We address the characterization of molecular nanomagnets at the quantum level and analyze the performance of local measurements in estimating the physical parameters in their spin Hamiltonians. To this aim, we compute key quantities in quantum estimation theory, such as the classical and the quantum Fisher information, in the prototypical case of a heterometallic antiferromagnetic ring. We show that local measurements, performed only on a portion of the molecule, allow a precise estimate of the parameters related to both magnetic defects and avoided level crossings.

We theoretically study a one-dimensional $p$-wave superconducting mesoscopic ring interrupted by a weak link and coupled inductively to a microwave cavity. We establish an input-output description for the cavity field in the presence of the ring, and identify the electronic contributions to the cavity response and their dependence on various parameters, such as the magnetic flux, chemical potential, and cavity frequency. We show that the cavity response is $4\pi$ periodic as a function of the magnetic flux in the topological region, stemming from the so-called fractional Josephson current carried by the Majorana fermions, while it is $2\pi$ periodic in the nontopological phase, consistent with the normal Josephson effect. We find a strong dependence of the signal on the cavity frequency, as well as on the parity of the ground state. Our model takes into account fully the interplay between the low-energy Majorana modes and the gapped bulks states, which we show is crucial for visualizing the evolution of the Josephson effect during the transition from the topological to the trivial phase.

We report on spectroscopy study of elementary magnetic excitations in an Ising-like antiferromagnetic chain compound ${\mathrm{SrCo}}_2{\mathrm{V}}_2{\mathrm{O}}_8$ as a function of temperature and applied transverse magnetic field up to 25 T. An optical as well as an acoustic branch of confined spinons, the elementary excitations at zero field, are identified in the antiferromagnetic phase below the Néel temperature of 5 K and described by a one-dimensional Schrödinger equation. The confinement can be suppressed by an applied transverse field and a quantum disordered phase is induced at 7 T. In this disordered paramagnetic phase, we observe three emergent fermionic excitations with different transverse-field dependencies. The nature of these modes is clarified by studying spin dynamic structure factor of a 1D transverse-field Heisenberg-Ising $\left(XXZ\right)$ model using the method of infinite time evolving block decimation. Our work reveals emergent quantum phenomena and provides a concrete system for testifying theoretical predications of one-dimension quantum spin models.

Realization and design of topological insulators emerging from electron correlations, called topological Mott insulators (TMIs), is pursued by using mean-field approximations as well as multi-variable variational Monte Carlo (MVMC) methods for Dirac electrons on honeycomb lattices. The topological insulator phases predicted in the previous studies by the mean-field approximation for an extended Hubbard model on the honeycomb lattice turn out to disappear, when we consider the possibility of a long-period charge-density-wave (CDW) order taking over the TMI phase. Nevertheless, we further show that the TMI phase is still stabilized when we are able to tune the Fermi velocity of the Dirac point of the electron band. Beyond the limitation of the mean-field calculation, we apply the newly developed MVMC to make accurate predictions after including the many-body and quantum fluctuations. By taking the extrapolation to the thermodynamic and weak external field limit, we present realistic criteria for the emergence of the topological insulator caused by the electron correlations. By suppressing the Fermi velocity to a tenth of that of the original honeycomb lattice, the topological insulator emerges in an extended region as a spontaneous symmetry breaking surviving competitions with other orders. We discuss experimental ways to realize it in a bilayer graphene system.

We formulate a bulk spectroscopic method for direct measurement of the topological invariant of nondegenerate band crossings in Weyl semimetals. We demonstrate how polarization-resolved resonant x-ray scattering captures the winding of the Berry flux around Weyl nodes. The spectra obtained by the proposed strategy feature an integer number of zero crossings that is directly related to the topological charge of the measured nodes. We benchmark the proposed protocol on TaAs, using realistic low-energy models derived from density-functional theory to accurately represent the states close to the Fermi level, including sizable deviations from the idealized linear dispersion. We conclude that the proposed measurement, which is within the reach of current experimental setups, yields direct signatures of nontrivial band topology in the spectroscopy of three-dimensional bulk matter.

