Implementation of geometric nonlinearity in microelectromechanical systems (MEMS) resonators offers a flexible and efficient design to overcome the limitations of linear MEMS by utilizing beneficial nonlinear characteristics not attainable in a linear setting. Integration of nonlinear coupling elements into an otherwise purely linear microcantilever is one promising way to intentionally realize geometric nonlinearity. Here, we demonstrate that a nonlinear, heterogeneous microresonator system, consisting of a silicon microcantilever with a polymer attachment exhibits strong nonlinear hardening behavior not only in the first flexural mode but also in the higher modes (i.e., second and third flexural modes). In this design, we deliberately implement a drastic and reversed change in the axial vs bending stiffness between the Si and polymer components by varying the geometric and material properties. By doing so, the resonant oscillations induce the large axial stretching within the polymer component, which effectively introduces the geometric stiffness and damping nonlinearity. The efficacy of the design and the mechanism of geometric nonlinearity are corroborated through a comprehensive experimental, analytical, and numerical (finite element) analysis on the nonlinear dynamics of the proposed system.

Entropy is a fundamental concept in equilibrium statistical mechanics, yet its origin in the nonequilibrium dynamics of isolated quantum systems is not fully understood. A strong consensus is emerging around the idea that the stationary thermodynamic entropy is the von Neumann entanglement entropy of a large subsystem embedded in an infinite system. Also motivated by cold-atom experiments, here we consider the generalization to Rényi entropies. We develop a new technique to calculate the diagonal Rényi entropy in the quench action formalism. In the spirit of the replica treatment for the entanglement entropy, the diagonal Rényi entropies are generalized free energies evaluated over a thermodynamic macrostate which depends on the Rényi index and, in particular, is not the same state describing von Neumann entropy. The technical reason for this perhaps surprising result is that the evaluation of the moments of the diagonal density matrix shifts the saddle point of the quench action. An interesting consequence is that different Rényi entropies encode information about different regions of the spectrum of the postquench Hamiltonian. Our approach provides a very simple proof of the long-standing issue that, for integrable systems, the diagonal entropy is half of the thermodynamic one and it allows us to generalize this result to the case of arbitrary Rényi entropy.

In this work we present friction-force microscopy (FFM) lattice-resolved images acquired on the (100) facet of the semiconductor organic oligomer ($2Z,2^{\prime }Z)-3,3^{\prime }$-(1,4-phenylene)bis(2-(4-butoxyphenyl)acrylonitrile) ($\beta$-DBDCS) crystal in water at room temperature. Stick-slip contrast, lateral contact stiffness, and friction forces are found to depend strongly on the sliding direction due to the anisotropic packing of the molecular chains forming the crystal surface along the [010] and [001] directions. The anisotropy also causes the maximum value of the normal force applicable before wearing to increase by a factor of 3 when the scan is performed along the [001] direction on the (100) face. Altogether, our results contribute to achieving a better understanding of the molecular origin of friction anisotropy on soft crystalline surfaces, which has been often hypothesized but rarely investigated in the literature.

Oxygen vacancies play an important role in controlling the physical properties of a perovskite oxide. We report alterations in the electronic properties of a cubic perovskite oxide, namely, ${\mathrm{KTaO}}_3$, as a function of oxygen vacancies. The conducting surface of the ${\mathrm{KTaO}}_3$ single-crystal substrate has been realized via ${\mathrm{Ar}}^{+}$ irradiation. The band gap changes as a function of conductivity which is controlled by irradiation time, indicating the formation of defect states. Kelvin probe force microscopy suggests a sharp increase in the work function upon ${\mathrm{Ar}}^{+}$ irradiation for a short period of time followed by a monotonic decrease, as we increase the irradiation time. Our experimental findings along with theoretical simulations suggest a significant surface dipole contribution and an unusual change in the electronic band line-up of ${\mathrm{KTaO}}_3$ due to oxygen vacancies.

We study a microscopic model of interacting fermions in a ladder setup, where the total number of particles is conserved. At a special point, the ground state is known and gives rise to a topological state of matter with edge modes obeying the statistics of Ising anyons. Using a combination of bosonization as well as full scale numerical density-matrix renormalization group analysis, we map out the full phase diagram. We find that the topological phase survives in an extended parameter regime. Remarkably, an additional symmetry is required to protect the topological phase.

