We study the single-orbital Hubbard model on the half-filled decorated honeycomb lattice. In the noninteracting theory at half filling the Fermi energy lies within a flat band where strong correlations are enhanced. The lattice is highly frustrated. We find a correlation driven first-order metal-insulator transition to two different insulating ground states—a dimer valence bond solid Mott insulator when intertriangle correlations dominate, and a broken $C_3$-symmetry antiferromagnet that arises from frustration when intratriangle correlations dominate. The metal-insulator transitions into these two phases have very different characters. The metal-broken $C_3$ antiferromagnetic transition is driven by spontaneous $C_3$ symmetry breaking that lifts the topologically required degeneracy at the Fermi energy and opens an energy gap in the quasiparticle spectrum. The metal-dimer valence bond solid transition breaks no symmetries of the Hamiltonian. It is caused by strong correlations renormalizing the electronic structure into a phase that is adiabatically connected to both the trivial band insulator and the ground state of the spin-1/2 Heisenberg model in the relevant parameter regime. Therefore, neither of these metal-insulator transitions can be understood in either the Brinkmann-Rice or Slater paradigms.

We show that an introduction of disorder in a controlled way using 1 MeV argon (Ar) ion irradiation, suppresses the correlation driven metal-insulator transition (MIT) in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films. The films make a crossover to a heavily disordered conductor governed by weak localization (WL) and at even higher disorder, an Anderson localized state. The disorder (atomic displacement up to 2% of the total atoms) in the $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films was created using 1 MeV ${\mathrm{Ar}}^{4+}$ ion irradiation. We show that the pristine films of $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ exhibit an MIT with the conduction process being governed by variable range hopping (VRH). For disorder up to 1% of the displaced atoms or lower, the insulating state arising from a gap in the density of states (DOS) at the Fermi level ($E_F$) as in a Mott insulator is suppressed and the conduction in the film shows a WL behavior with finite conductivity at temperature $T\to 0$. This behavior is expected in a disordered conductor that does not have a gap in DOS at $E_F$. At higher fluences the conductivity reduces substantially but the electrical conduction shows a power-law temperature dependence with a small but finite zero temperature conductivity $\sigma (T=0)$ which is expected in a solid with electrons that are Anderson localized. A similar experiment was performed on the La substituted $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ films (${\text{Nd}}_{1-x}{\text{La}}_x{\text{NiO}}_3$) with $x=0.3$ that are grown in the same way. La substitution in $\mathrm{Nd}\mathrm{Ni}{\mathrm{O}}_3$ suppresses the temperature driven transition and leads to a metallic state with critical composition at $x\approx 0.3$. The pristine as well as films irradiated with lowest fluence shows metallic or marginally metallic behavior grown on $\mathrm{La}\mathrm{Al}{\mathrm{O}}_3$ and $\mathrm{Sr}\mathrm{Ti}{\mathrm{O}}_3$ substrates, respectively. However, at higher fluences they too exhibit a convergence in electronic transport and $\sigma$ shows a power-law temperature dependence at low $T$ with $\sigma (T=0)\ne 0$. Evidence of suppression of correlated behavior can also be seen in the irradiated films where the non-Gaussian nature of resistance fluctuation at $T\approx T_{\text{MI}}$, a signature of correlated electron systems, is suppressed on irradiation that leads to collapse of the MIT. Evidence for progressing disordering of the films on irradiation were observed in Raman spectroscopy as well as x-ray studies that show the basic integrity of the ${\text{NiO}}_6$ octahedra is preserved and the structure retains its crystallinity.

We study longitudinal electric and thermoelectric transport coefficients of Dirac fermions on a simple lattice model where tuning of a single parameter enables us to change the type of Dirac cones from type I to type II. We pay particular attention to the behavior of the critical situation, i.e., the type-III Dirac cone. We find that the transport coefficients of the type-III Dirac fermions behave as the limiting case of neither type I nor type II. On the one hand, the qualitative behaviors of the type-III case are similar to those of the type-I case. On the other hand, the transport coefficients do not change monotonically upon increasing the tilting; namely, the largest thermoelectric response is obtained not for the type-III case but for the optimally tilted type-I case. For the optimal case, sizable transport coefficients are obtained; for example, the dimensionless figure of merit is 0.18.

