Quantum spin liquid (QSL) state has attracted a great deal of attention as a precursor of high-temperature superconductor. The promising QSL candidates such as $\alpha \text{−}{\mathrm{RuCl}}_3, {\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ with honeycomb structure are insulators or semiconductors. An insulator-metal transition (IMT) is necessary for hosting superconducting state. However, all of them have a robust insulating state, even under pressure. Here, an alternative candidate of QSL, ${\mathrm{Cu}}_2{\mathrm{IrO}}_3$ is investigated under high pressure and low temperature through x-ray diffraction and electric properties measurement. A dimerization occurs at 7.1 GPa which is similar to $\alpha \text{−}{\mathrm{RuCl}}_3$ and $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$, accompanied with an anomaly of resistance at $\sim 5.0\mathrm{GPa}$. The IMT appears under a pressure of $\sim 13.8\mathrm{GPa}$, which is absent in $\alpha \text{−}{\mathrm{RuCl}}_3, {\mathrm{Na}}_2{\mathrm{IrO}}_3$ and $\alpha \text{−}{\mathrm{Li}}_2{\mathrm{IrO}}_3$ under pressure. It is related to the crystal structure transition at $\sim 11.1\mathrm{GPa}$, which induces the decline of interlayer distance. Furthermore, a strange crossover from negative to positive magnetoresistance is observed under pressure of 21.8 to 24.2 GPa at 2 K. This may be due to the valence change of ${\mathrm{Cu}}^{+}$ to ${\mathrm{Cu}}^{2+}$, as a result of the decreasing of interlayer distance and the shortening of O–Cu–O bond under pressure.

We studied the ferroelectricity and magnetic ordering of multiferroic orthorhombic perovskite ${\mathrm{YMnO}}_3$ using magnetic and dielectric measurements and polarized/unpolarized neutron diffraction experiments with a single crystal. The ferroelectric polarization of a relatively large value $P\simeq 0.55\mu \mathrm{C}/{\mathrm{cm}}^2$ was observed below $T=29\mathrm{K}$. The magnetic $\mathit{k}$ vector was identified as $\mathit{k}=(0,0.487,0)$ in the low-temperature ferroelectric phase. Based on the spherical neutron polarimetry experiment, the magnetic structure was determined to be of collinear $E$-type ordering with spins along the $b$ axis. The $E$-type state did not coexist with other states in the low-temperature ferroelectric phase. A simple domain structure model indicated that a single collinear $E$-type structure with a small incommensurability can be realized as the magnetic ground state of ${\mathrm{YMnO}}_3$.

Superconducting high entropy alloys (HEAs) are emerging as an exotic class of superconducting materials, providing a unique opportunity to understand the complex interplay of disorder and superconductivity. We report the synthesis and detail bulk and microscopic characterization of a ${\mathrm{Nb}}_{60}{\mathrm{Re}}_{10}{\mathrm{Zr}}_{10}{\mathrm{Hf}}_{10}{\mathrm{Ti}}_{10}$ HEA alloy using transport, magnetization, specific heat, and muon spin rotation/relaxation ($\mu \mathrm{SR}$) measurements. Bulk superconductivity with transition temperature $T_C=5.7$ K is confirmed by magnetization, resistivity, and heat capacity measurements. Zero-field $\mu \mathrm{SR}$ measurements confirm that the superconducting state preserves time-reversal symmetry, and transverse-field measurements show that the superfluid density is well described by an isotropic $s$-wave model.

We study the ground-state properties of a spin-1 Heisenberg model on a square lattice with the first- and second-nearest-neighbor antiferromagnetic couplings $J_1$ and $J_2$ and a three-spin scalar chirality term $J_{\chi }$. Using the density matrix renormalization group calculation, we map out a global phase diagram including various magnetic order phases and an emergent quantum spin liquid phase. The nature of the spin liquid is identified as a bosonic non-Abelian Moore-Read state from the fingerprint of the entanglement spectra and identification of a full set of topological sectors. We further unveil a stripe magnetic order coexisting with this spin liquid. Our results not only establish a rare example of non-Abelian spin liquids in simple spin systems but also demonstrate the coexistence of fractionalized excitations and magnetic order beyond mean-field descriptions.