To understand the electronic states on the boundary region between the commensurate antiferromagnetic (II) phase and the incommensurate spin-density-wave phase in the generalized phase diagram for one-dimensional organic conductors, we performed antiferromagnetic resonance (AFMR) and nuclear magnetic resonance (NMR) measurements for ${\left(\mathrm{TMTTF}\right)}_2\mathrm{Br}$. The angular dependence of the AFMR fields at 1.5 K is different from that at 4.8 K, and the temperature dependence of the two AFMR modes is enhanced below 5 K. Furthermore, ${}^2\mathrm{D}$(Deuterium)-NMR measurements were performed for deuterated ${\left(\mathrm{TMTTF}\text{−}{d}_{12}\right)}_2\mathrm{Br}$ to investigate charge distribution by quadrupole splitting at low temperatures. We found that the ${}^2\mathrm{D}$-NMR spectrum changes at 4 K in the antiferromagnetic phase. Successive phase transition and a possible magnetic structure are discussed.

We study a tight-binding model of interacting Majorana (Hermitian) modes on a square lattice. The model may have an experimental realization in a superconducting-film–topological-insulator heterostructure in a magnetic field. We find a rich phase diagram, as a function of interaction strength, including an emergent superfluid phase with spontaneous breaking of an emergent $\mathrm{U}\left(1\right)$ symmetry, separated by a supersymmetric transition from a gapless normal phase.

We report magnetotransport measurements of PrSb in high magnetic fields. Our results show that PrSb exhibits large magnetoresistance at low temperatures. Meanwhile angle-dependent magnetoresistance measurements were used to probe the Fermi surface via Shubnikov–de Haas oscillations. We found that the angular dependence of the oscillation frequency of the $\alpha$ branch can be explained well by a model for a two-dimensional-like Fermi surface, whereas the effective mass of this branch as a function of angle shows a fourfold signature. The evolution of the Fermi surface with increasing magnetic field also was studied up to 32 T. A continuous increase in the oscillation frequency up to 14 T is observed before it becomes constant at higher fields. Meanwhile our analysis of the residual Landau index from the high-field data reveals a zero Berry phase and therefore trivial topology of the Fermi surface.

We present a theoretical analysis of the role that strain plays on the electronic structure of chromium nitride (CrN) crystals. We use local spin-density approximation + $U$ calculations to study the elastic constants, deformation potentials, and strain dependence of electron and hole masses near the fundamental gap. We consider the lowest energy antiferromagnetic models believed to describe CrN at low temperatures, and apply strain along different directions. We find relatively large deformation potentials for all models, and find increasing gaps for tensile strain along most directions. Most interestingly, we find that compressive strains should be able to close the relatively small indirect gap $(\simeq 100$ meV) at moderate amplitudes $\simeq 1.3\%$. We also find large and anisotropic changes in the effective masses with strain, with principal axes closely related to the magnetic ordering of neighboring layers in the antiferromagnet. It would be interesting to consider the role that these effects may have on typical film growth on different substrates, and the possibility of monitoring optical and transport properties of thin films as strain is applied.

We study the properties of electron excitation and emission of a finite carbon nanotube (CNT) under a linearly polarized femtosecond laser using real-time time-dependent density-functional theory. We find a plasmon resonance in the near-infrared region of the optical absorption spectrum that is highly sensitive to the laser polarization direction. The laser polarization direction dependence of the plasmon excitation obtained in the present study is consistent with the properties of polarized optical absorption observed in experiments. In electron emission that occurs subsequent to electronic excitation, the yield of emitted electrons also shows a laser polarization direction dependence, and the kinetic energy spectrum of the emitted electrons is broadened in the high-energy regime due to the electron acceleration caused by plasmon-induced electric-field enhancement. The findings of the present study are of crucial importance for understanding the laser-CNT interaction, which is a prerequisite for applications of CNTs in electron emitters, nanodevices, and optoelectronic components and sensors.

Recent experimental searches for signatures of Majorana-type excitations in proximitized semiconducting nanowires involve conductance spectroscopy, where the evidence sought after is a robust zero-bias peak (in longer wires) and its characteristic field-dependent splitting (in shorter wires). Although experimental results partially confirm the theoretical predictions, commonly observed discrepancies still include (i) a zero-bias peak that is significantly lower than the predicted value of $2e^2/h$ and (ii) the absence of the expected “Majorana oscillations” of the lowest-energy modes at higher magnetic fields. Here, we investigate how the inevitable presence of a normal drain lead connected to the hybrid wire can affect the conductance spectrum of the hybrid wire. We present numerical results using a one-band model for the proximitized nanowire, where the superconductor is considered to be in the diffusive regime, described by semiclassical Green functions. We show how the presence of the normal drain could (at least partially) account for the observed discrepancies, and we complement this with analytic results providing more insights in the underlying physics.

