We analyze the effect of disorder on the weak-coupling instabilities of a quadratic band crossing point (QBCP) in two-dimensional Fermi systems, which, in the clean limit, display interaction-driven topological insulating phases. In the framework of a renormalization group procedure, which treats fermionic interactions and disorder on the same footing, we test all possible instabilities and identify the corresponding ordered phases in the presence of disorder for both single-valley and two-valley QBCP systems. We find that disorder generally suppresses the critical temperature at which the interaction-driven topologically nontrivial order sets in. Strong disorder can also drive a topological phase transition into a topologically trivial insulating state.

We study the critical properties of the ${\mathrm{QED}}_3$-Gross-Neveu model with $2N$ flavors of two-component Dirac fermions coupled to a massless scalar field and a U(1) gauge field. For $N=1$, this theory has recently been suggested to be dual to the SU(2) noncompact ${\mathrm{CP}}^1$ model that describes the deconfined phase transition between the Néel antiferromagnet and the valence bond solid on the square lattice. For $N=2$, the theory has been proposed as an effective description of a deconfined critical point between chiral and Dirac spin liquid phases, and may potentially be realizable in spin-$1/2$ systems on the kagome lattice. We demonstrate the existence of a stable quantum critical point in the ${\mathrm{QED}}_3$-Gross-Neveu model for all values of $N$. This quantum critical point is shown to escape the notorious fixed-point annihilation mechanism that renders plain ${\mathrm{QED}}_3$ (without scalar-field coupling) unstable at low values of $N$. The theory exhibits an upper critical space-time dimension of four, enabling us to access the critical behavior in a controlled expansion in the small parameter $\epsilon =4-D$. We compute the scalar-field anomalous dimension ${\eta }_{\varphi }$, the correlation-length exponent $\nu$, as well as the scaling dimension of the flavor-symmetry-breaking bilinear $\overline{\psi }{\sigma }^z\psi$ at the critical point, and compare our leading-order estimates with predictions of the conjectured duality.

Color-code quantum computation seamlessly combines Majorana-based hardware with topological error correction. Specifically, as Clifford gates are transversal in two-dimensional color codes, they enable the use of the Majoranas' non-Abelian statistics for gate operations at the code level. Here, we discuss the implementation of color codes in arrays of Majorana nanowires that avoid branched networks such as T junctions, thereby simplifying their realization. We show that, in such implementations, non-Abelian statistics can be exploited without ever performing physical braiding operations. Physical braiding operations are replaced by Majorana tracking, an entirely software-based protocol which appropriately updates the Majoranas involved in the color-code stabilizer measurements. This approach minimizes the required hardware operations for single-qubit Clifford gates. For Clifford completeness, we combine color codes with surface codes, and use color-to-surface-code lattice surgery for long-range multitarget CNOT gates which have a time overhead that grows only logarithmically with the physical distance separating control and target qubits. With the addition of magic state distillation, our architecture describes a fault-tolerant universal quantum computer in systems such as networks of tetrons, hexons, or Majorana box qubits, but can also be applied to nontopological qubit platforms.

We present the electromagnetic scattering theory for a finite-length nanowire with an embedded mesoscopic object. The theory is based on a synthesis of the integral equation technique of classical electrodynamics and the quantum transport formalism. We formulate Hallén-type integral equations, where the canonical integral operators from wire antenna theory are combined with special terms responsible for the mesoscopic structure. The theory is applied to calculate the polarizability of a finite-length single-walled carbon nanotube (CNT) with a short low-conductive section (LCS) in the microwave and subterahertz ranges. The LCS is modeled as a multichannel two-electrode mesoscopic system. The effective resistive sheet impedance boundary conditions for the scattered field are applied on the CNT surface. It is shown that the imaginary part of the polarizability spectrum has three peaks. Two of them are in the terahertz range, while the third is in the gigahertz range. The polarizability spectrum of the CNT with many LCSs has only one gigahertz peak, which shifts to low frequencies as the number of LCSs increases. The physical nature of these peaks is explained, and potential applications of nanoantennas are proposed.