The response of solids to temperature gradients is often described in terms of a gravitational analog: the effect of a space-dependent temperature is modeled using a space-dependent metric. We investigate the validity of this approach in describing the bulk response of quantum Hall states and other gapped chiral topological states. To this end, we consider the prototypical Haldane model in two different cases of (i) a space-dependent electrostatic potential and gravitational potential and (ii) a space-dependent temperature and chemical potential imprinted by a weak coupling to noninteracting electron baths or phonons. We find that the thermal analog applied to theses cases is invalid; while a space-dependent gravitational potential induces transverse energy currents proportional to the third derivative of the gravitational potential, the response to an analogous temperature profile vanishes in limit of weak coupling to the thermal bath. Similarly, the Einstein relation, the analogy between the electrostatic potential and the internal chemical potential, is not valid in such a setup.

We apply the loop diagrammatic method for linear and nonlinear optics to the calculation of the sum-frequency response of a molecule-nanostructure composite system. The presence of the nanostructure modifies the molecular response through dipolar energy exchange, and the molecular hyperpolarizability is factorized by nanostructure response functions of increasing orders. We provide a general method to transform these functions into products of first-order nanostructure polarizabilities, accounting for enhancements of the molecular response by coupling to plasmonic or excitonic resonances. In particular, we show how the diagrams may be directly read to determine the response functions and their factorization without explicit calculation. The methodology provides a frame for various applications to other systems, interactions, and nonlinear optical processes.

We set up a general microscopic theory for the transfer of energy, momentum, and angular momentum mediated by photons. Using the nonequilibrium Green's function method, we propose a unified Meir-Wingreen formalism for the energy emitted, force experienced, and torque experienced by the objects due to the fluctuating electromagnetic field. Our theory does not require the local thermal equilibrium that is the central assumption of the conventional theory of fluctuational electrodynamics (FE). The obtained formulas are valid for arbitrary objects as well as the environment without the requirement of reciprocity. To show the capability of our microscopic theory, we apply the general formulas to transport problems of graphene edges in both equilibrium and nonequilibrium situations. We show the local equilibrium energy radiation of graphene obeys the well-known $T^4$ law with a converged theoretical emissivity of $2.058\%$. In the ballistic nonequilibrium situation driven by chemical potential biases, we observe nonzero results for force and torque from the graphene edges, which go beyond the predictive ability of the FE theory. Our method is general and efficient for large systems, which paves the way for studying more complex transport phenomena in the nonequilibrium regime.

The environmental interaction is a fundamental consideration in any controlled quantum system. While an interaction with a dissipative bath can lead to decoherence, it can also provide desirable emergent effects including induced spin-spin correlations. In this Letter, we show that under quite general conditions, a dissipative bosonic bath can induce a long-range ordered phase, without the inclusion of any additional direct spin-spin couplings. Through a quantum-to-classical mapping and classical Monte Carlo simulation, we investigate the $T=0$ quantum phase transition of an Ising chain embedded in a bosonic bath with Ohmic dissipation. We show that the quantum critical point is continuous, Lorentz invariant with a dynamical critical exponent $z=1.07\left(9\right)$, has a correlation length exponent $\nu =0.80\left(5\right)$, and anomalous exponent $\eta =1.02\left(6\right)$, thus the universality class is distinct from the previously studied limiting cases. The implications of our results on experiments in ultracold atomic mixtures and qubit chains in dissipative environments are discussed.

We observe nonequilibrium dynamical magnetic behavior in the magnetically ordered phase of a Tsai-type Tb-Au-Si quasicrystal approximant system. The magnetic texture in the ordered phase is found to exhibit scalar spin chirality (SSC) order, inferring that SSC is the order parameter of the present magnetic system. We further find that the introduction of “pseudo-Tsai” clusters, associated with additional Tb atoms in the structure, induces spin-glass dynamics. We discuss the observed dynamical magnetic behavior in the Tb-Au-Si systems, considering the effect of the pseudo-Tsai clusters on the magnetic configuration and local spin chirality.

