Bundling and crosslinking of actin networks provide cells with tensile strength and mobility and play critical roles in diverse mechanical processes such as wounding healing and fertilization. One simple mechanism for bundling and crosslinking actin networks is using divalent counterions such as ${\mathrm{Mg}}^{2+}$ to form crossbridges between actin filaments. Counterion crossbridges can also be exploited in materials engineering to alter the mechanics of polyelectrolyte materials. As such, the mechanical properties that counterion crossbridges confer to actin networks needs to be explored. Here, we use optical tweezers microrheology to characterize the mechanical response of actin networks in the presence of varying concentrations of ${\mathrm{Mg}}^{2+}$. We couple mechanics to structure by using confocal microscopy to characterize the mobility and architecture of networks. We show that only modest bundling and network rearrangement is required to induce dramatic increases in the elasticity and stiffness of the networks. We further show that bundles are resilient to nonlinear forcing and exhibit surprisingly minimal dissipation above a critical counterion concentration. Finally, we demonstrate that while crosslinking of network fibers plays an important role in linear regime mechanics, filament bundling dictates the nonlinear response.

Topological spintronics aims to exploit the spin-momentum locking in the helical surface states of topological insulators for spin-orbit torque devices. We address a fundamental question that still remains unresolved in this context: Does the topological surface state alone produce the largest values of spin-charge conversion efficiency or can the strongly spin-orbit coupled bulk states also contribute significantly? By studying the Fermi level dependence of spin pumping in topological insulator/ferrimagnetic insulator bilayers, we show that the spin Hall conductivity is constant when the Fermi level is tuned across the bulk band gap, consistent with a full bulk band calculation. The results suggest a different perspective, wherein “bulk-surface correspondence” allows spin-charge conversion to be simultaneously viewed either as coming from the full bulk band, or from spin-momentum locking of the surface state.

Although the crystal structures of pure metals are material dependent, we theoretically find that entropy-driven docosahedral local structures are the most frequent in their melts and glasses, independent of their crystal structures. In terms of the docosahedral short-range order, we explain why body-centered-cubic metals have higher glass-forming ability than face-centered-cubic ones.

We show that microwave-driven NV centers can function as robust mode switches by utilizing a special degeneracy called an exceptional point (EP). While previous theoretical and experimental work on EP-based mode switches applies only to pure states, we develop here a general theory for switching between mixed states, statistical ensembles of different pure states, resulting from the interaction with the environment. Our theory is general and applicable to all leading platforms for quantum information processing and quantum technologies. However, our numerical simulations use empirical parameters of NV centers. We provide guidelines for coping with the main challenges for experimental realization of this protocol: decoherence and mixed-state preparation.

Motivated by the recent surge of field-driven phenomena discussed for Kitaev materials, in particular the experimental observation of a finite thermal Hall effect and theoretical proposals for the emergence of additional spin liquid phases beyond the conventional Kitaev spin liquid, we develop a theoretical understanding of the thermal Hall effect in honeycomb Kitaev materials in magnetic fields. Our focus is on gapless U(1) spin liquids with a spinon Fermi surface, which have been shown to arise as field-induced phases. We demonstrate that in the presence of symmetry-allowed second-neighbor Dzyaloshinskii-Moriya interactions these spin liquids give rise to a finite, nonquantized, thermal Hall conductance in a magnetic field. The microscopic origin of this thermal Hall effect can be traced back to an interplay of Dzyaloshinskii-Moriya interaction and Zeeman coupling, which generates an internal U(1) gauge flux that twists the motion of the emergent spinons. We argue that such a nonquantized thermal Hall effect is a generic response in Kitaev models for a range of couplings.

Studying a single atomic ion confined in a time-dependent periodic anharmonic potential, we find large amplitude trajectories stable for millions of oscillation periods in the presence of stochastic laser cooling. Accounting for the complexity of the laser cooling process we calculate the details of the effective dynamics away from thermal equilibrium. The competition between energy gain from the time-dependent drive and damping leads to the stabilization of such stochastic limit cycles. Instead of converging to the global minimum of the averaged potential, the steady-state phase-space distribution develops multiple peaks in the regions of phase space where the frequency of the motion is close to a multiple of the periodic drive. Such distinct nonequilibrium behavior can be observed in realistic radio-frequency traps with laser-cooled ions, suggesting that Paul traps offer a well-controlled test bed for studying the transport and dynamics of microscopically driven systems.

We report intrinsic spin decay length of an antiferromagnetic insulator. We found that at an antiferromagnetic/ferromagnetic interface, a spin current generated by spin pumping is strongly suppressed by two-magnon scattering. By eliminating the two-magnon contribution, we discovered that the characteristic length of spin decay in polycrystalline NiO changes by two orders of magnitude through the paramagnetic to antiferromagnetic transition. The spin decay length in the antiferromagnetic state is around 100 nm, which is an order of magnitude longer than that previously believed. These results provide a crucial piece of information for the fundamental understanding of the physics of spin transport.