We present a bosonization procedure, which replaces fermions with generalized spin variables subject to local constraints. It requires that the number of Majorana modes per lattice site matches the coordination number modulo two. If this condition is not obeyed, then bosonization introduces additional fermionic excitations not present in the original model. In the case of one Majorana mode per site on a honeycomb lattice, we recover a sector of Kitaev's model. We discuss also decagonal and rectangular geometries and present bosonization of the Hubbard model. For geometries with a boundary we find that certain fermionic edge modes naturally emerge. They are of different nature than edge modes encountered in topological phases of matter. Euclidean representation for the unconstrained version of a spin system of the type arising in our construction is derived and briefly studied by computing some exact averages for small volumes.

We use hybrid density functional calculations to assess $n$-type doping in monoclinic $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloys. We focus on silicon, the most promising donor dopant, and study the structural properties, formation energies, and charge-state transition levels of its various configurations. We also explore the impact of carbon and hydrogen, which are common impurities in metal-organic chemical vapor deposition (MOCVD). In ${\mathrm{Ga}}_2{\mathrm{O}}_3, {\mathrm{Si}}_{\mathrm{Ga}}$ is an effective shallow donor, but in ${\mathrm{Al}}_2{\mathrm{O}}_3{\mathrm{Si}}_{\mathrm{Al}}$ acts as a $\mathit{DX}$ center with a ($+/-$) transition level in the band gap. Interstitial hydrogen acts as a shallow donor in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ but behaves as a compensating acceptor in $n$-type ${\mathrm{Al}}_2{\mathrm{O}}_3$. Interpolation indicates that Si is an effective donor in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ up to 70% Al, but it can be compensated by hydrogen already at 1% Al. We also assess the diffusivity of hydrogen and study complex formation. ${\mathrm{Si}}_{\mathrm{cation}}\text{−}\mathrm{H}$ complexes have relatively low binding energies. Substitutional carbon on a cation site acts as a shallow donor in ${\mathrm{Ga}}_2{\mathrm{O}}_3$, but can be stable in a negative charge state in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ when $x>5\%$. Substitutional carbon on an oxygen site (${\mathrm{C}}_{\mathrm{O}}$) always acts as an acceptor in $n$-type $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$, but will incorporate only under relatively oxygen-poor conditions. ${\mathrm{C}}_{\mathrm{O}}\text{−}\mathrm{H}$ complexes can actually incorporate more easily, explaining observations of carbon-related compensation in ${\mathrm{Ga}}_2{\mathrm{O}}_3$ grown by MOCVD. We also investigate ${\mathrm{C}}_{\mathrm{cation}}\text{−}\mathrm{H}$ complexes, finding they have high binding energies and act as compensating acceptors when $x>56\%$; otherwise the hydrogen just passivates the unintentional carbon donors. C-H complex formation explains why MOCVD-grown ${\mathrm{Ga}}_2{\mathrm{O}}_3$ can exhibit record-low free-carrier concentrations, in spite of the unavoidable incorporation of carbon. Our study highlights that, while Si is in principle a suitable shallow donor in $({\mathrm{Al}}_x{\mathrm{Ga}}_{1-x}{)}_2{\mathrm{O}}_3$ alloys up to high Al compositions, control of unintentional impurities is essential to avoid compensation.

In bilayer graphene, the application of a perpendicular electric field breaks the inversion symmetry to open a bulk band gap to harbor the quantum valley Hall effect. When the field varies spatially, a topologically confined mode (also named the zero-line mode) arises along the zero-field line. In this work, we theoretically investigate the electronic transport properties of the multichannel zero-line systems. The finite-size effect in topological systems (e.g., quantum Hall effect, topological insulators) often induces a topologically trivial gap to realize a normal insulator. To our surprise, we find that the coupling between neighboring zero lines can give rise to striking electronic properties depending on the number of channels $m$, i.e., a trivial band gap for even $m$, whereas a nontrivial gapless mode for odd $m$. We further show that these findings apply to various ribbon orientations. A general effective model is constructed to provide a clear physical picture of the emergence of gapless modes. In the end, a gate-tunable device is proposed to function as a switch with controllable current partitions. We believe that our findings are experimentally accessible, and have potential practical applications in designing multifunctional valley-based electronics.