The kagome metal compounds $A{\mathrm{V}}_3{\mathrm{Sb}}_5$ ($A=\text{K}$, Rb, and Cs) feature a wealth of phenomena including nontrivial band topology, charge density wave (CDW), and superconductivity. One intriguing property is the time-reversal symmetry breaking in the CDW state without local moments, which leads to anomalous transport responses. Here, we report the investigation of magnetothermoelectric effects on high-quality ${\mathrm{CsV}}_3{\mathrm{Sb}}_5$ single crystals. A large anomalous Nernst effect (ANE) is observed at temperatures below 30 K and can be enhanced by the high mobility. Multiple Fermi surfaces with small effective masses are revealed by quantum oscillations in the Nernst and Seebeck signals. Furthermore, we discover a magnetic breakdown effect across the two smallest Fermi surfaces, with a gap around 20 meV between them. We propose that the two Fermi surfaces are split from a Dirac band by the CDW gap. These results indicate the large ANE originates from the CDW modulated nontrivial band structure as well as the extrinsic contributions. A second phase transition below the CDW transition temperature is also suggested by the strange temperature dependence of the ANE.

We report a detailed theoretical and experimental study on the vanadium monotelluride VTe, which crystallizes in the NiAs hexagonal structure. First-principles calculations reveal a complex hierarchy of magnetic interactions and energy scales, with the ground state theoretically determined as an ($\frac{1}{2}$, 0, $\frac{1}{2}$) antiferromagnetic ordering with insulating character and a band gap of 0.5 eV. Experimental synthesis and characterization efforts find a substantially off-stoichiometric orthorhombic structure (a defect NiAs structure) with composition ${\mathrm{V}}_{0.85}\mathrm{Te}$, and an apparent Néel point of some 45 K. First-principles calculations find good agreement with the observed Néel point. We also give an extended examination of the effects of off-stoichiometry on the calculated energetics, finding significant volume-related effects. Our first-principles calculations find the stoichiometric phase VTe to have a negative vanadium defect formation energy of over 1 eV, thus explaining the formation of the off-stoichiometric phase. Finally, we provide a structural explanation for the formation of defect structures in this and numerous other NiAs-structure materials.

The control of spin waves in periodic magnetic structures has facilitated the realization of many functional magnonic devices, such as band stop filters and magnonic transistors, where the geometry of the crystal structure plays an important role. Here, we report on the magnetostatic mode formation in an artificial magnetic structure, going beyond the crystal geometry to a fractal structure, where the mode formation is related to the geometric scaling of the fractal structure. Specifically, the precessional dynamics was measured in samples with structures going from simple geometric structures toward a Sierpinski carpet and a Sierpinski triangle. The experimentally observed evolution of the precessional motion could be linked to the progression in the geometric structures that results in a modification of the demagnetizing field. Furthermore, we have found sets of modes at the ferromagnetic resonance frequency that form a scaled spatial distribution following the geometric scaling. Based on this, we have determined the two conditions for such mode formation to occur. One condition is that the associated magnetic boundaries must scale accordingly, and the other condition is that the region where the mode occurs must not coincide with the regions for the edge modes. This established relationship between the fractal geometry and the mode formation in magnetic fractals provides guiding principles for their use in magnonics applications.

It is well established that it is possible to switch certain antiferromagnets electrically, yet the interplay of Néel-spin-orbit torques and thermal activation is only poorly understood. Combining ab initio calculations and atomistic spin dynamics simulations we develop a multiscale model to study the current-induced switching in ${\mathrm{Mn}}_2\mathrm{Au}$. We compute from first principles the strength and direction of the electrically induced magnetic moments, caused by the Rashba-Edelstein effect, and take these into account in atomistic spin dynamics simulations. Our simulations reveal the switching paths as well as the timescales for switching. The size of the induced moments, however, turns out to be insufficient to lead to fully deterministic switching. Instead, we find that a certain degree of thermal activation is required to help overcome the relevant energy barrier.