The anomalous Nernst effect (ANE) is one of the most important mechanisms to explore the anomalous Hall heat current in ferromagnets. In this work, we studied the ANE in various ferromagnetic materials with in-plane anisotropy. Surprisingly, we show that the thickness dependence of the ANE on the magnitude and sign is nontrivial, even in conventional ferromagnetic metals (FMs), including Fe, Co, Ni, and Py ($\mathrm{N}{\mathrm{i}}_{80}\mathrm{F}{\mathrm{e}}_{20}$). While the sign of the ANE of Fe is opposite to that of Co, Ni, and Py in thicker films, it can even be reversed via decreasing thickness. Most importantly, the anomalous Nernst angles ${\theta }_{\mathrm{ANE}}$ for these FMs show a unified behavior. They can be significantly enhanced by up to one order of magnitude in ultrathin films. By systematically studying the thickness dependence of the electrical and thermal transport properties, we show that the enhanced ANE of FMs is dominated by spin-orbit coupling through the intrinsic and side-jump mechanisms.

The method of counting loops for calculating the partition function of the Ising model on the two dimensional square lattice is extended to lacunary planar lattices, especially scale invariant fractal lattices, the Sierpiński carpets with different values of the scale invariance ratio and of the number of deleted sites. The critical temperatures $T_c$ are exactly calculated for finite iteration steps $k$ up to $k=8$, for a range of the scale invariance ratio $n$ from 3 to 1000 and of the number of deleted sites from ${(n-2)}^2$ to ${(n-10)}^2$. The critical parameters $v_c=tanh(1/T_c)$ satisfy power laws of exponent -$k$, allowing the asymptotic extrapolation to the fractal limit $k\to \infty$. The extrapolated values of the critical temperature fit quite reliably power laws versus the fractal dimension. Results are compared to previous estimations obtained by numerical methods. Thermodynamical functions are also calculated and the fractal spectra of the Ising partition functions on several examples of Sierpiński carpets are illustrated.

X-ray magnetic circular dichroism (XMCD) and x-ray magnetic linear dichroism (XMLD) have been used to investigate the Fe magnetic response during the spin-reorientation transition (SRT) in $\mathrm{TmFe}{\mathrm{O}}_3$. Comparing the Fe XMLD results with neutron-diffraction and magnetization measurements on the same sample indicates that the SRT has an enhanced temperature range in the near surface region of approximately 82 to 120 K compared to approximately 82 to 92 K in bulk. This view is supported by complementary resonant soft x-ray-diffraction experiments at the Tm $M_5$ edge. These measurements find an induced magnetic moment on the Tm sites, which is well described by a dipolar mean-field model originating from the Fe moments. Even though such a model can describe the $4f$ response in the experiments, it is insufficient to describe the SRT even when considering a change in the $4f$ anisotropy. Moreover, the results of the Fe XMCD show a different temperature evolution through the SRT, the interpretation of which is hampered by additional spectral shape changes of the XCMD signal.

Chiral spin liquids (CSLs) in three dimensions and thermal phase transitions to paramagnet are studied by unbiased Monte Carlo simulations. For an extension of the Kitaev model to a three-dimensional tricoordinate network dubbed the hypernonagon lattice, we derive low-energy effective models in two different anisotropic limits. We show that the effective interactions between the emergent $Z_2$ degrees of freedom called fluxes are unfrustrated in one limit, while highly frustrated in the other. In both cases, we find a first-order phase transition to the CSL, where both time-reversal and parity symmetries are spontaneously broken. In the frustrated case, however, the CSL state is highly exotic—the flux configuration is subextensively degenerate while showing a directional order with broken $C_3$ rotational symmetry. Our results provide two contrasting archetypes of CSLs in three dimensions, both of which allow approximation-free simulation for investigating the thermodynamics.

