consistent with symmetry only on the surface of a three-dimensional higher-order insulator/superconductor. Our results show that the interacting manifestation of higher-order topology is the appearance of “anomalous gapped boundaries” between distinct topological orders whose quasiparticles are related by inversion, possibly in conjunction with other protecting symmetries such as time-reversal symmetry and charge conservation.

Electronic correlation can strongly influence the electronic properties of two-dimensional (2D) materials with open $d$ or $f$ orbitals. Herein, by taking single-layer (SL) $H\text{−}\mathrm{Fe}{\mathrm{Br}}_2$ as a representative of the SL $H\text{−}\mathrm{Fe}{\mathrm{X}}_2$ (X = Cl, Br, I) family, we investigated the electronic correlation effects in the magnetic anisotropy and electronic topology of such a system based on first-principles calculations with the density functional theory $+U$ approach. Our result is that the magnetic anisotropy energy (MAE) of SL $H\text{−}\mathrm{Fe}{\mathrm{Br}}_2$ shows a nonmonotonic evolution behavior with increasing electronic correlation strength, which is mainly due to the competition between different element-resolved MAEs of Fe and Br. Further investigations show that the evolution of element-resolved MAE arises from the variation of the spin-orbital coupling interaction between different orbitals in each atom. Moreover, tuning the strength of the electronic correlation can drive the occurrence of band inversions, causing the system to undergo multiple topological phase transitions and resulting in a quantum anomalous valley Hall effect. These exotic properties are universal for the SL $H\text{−}\mathrm{Fe}{\mathrm{X}}_2$ (X = Cl, Br, I) family. Our work sheds light on the role of electronic correlation effects in tuning magnetic and electronic structures in the SL $H\text{−}\mathrm{Fe}{\mathrm{X}}_2$ (X = Cl, Br, I) family, which could guide advances in the development of new spintronics and valleytronics devices based on these materials.

We develop a theory of fluctuation-driven phenomena in thermal transport in graphene double-layers. We work in the regime of electron hydrodynamics and focus on the double-charge neutrality point. Although at the neutrality point charge transport is decoupled from the hydrodynamic flow, thermal fluctuations of electron density cause both drag and heat transfer between the layers. The thermal transport in the bilayer system is governed by these two phenomena. We express the drag friction coefficient and the interlayer thermal conductivity in terms of the interlayer distance and the intrinsic conductivity of the electron liquid. We then obtain the thermal conductance matrix and determine the spatial dependence of the hydrodynamic velocity and temperature in the system. For shorter system the thermal drag resistance is determined by drag. In longer systems the situation of perfect thermal drag is realized, in which the hydrodynamic velocities in both layers become equal in the interior of the systems. Estimates are given for the monolayer and bilayer graphene devices. The predictions of our theory can be tested by the high-resolution thermal imaging and Johnson-Nyquist nonlocal noise thermometry.

The concept of bound states in the continuum (BICs) in a simple cavity attracts much interest in recent works in wave physics. The BICs are perfectly confined modes with an infinite lifetime that reside inside the continuous spectrum of radiative modes, but they remain totally decoupled from it. There exist several types of BICs based on their physical origin: one of the most interesting types is Friedrich-Wintgen (FW) BICs which result from the destructive interference of two resonant modes belonging to the same cavity. Here, we investigate theoretically and experimentally the existence of FW BICs in a side-coupled loop. The cavity is made of a loop of length $2d=d_2+d_3$ connected to a stub of length $d_4$. The whole cavity is attached vertically to two semi-infinite waveguides by a wire of length $d_1$. We demonstrate that the BICs can be induced either by the loop-stub system or by the two arms of lengths $d_2$ and $d_3$ of the loop for specific geometrical parameters. When a perturbation in the system produces a deviation from the BIC condition, the latter transforms to either electromagnetically induced transparency (EIT) or reflection (EIR) or Autler-Townes splitting (ATS) resonances. Both EIT and ATS exhibit similar features in the transmission spectrum, namely, a transparency window; however, they have different physical origins. Therefore, EIT and ATS resonances are fitted with corresponding analytical model expressions, revealing good agreements. The Akaike's information criterion is then used to quantitatively discern EIT from ATS and the transition from ATS to EIT is also carried out. Our theoretical results are obtained by means of the Green's function method which enables us to obtain the transmission and reflection coefficients, dispersion relations, as well as density of states and scattering matrix. An experimental validation of all these results is performed in the radio-frequency domain using coaxial cables.