The systematic diagnosis of band topology enabled by the method of “symmetry indicators” underlies the recent advances in the search for new materials realizing topological crystalline insulators. Such an efficient method has been missing for superconductors because the quasiparticle spectrum in the superconducting phase is not usually available. In this work, we establish symmetry indicators for weak-coupling superconductors that detect nontrivial topology based on the representations of the metallic band structure in the normal phase, assuming a symmetry property of the gap function. We demonstrate the applications of our formulas using examples of tight-binding models and density-functional-theory band structures of realistic materials.

Upcoming searches for the stochastic background of inflationary gravitational waves (GWs) offer the exciting possibility to probe the evolution of our Universe prior to big bang nucleosynthesis. In this spirit, we explore the sensitivity of future GW observations to a broad class of beyond-the-standard-model scenarios that lead to a nonstandard expansion history. We consider a new scalar field whose coherent oscillations dominate the energy density of the Universe at very early times, resulting in a scalar era prior to the standard radiation-dominated era. The imprint of this scalar era on the primordial GW spectrum provides a means to probe well-motivated yet elusive models of particle physics. Our work highlights the complementarity of future GW observatories across the entire range of accessible frequencies.

$\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ has recently been established as an intrinsic antiferromagnetic (AFM) topological insulator—an ideal platform to create quantum anomalous Hall insulator and axion insulator states. We performed comprehensive studies on the structure, nontrivial surface state, and magnetotransport properties of this material. Our results reveal an intrinsic anomalous Hall effect arising from a noncollinear spin structure for the magnetic field parallel to the $c$ axis. We observed negative magnetoresistance under arbitrary field orientation below and above the Néel temperature $\left(T_{\mathrm{N}}\right)$, providing clear evidence for strong spin fluctuation-driven spin scattering in both the AFM and paramagnetic states. Furthermore, we found that the nontrivial surface state opens a large gap $(\sim 85\mathrm{meV})$ even far above $T_{\mathrm{N}}$. Our findings demonstrate that the bulk band structure of $\mathrm{MnB}{\mathrm{i}}_2\mathrm{T}{\mathrm{e}}_4$ is strongly coupled with the magnetic property and that a net Berry curvature in momentum space can be created in the canted AFM state. In addition, our results imply that the gap opening in the surface states is intrinsic, likely caused by the strong spin fluctuations in this material.

Fracton topological phases host fractionalized topological quasiparticles with restricted mobility, with promising applications to fault-tolerant quantum computation. While a variety of exactly solvable fracton models have been proposed, there is a need for platforms to realize them experimentally. We show that a rich set of fracton phases emerges in interacting Majorana band models whose building blocks are within experimental reach. Specifically, our Majorana constructions overcome a principal obstacle, namely, the implementation of the complicated spin cluster interactions underlying fracton stabilizer codes. The basic building blocks of the proposed constructions include Coulomb blockaded Majorana islands and weak interisland Majorana hybridizations. This setting produces a wide variety of fracton states and promises numerous opportunities for probing and controlling fracton phases experimentally. Our approach also reveals the relation between fracton phases and Majorana fermion codes and further generates a hierarchy of fracton spin liquids.

We study the vibrational spectra and the specific heat of disordered systems using an effective hydrodynamic framework. We consider the contribution of diffusive modes, i.e., the “diffusons,” to the density of states and the specific heat. We prove analytically that these modes provide a constant term to the vibrational density of states $g\left(\omega \right)$. This contribution is dominant at low frequencies, with respect to the Debye propagating modes. We compare our results with numerical simulations data and random matrix theory. Finally, we compute the specific heat and we show the existence of a linear in $T$ scaling $C\left(T\right)\sim cT$ at low temperatures due to the diffusive modes. We analytically derive the coefficient $c$ in terms of the diffusion constant $D$ of the quasilocalized modes and we obtain perfect agreement with numerical data. The linear in $T$ behavior in the specific heat is stronger as the modes are more localized and crosses over to a $T^3$ (Debye) regime at a temperature $T^{*}\sim \sqrt{v^3/D}$, where $v$ is the speed of sound. Our results suggest that the anomalous properties of glasses and disordered systems can be understood effectively within a hyrodynamic approach, which accounts for diffusive quasilocalized modes generated via disorder-induced scattering.

A recent experimental realization of a quantum degenerate gas of ${}^{40}\mathrm{K}{}^{87}\mathrm{Rb}$ molecules opens up prospects of exploring strong dipolar Fermi gases and many-body phenomena arising in that regime. Here, we derive a mean-field variational approach based on the Wigner function for the description of the ground-state properties of such systems. We show that the stability of dipolar fermions in a general harmonic trap is universal as it only depends on the trap aspect ratios and the dipoles' orientation. We calculate the species-independent stability diagram and the deformation of the Fermi surface (FS) for polarized molecules, whose electric dipoles are oriented along a preferential direction. Compared to atomic magnetic species, the stability of a molecular electric system turns out to strongly depend on its geometry and the FS deformation significantly increases.