Motivated by recent experimental realizations of exotic phases of matter on programmable quantum simulators, we carry out a comprehensive theoretical study of quantum phase transitions in a Rydberg atom array on a square lattice, with both open and periodic boundary conditions. In the bulk, we identify several first-order and continuous phase transitions by performing large-scale quantum Monte Carlo simulations and develop an analytical understanding of the nature of these transitions using the framework of Landau-Ginzburg-Wilson theory. Remarkably, we find that under open boundary conditions, the boundary itself undergoes a second-order quantum phase transition, independent of the bulk. These results explain recent experimental observations and provide important insights into both the adiabatic state preparation of novel quantum phases and quantum optimization using Rydberg atom array platforms.

Phonon-related behaviors such as hot-carrier bottleneck and thermooptic coefficients stimulate the study of lattice dynamics and thermal conductivity ($\kappa$) in perovskites. Herein, we report ultralow thermal conductivities of synthesized highly oriented samples of ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$ ($\sim$ 0.45 W/mK at 300 K) and ${\mathrm{Cs}}_2{\mathrm{SnI}}_2{\mathrm{Cl}}_2$ ($\sim$ 0.60 W/mK at 300 K). Phonon frequency renormalization of the octahedral rotational soft modes in ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$ triggered by the fourth-order anharmonicity considerably amends the three- and four-phonon linewidths, significantly altering the thermal transport property of ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$. Furthermore, the phonon gas model partially breaks down, and both normal and diffuson-like phonons must be considered to capture the nearly temperature-independent $\kappa$ of ${\mathrm{Cs}}_2{\mathrm{PbI}}_2{\mathrm{Cl}}_2$. We also find these unusual phonon behaviors in other layered inorganic perovskites such as ${\mathrm{Cs}}_2{\mathrm{PbBr}}_2{\mathrm{Cl}}_2$ and ${\mathrm{Cs}}_2{\mathrm{SnI}}_2{\mathrm{Cl}}_2$. Our results rationalize the crucial high-order phonon interactions and two-channel lattice thermal transports in perovskites with ultralow $\kappa$.

Hybrid transition metal oxides continue to attract attention due to their multiple degrees of freedom (e.g., lattice, charge, spin, and orbital) and versatile properties. Here we investigate the magnetic and electronic properties of the newly synthesized double perovskite ${\mathrm{Ba}}_2{\mathrm{NiIrO}}_6$ using crystal-field theory, superexchange model analysis, density functional calculations, and parallel tempering Monte Carlo (PTMC) simulations. Our results indicate that ${\mathrm{Ba}}_2{\mathrm{NiIrO}}_6$ has the ${\mathrm{Ni}}^{2+}$ ($t_{2g}^6{e}_g^2$)-${\mathrm{Ir}}^{6+}$ ($t_{2g}^3$) charge states. The first nearest-neighboring ${\mathrm{Ni}}^{2+}-{\mathrm{Ir}}^{6+}$ ions prefer a ferromagnetic coupling, as expected from the Goodenough-Kanamori-Anderson rules, which contradicts the experimental antiferromagnetic (AF) order in ${\mathrm{Ba}}_2{\mathrm{NiIrO}}_6$. We find that the strong second nearest-neighboring (2NN) AF couplings are frustrated in the fcc sublattices, and they play a major role in determining the observed AF ground state. We also prove that the $J_{\mathrm{eff}}$ = 3/2 and $J_{\mathrm{eff}}$ = 1/2 states induced by spin-orbit coupling, which would be manifested in low-dimensional (e.g., layered) iridates, are, however, not the case for cubic ${\mathrm{Ba}}_2{\mathrm{NiIrO}}_6$. Our PTMC simulations show that when the long-range (2NN and 3NN) AF interactions are included, an AF transition with $T_{\mathrm{N}}$ = 66 K would be obtained, and it is well comparable with the experimental 51 K. Meanwhile, we propose a possible 2$\times 2\times 2$ noncollinear AF structure for ${\mathrm{Ba}}_2{\mathrm{NiIrO}}_6$.