Shubnikov–de Hass oscillations are directly observed in undoped antiferromagnetic topological insulator $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$. With increasing magnetic fields, the oscillation period decreases gradually in the magnetic transition from canted antiferromagnetism to ferromagnetism and then saturates in high magnetic fields, indicating the field-induced evolution of the band structure. From the analysis of the high-field oscillations, a nontrivial Berry phase and a small effective mass are extracted. Furthermore, rotating the magnetization of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$ can lead to a splitting of the high-field oscillations, which suggests the enhanced asymmetry of the Weyl cones in tilted fields. Therefore, the observation of these magnetically tunable quantum oscillations clearly demonstrates the indispensable role of field in tuning the band structure or physical properties of $\mathrm{Mn}{\mathrm{Bi}}_2{\mathrm{Te}}_4$.

The competition and interplay between charge-density wave and superconductivity have become a central subject for quasi-two-dimensional compounds. Some of these materials, such as the transition-metal dichalcogenides, exhibit strong electron-phonon coupling, an interaction that may favor both phases, depending on the external parameters, such as hydrostatic pressure. In view of this, here we analyze the single-band $t\text{−}{t}^{\prime }$ Holstein model in the square lattice, adding a next-nearest neighbor hopping $t^{\prime }$ in order to play the role of the external pressure. To this end, we perform unbiased quantum Monte Carlo simulations with an efficient inversion sampling technique appropriately devised for this model. Such a methodology drastically reduces the autocorrelation time and increases the efficiency of the Monte Carlo approach. By investigating the charge-charge correlation functions, we obtain the behavior of the critical temperature as a function of $t^{\prime }$ and, from compressibility analysis, we show that a first-order metal-to-insulator phase transition occurs. We also provide a low-temperature phase diagram for the model.

Crystal symmetry plays a critical role in thermal transport in solids. One example is mirror symmetry in monolayer graphene that promotes thermal transport by forbidding certain phonon scattering channels. Here, we show an interesting counterexample in monolayer ${\mathrm{Ta}}_2\mathrm{C}{\mathrm{S}}_2$ that inversion symmetry breaking can abnormally enhance lattice thermal conductivity of two-dimensional (2D) functionalized MXenes based on the phonon Boltzmann transport theory combined with first-principles calculations. We find inversion symmetry breaking not only affects scattering channels but also redistributes the charge and changes the anharmonic phonon properties. Both atomic vibrations in real space and phonon scattering rates in the reciprocal space confirm the significantly reduced lattice anharmonicity in an asymmetric ${\mathrm{Ta}}_2\mathrm{C}{\mathrm{S}}_2$ sheet, leading to substantially enhanced thermal conductivity after inversion symmetry breaking. The physical origin for the variation of anharmonic properties induced by symmetry is discussed in detail. In this paper, we provide insight into the complex role of symmetry on thermal transport in 2D functionalized MXenes.

Surface energy is a key quantity that controls many physical properties of materials, yet determining its value at the nanoscale remains challenging. By using $N$-body interatomic potentials and performing analytical calculations, we develop a robust approach to determine the surface energy of metallic nanoparticles as a function of particle size and temperature. A strong increase in the surface energy is obtained when the size of the nanoparticle decreases in both the solid and liquid states. However, we show how the use of the classical spherical approximation to characterize the surface area of a nanoparticle leads to an almost constant surface energy with size as usually done to characterize the thermodynamic and kinetic properties of NPs in many works. We then propose a correction of the spherical approximation that is particularly useful for small size nanoparticles to improve the different models developed in the literature so far.

We study the confinement of Dirac fermions in armchair graphene nanoribbons by means of electrostatic quantum dots. We provide an analytically feasible model where some bound states can be found explicitly. We show that the energies of these bound states belong either to the gap of valence and conducting bands or they represent bound states in the continuum whose energies are embedded in the continuous spectrum. The solutions satisfying armchair boundary conditions are found in an elegant manner with the use of specific projection operators.

A single-crystal sheet of graphene is synthesized on the low-symmetry substrate Ir(110) by thermal decomposition of ${\mathrm{C}}_2{\mathrm{H}}_4$ at 1500 K. Using scanning tunneling microscopy, low-energy electron diffraction, angle-resolved photoemission spectroscopy, and ab initio density functional theory, the structure and electronic properties of the adsorbed graphene sheet and its moiré with the substrate are uncovered. The adsorbed graphene layer forms a wave pattern of nanometer wavelength with a corresponding modulation of its electronic properties. This wave pattern is demonstrated in density functional theory calculations to enable the templated adsorption of naphthalene molecules, and in experiment to uniaxially align sandwich-molecular wires composed of Eu and cyclooctatetraene molecules.

