In this work we present an implementation of the analytical string theory recently applied to the description of glasses. These are modeled as continuum media with embedded elastic string heterogeneities, randomly located and randomly oriented, which oscillate around a straight equilibrium position with a fundamental frequency depending on their length. The existence of a length distribution is reflected then in a distribution of oscillation frequencies which is responsible for the boson peak in the glass density of states. Previously, it has been shown that such a description can account for the elastic anomalies reported at frequencies comparable with the boson peak: the strong phonon scattering and the negative dispersion in the sound velocity, as a result of the interference of the string oscillations with propagating sound plane waves. Here we start from the generalized hydrodynamics to determine the dynamic correlation function $S(k,\omega )$ associated with the coherent, dispersive, and attenuated sound waves resulting from such interference. We show that once the vibrational density of states has been measured, we can use it for unambiguously fixing the string length distribution inherent to a given glass. The density-density correlation function obtained using such distribution is strongly constrained, and able to account for the experimental data collected on two prototypical glasses with very different microscopic structure and fragility: glycerol and silica. The obtained string length distribution is compatible with the typical size of elastic heterogeneities previously reported for silica and supercooled liquids, and the atomic motion associated with the string dynamics is consistent with the soft modes recently identified in large-scale numerical simulations as nonphonon modes responsible for the boson peak. The theory is thus in agreement with the most recent advances in the understanding of the glass-specific dynamics and offers an appealing, simple understanding of the microscopic origin of the latter, while raising new questions on the universality or material specificity of the string distribution properties.

We present an in-depth analysis of muon-spin spectroscopy measurements of ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ under the effect of the ${\mathrm{Eu}}_{1–x}{\mathrm{Bi}}_x$ isovalent and diamagnetic substitution as well as of external pressure. Our results evidence an anomalous correlation between the magnetic volume fraction and the order parameter only for stoichiometric ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$, pointing towards highly unconventional properties of the magnetic phase developing therein. We argue that magnetism in ${\mathrm{Eu}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ develops based on the nucleation of magnetic droplets at $T_N$, whose successive growth is limited by the need of a continuous generation of magnetic hedgehog monopoles.

Raman spectroscopy has been applied to investigate the lattice and magnetic excitations of the honeycomb Kitaev material $\alpha \text{−}\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$. The temperature-dependent Raman spectra give evidence for an unusual magnetic excitation, typified by a broad continuum. The integrated intensity of this Raman continuum does not follow the bosonic statistics expected for the conventional insulating magnets, which indicates the emergence of itinerant Majorana fermions from spin fractionalization in $\alpha \text{−}\mathrm{L}{\mathrm{i}}_2\mathrm{Ir}{\mathrm{O}}_3$. In addition, the Raman results under pressures resolve one critical pressure at $P_c=4.1\mathrm{GPa}$ with structural and magnetic phase transitions, signaled by the in-plane Ir-Ir bond dimerization and magnetic collapse, and the system turns into a correlated band insulator above $P_c$.

Around discontinuous (first-order) magnetic phase transitions, the strong caloric response of materials to the application of small fields is widely studied for the development of solid-state refrigeration. Typically strong magnetostructural coupling drives such transitions and the attendant substantial hysteresis dramatically reduces the cooling performance. In this context, we describe a purely electronic mechanism which pilots a first-order paramagnetic-ferromagnetic transition in divalent lanthanide compounds and which explains the giant nonhysteretic magnetocaloric effect recently discovered in a ${\mathrm{Eu}}_2\mathrm{In}$ compound. There is a positive feedback between the magnetism of itinerant valence electrons and the ferromagnetic ordering of local $f$-electron moments, which appears as a topological change to the Fermi surface. The origin of this electronic mechanism stems directly from Eu's divalency, which explains the absence of a similar discontinuous transition in ${\mathrm{Gd}}_2\mathrm{In}$.