Donor electron spins in semiconductors make exceptional qubits because of their long coherence times and compatibility with industrial fabrication techniques. Despite many advances in donor-based qubit technology, it remains difficult to selectively manipulate single-donor electron spins. Here, we show that by replacing the prevailing semiconductor host material (silicon) with germanium, donor electron spin qubits can be electrically tuned by more than an ensemble linewidth, making them compatible with gate-addressable quantum computing architectures. Using $X$-band pulsed electron spin resonance, we measured the Stark effect for donor electron spins in germanium. We resolved both spin-orbit and hyperfine Stark shifts and found that at 0.4 T, the spin-orbit Stark shift dominates. The spin-orbit Stark shift is highly anisotropic, depending on the electric field orientation relative to the crystal axes and external magnetic field. When the Stark shift is maximized, the spin-orbit Stark parameter is four orders of magnitude larger than in silicon. At select orientations a hyperfine Stark effect was also resolved and is an order of magnitude larger than in silicon. We report the Stark parameters for ${}^{75}\mathrm{As}$ and ${}^{31}\mathrm{P}$ donor electrons and compare them to the available theory. Our data reveal that ${}^{31}\mathrm{P}$ donors in germanium can be tuned by at least four times the ensemble linewidth, making germanium an appealing new host material for spin qubits that offers major advantages over silicon.

Simulations of propagation of light beams in specially designed multilayer semiconductor structures (one-dimensional photonic crystals) with embedded quantum wells reveal characteristic optical properties of resonant hyperbolic metamaterials. A strong dependence of the refraction angle and the optical beam spread on the exciton radiative lifetime is revealed. We demonstrate the strong negative refraction of light and the control of the group velocity of light by an external bias through its effect upon the exciton radiative properties.

The electron momentum density obtained from the Schwinger-like mechanism is evaluated for a graphene sample immersed in a homogeneous time-dependent electric field. Based on the analogy between graphene low-energy electrons and quantum electrodynamics (QED), numerical techniques borrowed from strong field QED are employed and compared to approximate analytical approaches. It is demonstrated that for some range of experimentally accessible parameters, the pair production proceeds by sequences of adiabatic evolutions followed by nonadiabatic Landau-Zener transitions, reminiscent of the Kibble-Zurek mechanism describing topological defect density in second order phase transitions. For some field configurations, this yields interference patterns in momentum space which are explained in terms of the adiabatic-impulse model and the Landau-Zener-Stückelberg interferometry.

Excitons in transition-metal dichalcogenides can be dynamically manipulated using electrostatic fields. Analyzing both in-plane and out-of-plane fields, I compute the exciton Stark shift and electroabsorption spectrum of monolayer $\mathrm{Mo}{\mathrm{S}}_2,\mathrm{MoS}{\mathrm{e}}_2,\mathrm{W}{\mathrm{S}}_2$, and $\mathrm{WS}{\mathrm{e}}_2$. The effect of in-plane fields is found to greatly surpass that of out-of-plane fields. In particular, if exciton binding is reduced through screening by surrounding dielectrics, such as in $\mathrm{Mo}{\mathrm{S}}_2$ encapsulated by hexagonal boron-nitride, the in-plane exciton polarizability exceeds the measured out-of-plane value by nearly two orders of magnitude. Accordingly, pronounced electroabsorption features are expected for fields as low as $10\mathrm{V}\mu {\mathrm{m}}^{-1}$.

We study the transport mechanisms taking place in a quantum spin Hall bar with an embedded quantum dot, where electrons localize and experience Coulomb interaction $U$ as well as spin-flip processes $\lambda$. We solve the problem with nonequilibrium Green functions. We focus on the linear-response regime and treat the many-body interactions with quantum Monte Carlo. The effects of $U$ and $\lambda$ are competitive and the induced transport takes place through different channels. The two mechanisms can be switched by changing the occupation of the dot with a gate voltage.