The poor heat conduction between carbon nanotubes (CNTs) has become a severe obstacle, hindering the applications for thermal management. Considering that the across-tube heat transport is limited by the small fraction of low-frequency phonons (LFPs), it is of great necessity to develop a strategy to excite LFPs without modifying the tube contacts. Herein, a mechanism of LFP resonance is proposed by assigning soft long-chain polymers (e.g., polyurethane and polyethylene) to the groove sites of contacted CNTs. The resonance takes places at multiple points and in wide ranges of frequencies and wave vectors, originating from the rich low-frequency vibrations localized at the soft segments of the polymer. More excited phonons lead to an extraordinary improvement on the across-tube heat conductance, by a factor of 79%, which opens a route to develop advanced CNT-based thermal functional materials.

We investigate the occurrence of many-body localization (MBL) on a spin-1/2 transverse-field Ising model defined on a Chimera connectivity graph with random exchange interactions and longitudinal fields. We observe a transition from an ergodic phase to a nonthermal phase for individual energy eigenstates induced by a critical disorder strength for the Ising parameters. Our result follows from the analysis of both the mean half-system block entanglement and the energy-level statistics. We identify the critical point associated with this transition using the maximum variance of the block entanglement over the disorder ensemble as a function of the disorder strength. The calculated energy density phase diagram shows the existence of a mobility edge in the energy spectrum. In terms of the energy-level statistics, the system changes from the Gaussian orthogonal ensemble for weak disorder to a Poisson distribution limit for strong randomness, which implies localization behavior. We then realize the time-independent disordered Ising Hamiltonian experimentally using a reverse annealing quench-pause-quench protocol on a $\mathrm{D}$-wave 2000Q programmable quantum annealer. We characterize the transition from the thermal to the localized phase through magnetization measurements at the end of the annealing dynamics, and the results are compatible with our theoretical prediction for the critical point. However, the same behavior can be reproduced using a classical spin-vector Monte Carlo simulation, which suggests that genuine quantum signatures of the phase transition remain out of reach using this experimental platform and protocol.

It is known that in chiral magnets with intrinsic inversion symmetry breaking, two spin waves moving in opposite directions can propagate at different velocities, exhibiting a phenomenon called magnetochiral nonreciprocity, which allows for realizations of certain spin logic devices such as a spin-wave diode. Here, we theoretically demonstrate that the spin-wave nonreciprocity can occur without intrinsic bulk chirality in easy-cone ferromagnets and easy-cone antiferromagnets. Specifically, we show that nonlocal injection of a spin current from proximate normal metals to easy-cone magnets engenders a nonequilibrium chiral spin texture, on top of which spin waves exhibit nonreciprocity proportional to the injected spin current. In particular, the easy-cone ferromagnet is shown to support the spin-wave nonreciprocity without an external field, in contrast to the previously known easy-plane ferromagnetic counterpart that requires an external field, thereby providing a field-free means to manipulate the spin-wave nonreciprocity. One notable feature of the nonreciprocal spin waves is their gapless nature, which can lead to a large thermal rectification effect at sufficiently low temperatures.

In insulating lanthanides, unquenched orbital momentum and weak crystal-field (CF) splitting of the atomic $J$ multiplet at lanthanide ions result in a highly ranked (multipolar) exchange interaction between them and a complex low-temperature magnetic order not fully uncovered by experiment. Explicitly correlated ab initio methods proved to be highly efficient for an accurate description of CF multiplets and magnetism of individual lanthanide ions in such materials. Here we extend this ab initio methodology and develop a first-principles microscopic theory of multipolar exchange interaction between $J$ multiplets in $f$-metal compounds. The key point of the approach is a complete account of Goodenough's exchange mechanism along with traditional Anderson's superexchange and other contributions, the former being dominant in many lanthanide materials. Application of this methodology to the description of the ground-state order in the neodymium nitride with rocksalt structure reveals the multipolar nature of its ferromagnetic order. We found that the primary and secondary order parameters (of $T_{1u}$ and $E_g$ symmetry, respectively) contain non-negligible $J$-tensorial contributions up to the ninth order. The calculated spin-wave dispersion and magnetic and thermodynamic properties show that they cannot be simulated quantitatively by confining to the ground CF multiplet on the Nd sites. Our results demonstrate that the ab initio approach to the low-energy Hamiltonian represents a powerful tool for the study of materials with complex magnetic order.