$\gamma \text{−}\mathrm{Fe}$ and related alloys are model systems of the coupling between structure and magnetism in solids. Since different electronic states (with different volumes and magnetic ordering states) are closely spaced in energy, small perturbations can alter which one is the actual ground state. Here, we demonstrate that the ferromagnetic state of $\gamma \text{−}\mathrm{Fe}$ nanoparticles is associated with a tetragonal distortion of the fcc structure. Combining a wide range of complementary experimental techniques, including low-temperature Mössbauer spectroscopy, advanced transmission electron microscopy, and synchrotron radiation techniques, we unambiguously identify the tetragonally distorted ferromagnetic ground state, with lattice parameters $a=3.76\left(2\right)\AA$ and $c=3.50\left(2\right)\AA$, and a magnetic moment of 2.45(5) ${\mu }_B$ per Fe atom. Our findings indicate that the ferromagnetic order in nanostructured $\gamma \text{−}\mathrm{Fe}$ is generally associated with a tetragonal distortion. This observation motivates a theoretical reassessment of the electronic structure of $\gamma \text{−}\mathrm{Fe}$ taking tetragonal distortion into account.

The rapid progress in quantum technology enables the implementation of artificial many-body systems with correlated photons and polaritons. A multiconnected Jaynes-Cummings (MCJC) lattice can be constructed by connecting qubits and cavities alternately. Such models can be realized with superconducting qubits coupled to superconducting microwave resonators or with quantum dots coupled to optical nanocavities. We study the physical properties of a one-dimensional MCJC lattice using the density-matrix renormalization-group method. This model has an intrinsic symmetry between the left and right qubit-cavity couplings. The competition between these couplings may drive the ground state either to a Mott-insulating phase or to a superfluid phase at integer fillings. We calculate the single-particle and density-density correlation functions, the correlation lengths in the Mott-insulating phase, and the Luttinger parameters in the superfluid phase and determine accurately the critical points that separate these two phases.

The magnetization reversal of CoCrPt thin films has been examined as a function of thickness using magneto-optical Kerr effect (MOKE) microscopy and first-order reversal curves (FORC) techniques. MOKE images show differentiated magnetization reversal regimes for different film thicknesses: while the magnetic domains in 10-nm-thick CoCrPt film resemble a fractal structure, a labyrinth stripe domain configuration is observed for 20-nm-thick films. Although FORC distributions for both cases show two main features related to irreversible processes (propagation and annihilation fields) separated by a mostly flat region, this method can nonetheless distinguish which magnetization reversal process is active according to the horizontal profile of the first FORC peak, or propagation field. A single-peak FORC profile corresponds to the fractal magnetization reversal, whereas a flat-peak FORC profile corresponds to the labyrinth magnetization reversal.

Under mechanical loading, crystalline solids deform elastically, and subsequently yield and fail via plastic deformation. Thus crystalline materials experience two mechanical regimes: elasticity and plasticity. Here, we provide numerical and theoretical evidence to show that metal nanoplates exhibit an intermediate mechanical regime that occurs between elasticity and plasticity, which we call the elastic instability regime. The elastic instability regime begins with a decrease in stress, during which the nanoplates fail via global, and not local, deformation mechanisms that are distinctly different from traditional dislocation-mediated plasticity. Because the nanoplates fail via elastic instability, the governing strength criterion is the ideal strength, rather than the yield strength, and as a result, we observe a unique “smaller is weaker” trend. We develop a simple surface-stress-based analytic model to predict the ideal strength of the metal nanoplates, which accurately reproduces the smaller is weaker behavior observed in the atomistic simulations.

The compounds ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$ and ${\mathrm{Ba}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$ are insulating dimerized antiferromagnets with ${\mathrm{Cr}}^{5+}$ magnetic ions. These spin-$\frac{1}{2}$ ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction that leads to a singlet ground state and gapped triplet states. We report on the effect on the magnetic properties of ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$ by introducing chemical disorder upon replacing Sr by Ba. Two single crystals of ${\mathrm{Ba}}_{3-x}{\mathrm{Sr}}_x{\mathrm{Cr}}_2{\mathrm{O}}_8$ with $x=2.9$ (3.33% of mixing) and $x=2.8$ (6.66% of mixing) were grown in a four-mirror-type optical floating-zone furnace. The magnetic properties of these compounds were studied by magnetization measurements. Inelastic neutron-scattering measurements on ${\mathrm{Ba}}_{0.1}{\mathrm{Sr}}_{2.9}{\mathrm{Cr}}_2{\mathrm{O}}_8$ were performed in order to determine the interaction constants and the spin gap for $x=2.9$. The intradimer interaction constant is found to be $J_0=5.332\left(2\right)\mathrm{meV}$, about 4% smaller than that of pure ${\mathrm{Sr}}_3{\mathrm{Cr}}_2{\mathrm{O}}_8$, whereas the interdimer exchange interaction $J_e$ is smaller by 7%. These results indicate a noticeable change in the magnetic properties by a random substitution effect.