We propose a theoretical approach based on an interferometer composed by two quantum dots asymmetrically coupled to isolated Majorana quasiparticles (MQPs), lying on the edges of two topological Kitaev chains, respectively, via couplings $(t+\mathrm{\Delta })$ and $(\mathrm{\Delta }-t)$. This setup enables us to probe MQPs in a quite distinct way from the zero-bias peak feature. Most importantly, the system behaves as a current switch made by two distinct paths: (i) for the upper dot connected to both chains, the device perceives both MQPs as an ordinary fermion and the current crosses solely the lower dot, since current in the upper dot is prevented due to the presence of the superconducting gap; and (ii) by suppressing slightly the hybridization of the upper dot with one chain, the current is abruptly switched to flow through this dot, once a trapped electron as a bound state in the continuum (BIC) [Phys. Rev. B 93, 165116 (2016)] appears in the lower dot. Such a current switch between upper and lower dots characterizes the quantum phase transition (QPT) proposed here, being the ratio $t/\mathrm{\Delta }$ the control parameter of the transition. This QPT is associated with a change from an ordinary fermionic excitation regime to a MQP in the interferometer, which enables not only the fundamental revealing of MQPs, but also yields a current switch assisted by them.

We derive an analytic expression for the geometric Hall viscosity of noninteracting electrons in a single graphene layer in the presence of a perpendicular magnetic field. We show that a recently derived formula in C. Hoyos and D. T. Son [Phys. Rev. Lett. 108, 066805 (2012)], which connects the coefficient of $q^2$ in the wave-vector expansion of the Hall conductivity ${\sigma }_{xy}\left(q\right)$ of the two-dimensional electron gas (2DEG) to the Hall viscosity and the orbital diamagnetic susceptibility of that system, continues to hold for graphene, in spite of the lack of Galilean invariance, with a suitable definition of the effective mass. We also show that, for a sufficiently large number of occupied Landau levels in the positive-energy sector, the Hall conductivity of electrons in graphene reduces to that of a Galilean-invariant 2DEG with an effective mass given by $\hslash k_F/v_F$ (cyclotron mass). Even in the most demanding case, i.e., when the chemical potential falls between the zeroth and the first Landau levels, the cyclotron mass formula gives results accurate to better than 1%. The connection between the Hall conductivity and the viscosity provides a possible avenue to measure the Hall viscosity in graphene.

Shock compression of silicon (Si) under extremely high pressures (>100 Mbar) was investigated by using two first-principles methods of orbital-free molecular dynamics (OFMD) and path integral Monte Carlo (PIMC). While pressures from the two methods agree very well, PIMC predicts a second compression maximum because of $1s$ electron ionization that is absent in OFMD calculations since Thomas–Fermi-based theories lack shell structure. The Kohn-Sham density functional theory is used to calculate the equation of state (EOS) of warm dense silicon for low-pressure loadings ($P$ < 100 Mbar). Combining these first-principles EOS results, the principal shock Hugoniot curve of silicon for pressures varying from ∼1 Mbar to above ∼10 Gbar was derived. We find that silicon is ∼20%, or more, softer than what was predicted by widely used EOS models. Existing high-pressure experimental data ($P$ ≈ 1–2 Mbar) seem to indicate this softening behavior of Si, which calls for future strong-shock experiments ($P>10$ Mbar) to benchmark our results.

We report on nanoscale pillar-based hypersonic phononic crystals in single crystal Z-cut lithium niobate. The phononic crystal is formed by a two-dimensional periodic array of nearly cylindrical nanopillars 240 nm in diameter and 225 nm in height, arranged in a triangular lattice with a 300-nm lattice constant. The nanopillars are fabricated by the recently introduced nanodomain engineering via laser irradiation of patterned chrome followed by wet etching. Numerical simulations and direct measurements using Brillouin light scattering confirm the simultaneous existence of nonradiative complete surface phononic band gaps. The band gaps are found below the sound line at hypersonic frequencies in the range 2–7 GHz, formed from local resonances and Bragg scattering. These hypersonic structures are realized directly in the piezoelectric material lithium niobate enabling phonon manipulation at significantly higher frequencies than previously possible with this platform, opening new opportunities for many applications in plasmonic, optomechanic, microfluidic, and thermal engineering.

