Although most quantum systems thermalize locally on short timescales independent of initial conditions, recent developments have shown this is not always the case. Lattice geometry and quantum mechanics can conspire to produce constrained quantum dynamics and associated glassy behavior, a phenomenon that falls outside the rubric of the eigenstate thermalization hypothesis. Constraints “fragment” the many-body Hilbert space due to which some states remain insulated from others and the system fails to attain thermal equilibrium. Such fragmentation is a hallmark of geometrically frustrated magnets with low-energy “icelike manifolds” exhibiting a broad range of relaxation times for different initial states. Focusing on the highly frustrated kagome lattice, we demonstrate these phenomena in the Balents-Fisher-Girvin Hamiltonian (easy-axis regime), and a three-coloring model (easy-plane regime), both with constrained Hilbert spaces. We study their level statistics and relaxation dynamics to develop a coherent picture of fragmentation in various limits of the XXZ model on the kagome lattice.

The ${120}^{\circ }$ antiferromagnetism (AFM) is widely believed as the magnetic ground state of the triangular systems because of the geometrical frustration. The emergence of novel magnetism, such as the row-wise AFM in Mn/Cu(111) and Sn/Si(111), reveals the importance of the longer-range hopping on magnetic competitions in realistic material systems. By utilizing advanced many-body techniques, we systematically studied the isotropic triangular Hubbard model with second nearest-neighbor hopping $t^{\prime }$, including both the single- and the two-particle responses. We found that both electronic and magnetic phase transitions show a clear dependence on $t^{\prime }/t$. Consequently, we observed a remarkable reentrance of the metal-insulator transition and a crossover between the ${120}^{\circ }$- and the row-wise AFM. The Fermi surface (FS) shows two distinct structures with the nesting vectors consistent with the magnetic correlations. When $t^{\prime }$ evolvs from 0 to 1, the correlated Fermi surface demonstrates a Lifschitz transition between the two nesting structures, and exotic phases like the featureless insulating state can be realized. Our work sheds light on the engineering of electronic and magnetic correlations of correlated triangular surfaces via longer-range hopping. The rich phase diagram and the high degree of tunability make the triangular lattice with longer-range hopping a more realistic platform to study the emergent magnetic competitions.

In two-dimensional semiconductors, the density fluctuation potential created by a screening of the photohole is intrinsically anisotropic because of the infinitely periodic planar arrangements of atoms. Modeling the anisotropic-localized screening with the cylindrical geometry, we investigate the emission delay of the photoelectron from $2s$ and $2p_z$ states of graphene in an angle-resolved mode of the attosecond streaking by solving the time-dependent Schrödinger equation. Strong angle dependencies in absolute emission delays of $2s$ and $2p_z$ states are obtained, from which the effects of the infrared-induced continuum transition are ruled out and those of the anisotropic screening could be solely extracted. Consequently, the anisotropic scattering induces photoelectrons to be emitted with substantial negative delays within small angles (i.e., $\theta \lesssim {\theta }_c$) from the normal direction to the atomic arrangement, that is, a conical electron emission in a very early stage of the photoemission.

The nitrogen-vacancy (NV) defect in diamond is an efficient quantum sensor of randomly fluctuating signals via relaxometry measurements. In particular, the longitudinal spin relaxation of the NV defect accelerates in the presence of magnetic noise with a spectral component at its electron spin resonance frequency. We look into this effect quantitatively by applying a calibrated and tunable magnetic noise on a single NV defect. We show that an increase of the longitudinal spin relaxation rate translates into a reduction of the photoluminescence (PL) signal emitted under continuous optical illumination, which can be explained using a simplified three-level model of the NV defect. This PL quenching mechanism offers a simple, all-optical method to detect magnetic noise sources at the nanoscale.

It is commonly assumed that topological phase transitions in topological superconductors are accompanied by a closing of the topological gap or a change of the symmetry of the system. We demonstrate that an unconventional topological phase transition with neither gap closing nor a change of symmetry is possible. We consider a nanoscopic length ladder of atoms on a superconducting substrate, comprising self-organized magnetic moments coupled to itinerant electrons. For a range of conditions, the ground state of such a system prefers helical magnetic textures, a self-sustaining topologically nontrivial phase. Abrupt changes in the magnetic order as a function of induced superconducting pairing or chemical potential can cause topological phase transitions without closing the topological gap. Furthermore, the ground state prefers either parallel or antiparallel configurations along the rungs, and the antiparallel configuration causes an emergent time reversal asymmetry protecting Kramers pairs of Majorana zero modes, but in a BDI topological superconductor. We determine the topological invariant and inspect the boundary Majorana zero modes.