Magnetic spinels (with chemical formula $A{X}_2{\mathrm{O}}_4$, with $X$ a $3d$ transition metal ion) that also have an orbital degeneracy are Jahn-Teller active and hence possess a coupling between spin and lattice degrees of freedom. At high temperatures, ${\mathrm{MgV}}_2{\mathrm{O}}_4$ is a cubic spinel based on ${\mathrm{V}}^{3+}$ ions with a spin $S=1$ and a triply degenerate orbital ground state. A structural transition occurs at $T_{\text{OO}}=63$ K to an orbitally ordered phase with a tetragonal unit cell followed by an antiferromagnetic transition of $T_N=42$ K on cooling. We apply neutron spectroscopy in single crystals of ${\mathrm{MgV}}_2{\mathrm{O}}_4$ to show an anomaly for intermediate wave vectors at $T_{\text{OO}}$ associated with the acoustic phonon sensitive to the shear elastic modulus $\left(C_{11}-C_{12}\right)/2$. On warming, the shear mode softens for momentum transfers near close to half the Brillouin zone boundary, but recovers near the zone center. High resolution spin-echo measurements further illustrate a temporal broadening with increased temperature over this intermediate range of wave vectors, indicative of a reduction in phonon lifetime. A subtle shift in phonon frequencies over the same range of momentum transfers is observed with magnetic fields. We discuss this acoustic anomaly in context of coupling to orbital and charge fluctuations.

Transition metal oxides are well known for their complex magnetic and electrical properties. When brought together in heterostructure geometries, they show particular promise for spintronics and colossal magnetoresistance applications. In this article, we propose a carrier-driven coupling mechanism in heterostructures composed of itinerant ferromagnetic materials. The coupling is mediated by charge carriers that strive to maximally delocalize through the heterostructure to gain kinetic energy. In doing so, they force a ferromagnetic or antiferromagnetic coupling between the constituent layers. To illustrate this, we focus on heterostructures composed of ${\mathrm{SrRuO}}_3$ and ${\mathrm{La}}_{1-x}{\mathrm{A}}_x{\mathrm{MnO}}_3$ (A = Ca/Sr). Using the minority-carrier nature of ${\mathrm{SrRuO}}_3$, we provide a simple explanation for antiferromagnetic alignment that is known to occur in multilayers. We present a phenomenological Kondo-lattice model which reproduces the known magnetization properties of multilayers. In addition, we discuss a quantum well model for heterostructures and argue that the spin-dependent density of states determines the nature of the coupling. As a smoking gun signature, we propose that bilayers with the same constituents will oscillate between ferromagnetic and antiferromagnetic coupling upon tuning the relative thicknesses of the layers. We present ab initio results that support this prediction.

We have carried out ${}^{29}\mathrm{Si}$ nuclear magnetic resonance of ${\mathrm{URu}}_2{\mathrm{Si}}_2$, which undergoes the so-called hidden order (HO) at $T_{\mathrm{HO}}=17.5$ K. The use of a ${}^{29}\mathrm{Si}$-enriched sample enabled us to obtain accurate NMR data up to 350 K and to extract the features of low energy magnetic correlations at high temperatures in this compound. Present results naturally indicate that ${\mathrm{URu}}_2{\mathrm{Si}}_2$ undergoes a crossover from localized to itinerant regimes with decreasing temperature and the HO occurs in the crossover region. We also discuss possible antiferromagnetic correlations remaining down to $T_{\mathrm{HO}}$ by extracting magnetic fluctuation components.