We investigate the short-time quantum critical dynamics in the imaginary-time relaxation processes of finite-size systems. Universal scaling behaviors exist in the imaginary-time evolution. In particular, the system undergoes a critical initial slip stage characterized by an exponent $\theta$, in which an initial power-law increase emerges in the imaginary-time correlation function when the initial state has a zero order parameter and a vanishing correlation length. Under different initial conditions, the quantum critical point and critical exponents can be determined from the universal scaling behaviors. We apply the method to the one- and two-dimensional transverse field Ising models using quantum Monte Carlo (QMC) simulations. In the one-dimensional case, we locate the quantum critical point at ${(h/J)}_c=1.00003\left(8\right)$ in the thermodynamic limit, and we estimate the critical initial slip exponent $\theta =0.3734\left(2\right)$ and the static exponent $\beta /\nu =0.1251\left(2\right)$ by analyzing data on chains of length $L=32–256$ and 48–256, respectively. For the two-dimensional square-lattice system, the critical coupling ratio is given by $3.04451\left(7\right)$ in the thermodynamic limit, while the critical exponents are $\theta =0.209\left(4\right)$ and $\beta /\nu =0.518\left(1\right)$ estimated by data on systems of size $L=24$–64 and 32–64, respectively. Remarkably, the critical initial slip exponents obtained in both models are notably distinct from their classical counterparts due to the essential differences between classical and quantum dynamics. The short-time critical dynamics and the imaginary-time relaxation QMC approach can be readily adapted to various models.

Brillouin light scattering is an established technique to study magnons, the elementary excitations of a magnet. Its efficiency can be enhanced by cavities that concentrate the light intensity. Here, we theoretically study inelastic scattering of photons by a magnetic sphere that supports optical whispering gallery modes in a plane normal to the magnetization. Magnons with low angular momenta scatter the light in the forward direction with a pronounced asymmetry in the Stokes and the anti-Stokes scattering strength, consistent with earlier studies. Magnons with large angular momenta constitute Damon-Eschbach modes which are shown to inelastically reflect light. The reflection spectrum contains either a Stokes or anti-Stokes peak, depending on the direction of the magnetization, a selection rule that can be explained by the chirality of the Damon-Eshbach magnons. The controllable energy transfer can be used to manage the thermodynamics of the magnet by light.

Recent preliminary experiments [A. Marakov et al., Phys. Rev. B 91, 094503 (2015).] using triplet-state ${\mathrm{He}}_2$ excimer molecules as tracers of the motion of the normal fluid have shown that, in thermal counterflow turbulence in superfluid ${}^4\mathrm{He}$, small-scale turbulence in the superfluid component is accompanied, above a critical heat flux, by partially coupled large-scale turbulence in both fluids, with an energy spectrum proportional to $k^{-m}$, where $m$ is greater than the Kolmogorov value of 5/3. Here we report the results of a more detailed study of this spectrum over a range of temperatures and heat fluxes using the same experimental technique. We show that the exponent $m$ varies systematically with heat flux but is always greater than 5/3. We interpret this as arising from the steady counterflow, which causes large-scale eddies in the two fluids to be pulled in opposite directions, giving rise to dissipation by mutual friction at all wave numbers, mutual friction tending also to oppose the effect of the counterflow. Comparison of the experimental results with a simple theory suggests that this process may be more complicated than we might have hoped, but experiments covering a wider range of heat fluxes, which are technically very difficult, will probably be required before we can arrive at a convincing theory.

Our detailed temperature-dependent synchrotron powder x-ray diffraction studies along with first-principles density-functional-theory-based calculations enable us to shed light on the origin of ferroelectricity in ${\mathrm{GdCrO}}_3$. The actual lattice symmetry is found to be the noncentrosymmetric orthorhombic $Pna{2}_1$ structure, supporting the polar nature of the system. Polar distortion is associated with the Gd displacements with respect to the oxygen cage. Our study reveals an intimate analogy between ${\mathrm{GdCrO}}_3$ and ${\mathrm{YCrO}}_3$. However, a distinctive difference exists because Gd is less displacive than Y, which results in an orthorhombic $Pna{2}_1$ structure in ${\mathrm{GdCrO}}_3$ in contrast to the monoclinic $P{2}_1$ structure in ${\mathrm{YCrO}}_3$ and, consequently, decreases its polar property. It is found that magnetic coupling between Gd $4f$ and Cr $3d$ plays an important role in ferroelectric distortion. A strong magneto-electric coupling is also revealed using Raman spectroscopy based analysis, indicating its relevance to ferroelectric modulation.