The basis for pulling a passive nanostructured material using laser light by establishing a transverse surface plasmon resonance on the back of a membrane is presented and supported with both theory and numerical simulations. The total force magnitude and direction can be regulated by electromagnetic resonant modes interacting with materials that can be adjusted using the material and geometry variables, and with plane-wave illumination under sinusoidal steady-state conditions. In the situations treated, the structured material is fixed in position and the force density and hence total force are determined numerically. Simulations indicate that a silicon nitride film coated with gold and patterned to produce a unit cell having a slot passing through both materials allows satisfactory surface wave control to facilitate a pulling force with visible wavelengths. When the plasmon surface wave on the back dominates, pulling occurs, and when the mode on the front has more energy, the result is pushing. It is also shown that pushing or pulling, regulated by varying the wavelength of the incident light, is possible, and an example is presented. This work motivates an experimental effort to investigate pulling with nanostructured media and offers a different paradigm in optomechanics.

We propose a simple and efficient real-space approach for the calculation of the ground-state energies of Wigner crystals in one, two, and three dimensions. To be precise, we calculate the first two terms in the asymptotic expansion of the total energy per electron which correspond to the classical energy and the harmonic correction due to the zero-point motion of the Wigner crystals. Our approach employs Clifford periodic boundary conditions to simulate the infinite electron gas and a renormalized distance to evaluate the Coulomb potential. This allows us to calculate the energies unambiguously and with a higher precision than those reported in the literature. Our results are in agreement with the literature values with the exception of harmonic correction of the two-dimensional Wigner crystal, for which we find a significant difference. Although we focus on the ground state, i.e., the triangular lattice and the bcc lattice in two and three dimensions, respectively, we also report the classical energies of several other common lattice structures.

Experimentally synthesized perovskite-type ${\mathrm{YRh}}_3\mathrm{B}$ with a $Pm\overline{3}m$ type structure was proposed as a topological material (TM) via first-principles calculations and the low-energy $k·p$ effective Hamiltonian, which has a quadratic contact triple point (QCTP) at point $\mathrm{\Gamma }$ and six pairs of open nodal lines (NLs) of the hybrid type. Clear surface states observed in the surface spectrum confirmed the topological states. When spin-orbit coupling was considered, the QCTP at $\mathrm{\Gamma }$ transferred to the quadratic-type Dirac nodal point (NP). Under $1\%$ tetragonal strained lattice constants, ${\mathrm{YRh}}_3\mathrm{B}$ hosted richer topological states, including a quadratic-type twofold degenerate NP, six pairs of open NLs of the hybrid type, and two closed NLs of type I and hybrid type. Moreover, it was proved that the NLs of ${\mathrm{YRh}}_3\mathrm{B}$ at its strained lattice constants contain all types of band-crossing points (BCPs) (i.e., type I, type II, and critical type). Such rich types of NP and NL states in one compound make it potentially applicable for multifunctional electronic devices as well as an appropriate platform to study entanglement among topological states.

A laser beam is focused onto the junction of a scanning tunneling microscope (STM) to probe the local photocurrent response of highly ordered $\mathrm{Mo}{\mathrm{Se}}_2$ islands. The islands, grown by physical vapor deposition onto Au(111), are characterized by uniform triangular shapes, Mo-terminated edges, and the typical 1.8 V band gap of monolayer $\mathrm{Mo}{\mathrm{Se}}_2$. The laser-induced signals are detected with nanometer spatial resolution by a lock-in amplifier, both in the form of point spectra and surface maps. To distinguish real photocurrents from current contributions, arising from periodic tip expansions due to the dissipated laser power, blind experiments are performed in which the tip height is modulated without laser irradiation. In addition, the expected current responses are simulated as a function of sample bias for the two scenarios. Comparison between experimental and simulated data indicates that periodic tip expansions dominate the detected current modulations and photoinduced signals are of minor importance. Our work provides various indicators to safely distinguish photo- and thermally induced current components in a laser-driven STM experiment and discusses means to enhance the desired photoresponse with respect to parasitic thermal effects.