We report a comprehensive investigation of the effects of quantum turbulence and quantized vorticity in superfluid ${}^4\mathrm{He}$ on the motion of a microelectromechanical systems (MEMS) resonator. We find that the MEMS is uniquely sensitive to quantum turbulence present in the fluid. To generate turbulence in the fluid, a quartz tuning fork (TF) is placed in proximity to the MEMS and driven at a large amplitude. We observe that at low velocity, the MEMS is damped by the turbulence, and that above a critical velocity, $v_c\simeq 5\mathrm{mm}{\mathrm{s}}^{-1}$, the turbulent damping is greatly reduced. We find that above $v_c$, the damping of the MEMS is reduced further for increasing velocity, indicating a velocity dependent coupling between the surface of the MEMS and the quantized vortices constituting the turbulence. We propose a model of the interaction between vortices in the fluid and the surface of the MEMS. The sensitivity of these devices to a small number of vortices and the almost unlimited customization of MEMS open the door to a more complete understanding of the interaction between quantized vortices and oscillating structures, which in turn provides a new route for the investigation of the dynamics of single vortices.

The ultimate challenge in the investigation of ferroelectric properties lies in the quantitative measurements of their polarization at the unit cell scale. Such investigations are commonly performed using an indirect approach, by measuring the atomic displacements from atomic resolution images. Differential phase-contrast (DPC) scanning transmission electron microscopy (STEM) allows mapping the electric field with atomic resolution. This unique capability offers a direct way to study the polar properties in ferroelectrics. However, the effects of ferroelectric polarization on the contrast of high-resolution DPC-STEM imaging have not been addressed so far. In this work, we perform a theoretical study on the origin of the DPC-STEM contrast in ferroelectric materials and propose a modified multislice algorithm for STEM image simulations. Our results demonstrate that the mesoscopic polarization induces asymmetries in the detected electric fields, which are in line with our previous experimental observations. Moreover, we discuss the dependence of the DPC-STEM sensitivity on the polar field amplitude, specimen thickness, and defocus, and provide a route to discriminate between mesoscopic polarization and specimen misorientation.

We show that finite lattices with arbitrary boundaries may support large degenerate subspaces, stemming from the underlying translational symmetry of the lattice. When the lattice is coupled to an environment, a potentially large number of these states remains weakly or perfectly uncoupled from the environment, realizing a new kind of bound states in the continuum. These states are strongly localized along particular directions of the lattice, which, in the limit of strong coupling to the environment, leads to spatially localized subradiant states.

We study an extended central spin model with an isotropic nearest-neighbor spin-exchange interaction among the bath spins. The system is controllable by external magnetic fields applied on the central spin and the bath, respectively. We construct a basis set of the Hilbert space and express the Hamiltonian of the extended model into a series of $2\times 2$ block matrices to obtain the exact solution of the model successfully. Therefrom, the coherence and the spin polarization of the central spin are investigated. We find that if the couplings among the bath spins are antiferromagnetic, the central spin has good coherence and polarization at low temperatures. Moreover, the decoherence is greatly suppressed at the critical point where the strengths of the central and the bath magnetic field take the same value. A dephasing phenomenon is identified when the initial state of the central spin is unpolarized.

We investigate diffusion of a quantum charged particle in a tight-binding lattice (TBL) under fluctuating noisy electric fields. Several closed-form expressions such as that of probability propagator, average current, and mean-squared displacement are derived. The particular effect of Gaussian noise in a 1D TBL is studied in detail and a comparison is made with the case of a telegraph noise. The Gaussian white noise and telegraph noise show up in different manifestations in the underdamped regime. On the contrary, when relaxation is rapid the noisy field averages out to zero yielding delocalization for both Gaussian and telegraphic noisy electric fields. Depending on the strength of noise and the external field one can observe different current characteristics.

We present a theory of nonlinear magnetic response of a chiral soliton lattice state in a monoaxial chiral helimagnet under an oscillating magnetic field. The chiral soliton lattice is stabilized by a static magnetic field applied perpendicular to the chiral axis. Just below the critical field strength, where an incommensurate-to-commensurate phase transition occurs, the soliton density becomes quite low and almost isolated $2\pi$ kinks are partitioned by vast ferromagnetic regions. We consider this highly nonlinear regime and demonstrate that internal deformations of each kink give rise to the nonlinear response in this regime.