The transition between a metallic and a pseudogap phase in high-$T_c$ cuprate superconductors, which is the subject of recent experimental investigations, is studied theoretically within the Hubbard model. We apply the cluster dynamical impurity approximation to the anisotropic Hubbard model on the square lattice at zero temperature and finite doping. This approach can detect a first-order transition between two metallic states: a pseudogap state at low doping with a depleted density of states at the Fermi level and a correlated metal at higher doping. This transition was seen first in the cluster dynamical mean-field theory at finite temperatures by Sordi et al. [Phys. Rev. Lett. 108, 216401 (2012)] on the isotropic Hubbard model. Here we investigate this transition at zero temperature and as a function of on-site interaction $U$, anisotropy $t_y/t_x$, and doping. We find a first-order transition line which ends at a critical point, which occurs around $t_y/t_x\sim 0.5$.

We propose a simple many-body based screening mixing strategy to considerably enhance the performance of the Bethe-Salpeter equation (BSE) approach for prediction of excitation energies of molecular systems. This strategy enables us to closely reproduce results of highly correlated equation of motion coupled cluster singles and doubles (EOM-CCSD) through optimal use of cancellation effects. We start from the Hartree-Fock (HF) reference state and take advantage of local density approximation (LDA) based random phase approximation (RPA) screening, denoted as $W_0$-RPA@LDA with $W_0$ as the dynamically screened interaction built upon LDA wave functions and energies. We further use this $W_0$-RPA@LDA screening as an initial screening guess for calculation of quasiparticle energies in the framework of $G_0{W}_0$@HF. The $W_0$-RPA@LDA screening is further injected into the BSE. By applying such an approach on a set of 22 molecules for which the traditional $GW$/BSE approaches fail, we observe good agreement with respect to EOM-CCSD references. The reason for the observed good accuracy of this mixing ansatz (scheme A) lies in an optimal damping of HF exchange effect through the $W_0$-RPA@LDA strong screening, leading to substantial decrease of typically overestimated HF electronic gap, and hence to better excitation energies. Further, we present a second multiscreening ansatz (scheme B), which is similar to scheme A with the exception that now the $W_0$-RPA@HF screening is used in the BSE in order to further improve the overestimated excitation energies of carbonyl sulfide (COS) and disilane $\left({\mathrm{Si}}_2{\mathrm{H}}_6\right)$. The reason for improvement of the excitation energies in scheme B lies in the fact that $W_0$-RPA@HF screening is less effective (and weaker than $W_0$-RPA@LDA), which gives rise to stronger electron-hole effects in the BSE.

We investigate the influence of film thickness and magnetism on the electronic structure of a strongly correlated prototypical system, MnO(001) films epitaxially grown on Ag(001), using angle-resolved photoemission spectroscopy (ARPES) and x-ray photoelectron spectroscopy (XPS) techniques. XPS measurements have confirmed the MnO stoichiometry as well as the layer-by-layer growth mode of the films, while the ARPES measurements have confirmed the high structural quality of the films by the observation of clear band dispersions devoid of any sample charging effects. For lower coverage films, finite-size effects/reduced correlation effects are observed, while from 3 ML coverages, bulklike band structures are formed. Detailed band mapping along $\overline{\mathrm{\Gamma }}\text{−}\overline{X}$ and $\overline{\mathrm{\Gamma }}\text{−}\overline{M}$ directions reveal a relative dispersion between Mn-derived $e_g$ and $t_{2g}$ states up to 1.2 eV, in contrast to previous ARPES results where maximum relative dispersions of $\pm 0.1$ eV were reported, while our results excellently match with the theoretically predicted dispersions. By comparing with theoretical results we show the hybridization between O $2p$ with the Mn $e_g$ states giving rise to the observed dispersions resulting from the highly dispersive nature of the O $2p$ bands. Furthermore, we show that during the paramagnetic (PM) to antiferromagnetic (AFM) transition, the valence band gets narrower with an enhancement of the ${}^{5}e_g$ peak intensity, along with the widening of the energy gap by 200 meV. Theoretically, Terakura et al. had predicted the possibility of a similar outcome due to the decrease of the effective $dd$ hopping between the oppositely magnetized cations during the PM to AFM-II transition because of the $dd\sigma$ type interaction, yet the experimental verification was absent in the literature. We believe that the exact same mechanism as predicted by Terakura et al. is responsible for the changes seen in the electronic structure during magnetic phase transition and argue that a similar trend can be seen for the prototype materials such as CoO and NiO.