We find that the Casimir pressure in peptide films deposited on metallic substrates is always repulsive which makes these films less stable. It is shown that by adding a graphene sheet on top of peptide film one can change the sign of the Casimir pressure by making it attractive. For this purpose, the formalism of the Lifshitz theory is extended to the case when the film and substrate materials are described by the frequency-dependent dielectric permittivities, whereas the response of graphene to the electromagnetic field is governed by the polarization tensor in $(2+1)$-dimensional space-time found in the framework of the Dirac model. Both pristine and gapped and doped graphene sheets are considered possessing some nonzero energy gap and chemical potential. According to our results, in all cases the presence of graphene sheet makes the Casimir pressure in peptide film deposited on a metallic substrate attractive starting from some minimum film thickness. The value of this minimum thickness becomes smaller with increasing chemical potential and larger with increasing energy gap and the fraction of water in peptide film. The physical explanation for these results is provided, and their possible applications in organic electronics are discussed.

In this study, atomic-scale origin of the Invar effect, which is nearly-zero thermal expansion observed in the Invar alloy ${\mathrm{Fe}}_{65}{\mathrm{Ni}}_{35}$, was investigated by reverse Monte Carlo analysis using complementary data sets of extended x-ray absorption fine structure and x-ray diffraction. The interatomic distances of the nearest neighboring Fe-Fe atomic pairs were $\sim 0.02$ Å longer than those of the Fe-Ni and Ni-Ni pairs at the minimum pressure in this study (0.6 GPa). The elongation in the Fe-Fe pairs was suppressed with increasing pressure, and the distances of the three pairs were comparable under pressures above the magnetic transition from ferromagnetic to paramagnetic phase at $P_{\text{c}}\approx 7$ GPa. Therefore, the Fe-Fe pairs dominantly contribute to the volume expansion due to the magnetovolume effect. Because a similar magnitude of elongation was observed in the Fe-Fe pairs of a non-Invar Fe-Ni alloy, we conclude that the Invar effect originates from the delicate balance between the number of Fe-Fe pairs and their elongation depending on the magnetization.

The dynamics of extended objects such as domain walls, domain bubbles, vortex structures, etc., can be described by their equations of motion associated with their effective mass and spring constant. Here we analytically derive the equations of motion for the polarization dynamics and elastodynamics for the structural responses of ferroelectric polar vortices, and theoretically extract their effective mass, spring constant, and mode frequencies. We demonstrate two subterahertz phonon modes and predicted their frequencies, both consistent with our recent experimental measurements and phase-field simulations. We show that elastic modulation of the energy function and spring constants leads to a condensation of a collective mode upon a second-order structural transition from symmetric to asymmetric vortices at a critical strain, analogous to the ferroelectric soft phonon mode at a ferroelectric transition. The present work offers a theoretical framework for predicting and manipulating the ultrafast collective dynamics of polar nanostructures.

We present a model compound with a spin-$\frac{1}{2}$ spatially anisotropic frustrated square lattice, in which three antiferromagnetic interactions and one ferromagnetic interaction are competing. We observe an unconventional gradual increase in the low-temperature magnetization curve reminiscent of the quantum critical behavior between gapped and gapless phases. In addition, the specific heat and electron spin resonance signals indicate one-dimensional characteristics. These results demonstrate quantum critical behavior associated with one dimensionalization caused by frustrated interactions in the spin-$\frac{1}{2}$ spatially anisotropic square lattice.

Green's functions are fundamental quantities that determine the linear responses of physical systems. The recent developments of non-Hermitian systems, therefore, call for Green's function formulas of non-Hermitian bands. This task is complicated by the high sensitivity of energy spectrums to boundary conditions, which invalidates the straightforward generalization of Hermitian formulas. Here, based on the non-Bloch band theory, we obtain simple Green's function formulas of general one-dimensional non-Hermitian bands. Furthermore, in the large-size limit, these formulas dramatically reduce to finding the roots of a simple algebraic equation. As an application, our formulation provides the desirable formulas for the defining quantities, the gain and directionality, of directional amplification. Thus our formulas provide an efficient guide for designing directional amplifiers.

Control of domain and domain wall configurations in antiferroelectrics is a necessary step towards practical use of new functionalities based on polar domain wall structures. Here we propose and demonstrate a domain engineering scheme that provides an antiferroelectric state with only one type of orientational domains and one type of walls. We demonstrate with in situ synchrotron diffraction experiment that in a material, where the transition from the high-symmetry nonpolar phase to that antiferroelectric occurs via crossing an intermediate, once the transition is passed under a moderate electric field, the final antiferroelectric domain state can be fully controlled. A theoretical analysis shows that such a phenomenon can be explained in terms of biquadratic coupling between the polarization and antiferroelectric order parameter. This analysis also suggests that the electric-field control of the antiferroelectric state may be possible in a more general case where the intermediate ferroelectric state is absent. Anisotropy of lattice excitations in the intermediate polar phase under electric field is uncovered by an inelastic x-ray scattering experiment, which indicates that lattice instability is a driving force of transformation towards antiferroelectric phase despite of a strong first character of the transition.