Perpendicular magnetic anisotropy at an interface between Fe and MgO with Co-phthalocyanine (CoPc) adsorption has been investigated. We find that the CoPc adsorption at an Fe/MgO interface increases the perpendicular magnetic anisotropy energy (PMA) while maintaining the voltage-controlled magnetic anisotropy effect. By performing x-ray magnetic circular dichroism spectroscopy measurements, we reveal that the CoPc adsorption induces hole accumulation at the Fe surface. The origin of the PMA enhancement is attributed to the increased orbital magnetic moment anisotropy of the Fe atoms.

Ferromagnetic (FM) films with higher magnetic moment density typically exhibit sizable in-plane magnetic anisotropy as a result of strong dipolar interactions, which hinder their application in state-of-the-art perpendicularly based magnetic devices. This study reports the effects of triggering the perpendicular magnetic anisotropy (PMA) of a Co/Fe film with strong in-plane magnetic anisotropy through nearby antiferromagnetic (AFM) fcc ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ and face-centered tetragonal (e-fct) Mn films. Our results demonstrate that fcc ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ films with a three-dimensional quadratic AFM spin structure can trigger robust PMA in a Co/Fe film through collinearlike AFM-FM exchange coupling at room temperature. Furthermore, by incorporating a monolayered ${\mathrm{Fe}}_{50}{\mathrm{Mn}}_{50}$ film into the Mn-Co interface or by reducing the temperature, PMA can also be triggered in a Co/Fe film by using e-fct Mn films with an in-plane-oriented AFM spin structure at room or low temperature through noncollinear exchange coupling across the AFM-FM interface. Our results clarify the exchange-coupling mechanisms, characteristic behaviors, and critical conditions for increasing control over antiferromagnet-induced PMA in FM films with high magnetic moment density but strong in-plane magnetic anisotropy, which is helpful for the development of next-generation perpendicularly based spintronic devices.

The effect of pressure ($P$) on magnetic susceptibility of ${\mathrm{CsCuCl}}_3$ was examined in magnetic fields (${\mu }_{0}H$) of up to 51 T using a proximity-detector oscillator (PDO), and the $H\text{−}P$ phase diagram of ${\mathrm{CsCuCl}}_3$ was constructed over the saturation field ($H_{\mathrm{sat}}$). We found that, with increasing $P, H_{\mathrm{sat}}$ increases and the up-up-down phase that appeared at $P=0.7\mathrm{GPa}$ widened. Based on comparison between the experimental and calculated $H\text{−}P$ phase diagrams, the Y phase was predicted to appear above 1.7 GPa. The interchain antiferromagnetic exchange interaction in the $ab$ plane was evaluated and found to increase with increasing $P$, which is consistent with a previous study under high pressure [D. Yamamoto et al., Nat. Commun. 12, 4263 (2021)]. Moreover, an anomaly was observed below $P=0.6\mathrm{GPa}$ just below $H_{\mathrm{sat}}$ and might be a new phase transition derived from nonlinear response caused by the PDO technique.

Parafermion modes are non-Abelian anyons which were introduced as ${\mathbb{Z}}_N$ generalizations of ${\mathbb{Z}}_2$ Majorana states. In particular, ${\mathbb{Z}}_3$ parafermions can be used to produce Fibonacci anyons, laying a path towards universal topological quantum computation. Due to their fractional nature, much of the theoretical work on ${\mathbb{Z}}_3$ parafermions has relied on bosonization methods or parafermionic quasiparticles. In this paper, we introduce a representation of ${\mathbb{Z}}_3$ parafermions in terms of purely fermionic models. We establish the equivalency of a family of lattice fermionic models written in the basis of the $t-J$ model with a Kitaev-like chain supporting free ${\mathbb{Z}}_3$ parafermionic modes at its ends. By using density matrix renormalization group calculations, we are able to characterize the topological phase transition and study the effect of local operators (doping and magnetic fields) on the spatial localization of the parafermionic modes and their stability. Moreover, we discuss the necessary ingredients towards realizing ${\mathbb{Z}}_3$ parafermions in strongly interacting electronic systems.