Using the first-principles calculations, we systematically investigated magnetic properties of a $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ (MBT) monolayer in contact with $\mathrm{II}{\mathrm{I}}_2\text{−}\mathrm{V}{\mathrm{I}}_3$ two-dimensional ferroelectric substrates. As the electric polarization in the ferroelectric films is switched, a semiconductor to half-metal transition occurs in MBT, and its magnetic easy axis in several cases reorients. As an example, the Curie temperature of $\mathrm{MBT}/\mathrm{I}{\mathrm{n}}_2\mathrm{S}{\mathrm{e}}_3$ can be enhanced to 21.5 or 32 K under the influence of electric polarization and biaxial strain, much higher than that of the pristine MBT monolayer, which is 14.5 K in our calculations. Our results suggest effective ways to tune magnetic properties of MBT and provide useful insights for the further development of these important van der Waals functional materials.

The high-harmonic spectrum of the Mott insulating Hubbard model has recently been shown to exhibit plateau structures with cutoff energies determined by $n\mathrm{th}$-nearest-neighbor doublon-holon recombination processes. The spectrum thus allows one to extract the on-site repulsion $U$. Here, we consider generalizations of the single-band Hubbard model and discuss the signatures of bosonic excitations in high-harmonic spectra. Specifically, we study an electron-plasmon model which captures the essential aspects of the dynamically screened Coulomb interaction in solids and a multiorbital Hubbard model with Hund coupling which allows one to analyze the effect of local spin excitations. For the electron-plasmon model, we show that the high-harmonic spectrum can reveal information about the screened and bare on-site interaction, the boson frequency, as well as the relation between boson coupling strength and boson frequency. In the multiorbital case, string states formed by local spin excitations result in an increase of the radiation intensity and cutoff energy associated with higher-order recombination processes.

We propose that a pertinently engineered double superconducting island connected to two spinless one-dimensional conducting leads can work as a tunable (iso)spin Kondo and charge Kondo system, with the lead index regarded as an effective isospin degree of freedom. We evidence how, by tuning a single gate voltage applied to the island, it is possible to make the system switch from the (iso)spin Kondo to the charge Kondo phase, passing across an intermediate phase, in which the Kondo impurity is effectively irrelevant for the low-temperature behavior of the system. Eventually, we evidence how to probe the various phases by measuring the ac conductance tensor of our system, by emphasizing the features that should allow to identify the onset of the so far quite elusive “charge-2” charge Kondo effect.

The modern theory of electric polarization has recently been extended to higher multipole moments, such as quadrupole and octupole moments. The higher electric multipole insulators are essentially topological crystalline phases protected by underlying crystalline symmetries. Henceforth, it is natural to ask what are the consequences of symmetry breaking in these higher multipole insulators. In this work, we investigate topological phases and the consequences of symmetry breaking in generalized electric quadrupole insulators. Explicitly, we generalize the Benalcazar-Bernevig-Hughes model by adding specific terms in order to break the crystalline and nonspatial symmetries. Our results show that chiral-symmetry breaking induces an indirect gap phase which hides corner modes in bulk bands, ruining the topological quadrupole phase. We also demonstrate that quadrupole moments can remain quantized even when mirror symmetries are absent in a generalized model. Furthermore, it is shown that topological quadrupole phase is robust against a unique type of disorder presented in the system.

The quantum anomalous Hall (QAH) effect, a condensed matter analog of the parity anomaly, is characterized by a quantized Hall conductivity in the absence of an external magnetic field. However, it has been recently shown that, even in the presence of Landau levels, the QAH effect can be distinguished from the conventional quantum Hall (QH) effect due to the parity anomaly. As a signature of this effect, we predicted coexistent counterpropagating QAH and QH edge states. In the present work, we generalize these findings to QAH insulators with broken particle-hole symmetry. In particular, we derive the connection to the spectral asymmetry, which is a topological quantity arising in the context of Dirac-like systems. Moreover, it is shown that, depending on the magnetic field direction, particle-hole asymmetry strengthens or weakens the hybridization of the counterpropagating QH and QAH edge states. Implications for ferro- or paramagnetic topological insulators are derived which pave the way towards identifying signatures of the QAH effect even in external magnetic fields.