We revisit the problem of characterizing band topology in dynamically stable quadratic bosonic Hamiltonians that do not conserve particle number. We show this problem can be rigorously addressed by a smooth and local adiabatic mapping procedure to a particle-number-conserving Hamiltonian. In contrast to a generic fermionic pairing Hamiltonian, such a mapping can always be constructed for bosons. Our approach shows that particle-nonconserving bosonic Hamiltonians can be classified using known approaches for fermionic models. It also provides a simple means for identifying and calculating appropriate topological invariants. We also explicitly study dynamically stable but non-positive-definite Hamiltonians (as arise frequently in driven photonic systems). We show that in this case, each band gap is characterized by two distinct invariants.

Modeling blocking temperature in molecular magnets has been a long-standing problem in the field of molecular magnetism. We investigate this problem using a kinetic Monte Carlo (kMC) approach on an assembly of 100 000 short molecular magnetic chains (SMMCs), each of six identical spins with nearest-neighbor anisotropic ferromagnetic exchange interactions. Each spin is also anisotropic with an uniaxial anisotropy. The site spin on these SMMCs takes values 1, $3/2$, or 2. Using the eigenstates of these SMMCs as the states of a Markov chain, we carry out a kMC simulation starting with an initial state in which all SMMCs are completely spin-polarized and assembled on a one-dimensional lattice so as to experience ferromagnetic spin-dipolar interaction with each other. From these simulations, we obtain the relaxation time ${\tau }_r$ as a function of temperature and the associated blocking temperature. We study this for different exchange anisotropy, on-site anisotropy, and strength of dipolar interactions. The magnetization relaxation times show non-Arrhenius behavior for weak on-site interactions. The energy barrier to magnetization relaxation increases with an increase in on-site anisotropy, exchange anisotropy, and strength of spin dipolar interactions, more strongly on the last parameter. In all cases, the barrier saturates at large on-site anisotropy. The barrier also increases with site spin. The large barrier observed in rare-earth single ion magnets can be attributed to large dipolar interactions due to short intermolecular distances, due to their small size and large spin of the rare-earth ion in the molecule.

Magnon transport through a magnetic insulator can be controlled by current-biased heavy-metal gates that modulate the magnon conductivity via the magnon density. Here, we report nonlinear modulation effects in $10\mathrm{nm}$ thick yttrium iron garnet (YIG) films. The modulation efficiency is larger than 40%/mA. The spin-transport signal at high dc current density $(2.2\times {10}^{11}\mathrm{A}/{\mathrm{m}}^2)$ saturates for a $400\mathrm{nm}$ wide Pt gate, which indicates that even at high current levels a magnetic instability cannot be reached in spite of the high magnetic quality of the films.

The electronic states near the Fermi level of recently discovered superconductor ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ consist primarily of the Cu $d_{x^2-y^2}$ and $d_{3z^2-r^2}$ orbitals. We investigate the electronic correlation effect and the orbital polarization of an effective two-orbital Hubbard model mimicking the low-energy physics of ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ in the hole-rich regime by utilizing the dynamical mean-field theory with the Lanczos method as the impurity solver. We find that the hole-overdoped ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ with $3d^8$ (${\mathrm{Cu}}^{3+}$) is in the orbital-selective Mott phase (OSMP) at half-filling, and the typical two-orbital feature remains in ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ when the electron filling approaches $n_e\sim 2.5$, which closely approximates to the experimental hole doping for the emergence of the high-$T_c$ superconductivity. We also obtain that the orbital polarization is very stable in the OSMP, and the multiorbital correlation can drive orbital polarization transitions. These results indicate that in hole-overdoped ${\mathrm{Ba}}_2{\mathrm{CuO}}_{4-\delta }$ the OSMP physics and orbital polarization, local magnetic moment, and spin or orbital fluctuations still exist. We propose that our present results are also applicable to ${\mathrm{Sr}}_2{\mathrm{CuO}}_{4-\delta }$ and other two-orbital cuprates, demanding an unconventional multiorbital superconducting scenario in hole-overdoped high-$T_c$ cuprates.