The smectic phase shows a layered structure, which is the most fundamental and universal order found in soft materials. However, previous technical limitations made it difficult to fully and microscopically observe the interlayer molecular motions in the smectic phase. We were able to directly measure the interlayer molecular motions by quasielastic scattering spectroscopy using Mössbauer gamma rays for $4^{\prime }-n$-octyl-4-cyanobiphenyl (8CB). From the measured motions, we evaluated the order parameter of the layer structure and explain the macroscopic diffusion coefficient in the microscopic viewpoint. Our methodology can be applied broadly to determine the layer order parameter of various soft materials.

Quantum entanglement may have various origins ranging from solely interaction-driven quantum correlations to single-particle effects. Here, we explore the dependence of entanglement on time-dependent single-particle basis transformations in fermionic quantum many-body systems, thus aiming at isolating single-particle sources of entanglement growth in quench dynamics. Using exact diagonalization methods, for paradigmatic nonintegrable models we compare to the standard real-space cut various physically motivated bipartitions. Moreover, we search for a minimal entanglement basis using local optimization algorithms, which at short to intermediate postquench times yields a significant reduction of entanglement beyond a dynamical Hartree-Fock solution. In the long-time limit, we identify an asymptotic universality of entanglement for weakly interacting systems, as well as a crossover from dominant real-space to momentum-space entanglement in Hubbard models undergoing an interaction quench. Finally, we discuss the relevance of our findings for the development of tensor-network-based algorithms for quantum dynamics.

Percolation on complex networks is used both as a model for dynamics $\mathit{on}$ networks, such as network robustness or epidemic spreading, and as a benchmark for our models $\mathit{of}$ networks, where our ability to predict percolation measures our ability to describe the networks themselves. In many applications, correctly identifying the phase transition of percolation on real-world networks is of critical importance. Unfortunately, this phase transition is obfuscated by the finite size of real systems, making it hard to distinguish finite-size effects from the inaccuracy of a given approach that fails to capture important structural features. Here, we borrow the perspective of smeared phase transitions and argue that observed discrepancies may be due to the complex mesoscopic structure of real networks rather than to finite-size effects only. We support and illustrate this claim by studying real and synthetic networks through the lens of local order parameters, message passing, and local susceptibility. Our results not only shed light on the nature of the percolation transition in complex networks but also provide two important insights on the numerical and analytical tools we use to study them. First, we propose a measure of local susceptibility to better detect both clean and smeared phase transitions by looking at the topological variability of the order parameter. Second, we discuss a shortcoming in state-of-the-art analytical approaches such as message passing, which can detect smeared transitions but not characterize their nature.

The detection of gravitational waves has established a new and very exciting field of astronomy in the past few years. To increase the number of detections and allow observation of a wider range of sources, several future gravitational wave detectors will operate at cryogenic temperatures. Recent investigations of a mirror in one of the cryostats of the Japanese KAGRA detector showed a decrease in reflectivity due to ice growth, induced by residual water molecules moving from the warm to the cold sections of the detector's vacuum system. Based on the optical measurements made in KAGRA, in this paper we calculate the implications of an ice layer on coating thermal noise for the planned European Einstein Telescope. We find coating thermal noise to oscillate, due to periodic reflectivity changes as the ice layer grows. The average coating thermal noise increases significantly over a time of one year with a larger increase at higher temperatures.

A complex conjugation of unitary quantum map is a second-order map (supermap) that maps a unitary operator $U$ to its complex conjugate $U^{*}$. First, we present a deterministic quantum protocol that universally implements the complex conjugation supermap when we are given a blackbox quantum circuit, guaranteed to implement some unitary operation, whose only known description is its dimension. We then discuss the complex conjugation supermap in the context of entanglement theory and derive a conjugation-based expression of the $G$ concurrence. Finally, we present a physical process involving identical fermions from which the complex conjugation protocol is derived as a simulation of the process using qudits.

In quantum computing, the indirect measurement of unitary operators such as the Hadamard test plays a significant role in many algorithms. However, in certain cases, the indirect measurement can be reduced to the direct measurement, where a quantum state is destructively measured. Here, we investigate under what conditions such a replacement is possible and develop a general methodology for trading an indirect measurement with sequential direct measurements. The results can be applied to construct quantum circuits to evaluate the analytical gradient, metric tensor, Hessian, and even higher order derivatives of a parametrized quantum state. Also, we propose a method to measure the out-of-time-order correlator based on the presented protocol. Our protocols can significantly reduce the depth of a quantum circuit by making the controlled operation unnecessary, and thus are suitable for quantum-classical hybrid algorithms on near-term quantum computers.

In this paper we calculate the optical forces and torques caused by the presence of a sizable magneto-optical effect. We find a conservative force proportional to the gradient of the spin density of the light field and an extinction force proportional to the helicity of the light field. The conservative interaction allows for a spin-selective, magnetic field based Stern-Gerlach experiment, capable of differentiating between right and left circular polarizations. We also prove that by using a spinless linearly polarized plane wave, the magneto-optical effect allows for the existence of a permanent nonreciprocal torque, proportional to the intensity of the light field.