The layered magnetic topological insulator $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ is a promising platform to realize the quantum anomalous Hall effect because its layers possess intrinsic ferromagnetism. However, it is not well understood why the high-spin $d^5$ magnetic ions $\mathrm{M}{\mathrm{n}}^{2+}$ forming the Mn-Te-Mn spin exchange paths prefer ferromagnetic (FM) coupling, contrary to the prediction of the Goodenough-Kanamori rule that a TM-$L$-TM spin exchange where TM and $L$ are a transition-metal magnetic cation and a main group ligand, respectively, is antiferromagnetic (AFM) even when the bond angle of the exchange path is 90°. Using density functional theory calculations, we show that the presence of $\mathrm{B}{\mathrm{i}}^{3+}$ ions is essential for the FM coupling in $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$. Then, using a tight-binding model Hamiltonian, we find that high-spin $d^5$ ions ($S=5/2$) in TM-$L$-TM spin exchange paths prefer FM coupling if the empty $p$ orbitals of a nonmagnetic cation $M$ (e.g., $\mathrm{B}{\mathrm{i}}^{3+}$ ion) hybridize strongly with those of the bridging ligand $L$ but AFM coupling otherwise.

Thanks to recent advances, the 4D quantum Hall (QH) effect is becoming experimentally accessible in various engineered setups. In this paper, we propose a type of 4D topological system that, unlike other 2D and 4D QH models, does not require complicated (artificial) gauge fields and/or time-reversal symmetry breaking. Instead, we show that there are 4D QH systems that can be engineered for spinless particles by designing the lattice connectivity with real-valued hopping amplitudes, and we explain how this physics can be intuitively understood in analogy with the 2D Haldane model. We illustrate our discussion with a specific 4D lattice proposal, inspired by the widely-studied 2D honeycomb and brickwall lattice geometries. This also provides a minimal model for a topological system in class AI, which supports nontrivial topological band invariants only in four spatial dimensions or higher.

Matrix product states and matrix product operators (MPOs) are one-dimensional tensor networks that underlie the modern density matrix renormalization group (DMRG) algorithm. The use of MPOs accounts for the high level of generality and wide range of applicability of DMRG. However, current algorithms for two-dimensional (2D) tensor network states, known as projected entangled-pair states, rarely employ the associated 2D tensor network operators, projected entangled-pair operators (PEPOs), due to their computational cost and conceptual complexity. To lower these two barriers, we describe how to reformulate a PEPO into a set of tensor network operators that resemble MPOs by considering the different sets of local operators that are generated from sequential bipartitions of the 2D system. The expectation value of a PEPO can then be evaluated on the fly using only the action of MPOs and generalized MPOs at each step of the approximate contraction of the 2D tensor network. This technique allows for the simpler construction and more efficient energy evaluation of 2D Hamiltonians that contain finite-range interactions, and provides an improved strategy to encode long-range interactions that is orders of magnitude more accurate and efficient than existing schemes.

Recent experiments have shown the signatures of Majorana bound states at the ends of magnetic chains deposited on a superconducting substrate. Here, we employ first-principles calculations to directly investigate the topological properties of $3d$ transition metal nanochains (i.e., Mn, Cr, Fe and Co). In contrast to the previous studies [Nadj-Perge et al., Science 346, 602 (2014) and Ruby et al., Nano Lett. 17, 4473 (2017)], we found the exact tight-binding models in the Wannier orbital basis for the isolated chains as well as for the surface–deposited wires. Based on these models, we calculate the topological invariant of ${\mathbb{Z}}_2$ phases for all systems. Our results for the isolated chains demonstrate the existence of the topological phase only in Mn and Co systems. We also considered a noncollinear magnetic order as a source of the nontrivial topological phase and found that this type of magnetic order is not a stable ground state in the Fe and Co isolated chains. Further studies showed that a coupling between the chain and substrate leads to strong modification of the band structure. Moreover, the analysis of the topological invariant indicates the possibility of emergence of the topological phase in all studied nanochains deposited on the Pb surface. Therefore, our results demonstrate an important role of the coupling between deposited atoms and a substrate for topological properties of nanosystems.

By using the self-consistent Born approximation, we investigate disorder effect induced by the short-range impurities on the band gap in two-dimensional Dirac systems with the higher order terms in momentum. Starting from the Bernevig-Hughes-Zhang (BHZ) model, we calculate the density of states as a function of the disorder strength. We show that due to quadratic corrections to the Dirac Hamiltonian, the band gap is always affected by the disorder even if the system is gapless in the clean limit. Finally, we explore the disorder effects by using an advanced effective Hamiltonian describing the side maxima of the valence subband in HgTe quantum wells. We show that the band gap and disorder-induced topological phase transition in the real structures may differ significantly from those predicted within the BHZ model.