We carried out inelastic neutron scattering experiments on a buckled honeycomb lattice antiferromagnet ${\mathrm{Ba}}_2{\mathrm{NiTeO}}_6$ exhibiting a stripe structure at a low temperature. Magnetic excitations are observed in the energy range of $\hslash \omega \lesssim 10$ meV having an anisotropy gap of 2 meV at 2 K. We perform spin-wave calculations to identify the spin model. The obtained microscopic parameters are consistent with the location of the stripe structure in the classical phase diagram. Furthermore, the Weiss temperature independently estimated from a bulk magnetic susceptibility is consistent with the microscopic parameters. The results reveal that a competition between the nearest-neighbor and next-nearest-neighbor interactions that together with a relatively large single-ion magnetic anisotropy stabilize the stripe magnetic structure.

Many researchers have shown an interest in Ge-based dilute magnetic semiconductors (DMSs) due to potential advantages for semiconductor spintronics applications. There has been great discussion about mechanisms of experimentally observed ferromagnetism in (Ge,Fe) and (Ge,Mn). We investigate the electronic structures, structural stabilities, magnetic exchange coupling constants, and Curie temperature of Ge-based DMSs, and clarify origins of the ferromagnetism, on the basis of density functional theory calculations. In both the (Ge,Fe) and (Ge,Mn) cases, the inhomogeneous distribution of the magnetic impurities plays an important role to determine the magnetic states; however, physical mechanisms of the ferromagnetism in these two materials are completely different. By the spinodal nanodecomposition, the Fe impurities in Ge gather together with keeping the diamond structure, so that the number of the first-nearest-neighbor Fe pairs with strong ferromagnetic interaction increases. Therefore, the Curie temperature drastically increases with the progress of the annealing. Our cluster expansion method clearly reveals that the other ordered compounds with different crystal structures such as ${\mathrm{Ge}}_3{\mathrm{Mn}}_5$ and ${\mathrm{Ge}}_8{\mathrm{Mn}}_{11}$ are easily generated in the (Ge,Mn) system. The estimated Curie temperature of ${\mathrm{Ge}}_3{\mathrm{Mn}}_5$ is in agreement with the observed Curie temperature in experiments. It should be considered that the precipitation of the ferromagnetic ${\mathrm{Ge}}_3{\mathrm{Mn}}_5$ clusters is an origin of high Curie temperature in (Ge,Mn).

We investigate the phase diagrams of the effective spin models derived from Fermi-Hubbard and Bose-Hubbard models with Rashba spin-orbit coupling, using string bond states, one of the quantum tensor network states methods. We focus on the role of quantum fluctuation effect in stabilizing the exotic spin phases in these models. For boson systems, and when the ratio between interparticle and intraparticle interaction $\lambda >1$, the out-of-plane ferromagnetic (FM) and antiferromagnetic (AFM) phases obtained from quantum simulations are the same as those obtained from the classic model. However, the quantum order-by-disorder effect reduces the classical in-plane XY-FM and XY-vortex phases to the quantum X/Y-FM and X/Y-stripe phase when $\lambda <1$. The spiral phase and skyrmion phase can be realized in the presence of quantum fluctuation. For the Fermi-Hubbard model, the quantum fluctuation energies are always important in the whole parameter regime. A general picture to understand the phase diagrams from a symmetry point of view is also presented.

We investigate theoretically the nature of laser-irradiated Kondo insulators. Using Floquet theory and the slave-boson approach, we study a periodic Anderson model and derive an effective model that describes laser-irradiated Kondo insulators. In this model, we find two generic effects induced by laser light. One is dynamical localization, which suppresses hopping and hybridization. The other is laser-induced hopping and hybridization, which can be interpreted as synthetic spin-orbit coupling or a magnetic field. The first effect drastically changes the behavior of the Kondo effect. In particular, the Kondo effect under laser light qualitatively changes its character depending on whether the hybridization is on-site or off-site. The second effect triggers topological phase transitions. In topological Kondo insulators, linearly polarized laser light realizes phase transitions between trivial, weak topological, and strong topological Kondo insulators. Moreover, circularly polarized laser light breaks time-reversal symmetry and induces Weyl semimetallic phases. Our results make it possible to dynamically control the Kondo effect and topological phases in heavy-fermion systems. We also discuss experimental setups to detect the signatures.

We predicted, using first principles calculation, that $A\mathrm{MgBi}$ ($A=$ K, Rb, Cs) are symmetry-protected topological semimetals near the boundary of type-I and type-II Dirac semimetal phases, dubbed topological critical Dirac semimetals. In-plane compressive strain or doping Rb or Cs into KMgBi can drive the transition between the two phases. An effective theory is developed to describe the bands near the Fermi energy, by which we calculate the surface Fermi arcs and the Landau levels throughout the transition. We predict the key features of critical Dirac semimetals that can be observed in photoemission, quantum oscillation, and transport measurements.