Crystallographic, electronic transport, thermal, and magnetic properties are reported for ${\mathrm{SrMn}}_2{\mathrm{As}}_2$ and ${\mathrm{CaMn}}_2{\mathrm{As}}_2$ single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal ${\mathrm{CaAl}}_2{\mathrm{Si}}_2$-type structure (space group $P\overline{3}m1$), in agreement with the literature. Electrical resistivity $\rho$ versus temperature $T$ measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for ${\mathrm{SrMn}}_2{\mathrm{As}}_2$ and 61 meV for ${\mathrm{CaMn}}_2{\mathrm{As}}_2$. In a local-moment picture, the ${\mathrm{Mn}}^{+2}3d^5$ ions are expected to have high-spin $S=5/2$ with spectroscopic splitting factor $g\approx 2$. Magnetic susceptibility $\chi$ and heat capacity $C_{\mathrm{p}}$ measurements versus $T$ reveal antiferromagnetic (AFM) transitions at $T{}_{\mathrm{N}}=120\left(2\right)$ K and 62(3) K for ${\mathrm{SrMn}}_2{\mathrm{As}}_2$ and ${\mathrm{CaMn}}_2{\mathrm{As}}_2$, respectively. The anisotropic $\chi (T\le T{}_{\mathrm{N}})$ data indicate that the hexagonal $c$ axis is the hard axis and hence that the ordered Mn moments are aligned in the $ab$ plane. The $\chi \left(T\right)$ data for both compounds and the $C_{\mathrm{p}}\left(T\right)$ for ${\mathrm{SrMn}}_2{\mathrm{As}}_2$ show strong dynamic short-range AFM correlations from $T_{\mathrm{N}}$ up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the $ab$ plane.

In this work, we study the electronic structure, lattice dynamics, and the electron-phonon interaction of the ScIrP compound using a first-principles method. The band structure has a complex three-dimensional structure with a hole pocket centered at the $\mathrm{\Gamma }$ point; the inclusion of spin-orbit coupling (SOC) removes the degeneracies of some bands near the Fermi level. The SOC also has a considerable effect on the phonon property of this material; it decreases the energy of some phonon modes and increases the strength of the dominant peaks of the Eliashberg spectral function. Thus it enhances the electron-phonon coupling constant as well as the superconductivity transition temperature $T_c$. The averaged logarithmic phonon frequency is estimated to be 161.619 K, and the electron-phonon coupling constant is 0.596. Setting the Coulomb pseudopotential ${\mu }^{*}=0.10$, the obtained $T_c$ from McMillan's equation with SOC is $\sim 3.6$ K, in good agreement with the experimentally measured $T_c=3.4$ K. This result demonstrates that the superconductivity in ScIrP should originate in the phonon-mediated electron-electron interaction with $s$-wave pairing symmetry.

We perform ab initio quantum Monte Carlo simulations of para-hydrogen $\left(p{\text{-H}}_2\right)$ at $T=0$ K confined in carbon nanotubes (CNT) of different radii. The radial density profiles show a strong layering of the $p{\text{-H}}_2$ molecules which grow, with increasing number of molecules, in solid concentric cylindrical shells and eventually a central column. The central column can be considered an effective one-dimensional (1D) fluid whose properties are well captured by the Tomonaga-Luttinger liquid theory. The Luttinger parameter is explicitly computed and interestingly it shows a nonmonotonic behavior with the linear density similar to what found for pure 1D ${}^3\mathrm{He}$. Remarkably, for the central column in a (10,10) CNT, we found an ample linear density range in which the Luttinger liquid is (i) superfluid and (ii) stable against a weak disordered external potential, as the one expected inside realistic pores. This superfluid behavior could be experimentally revealed in bundles of carbon nanotubes, where deviations from classical inertial values associated with superfluid density could be measured by using quartz crystal microbalance techniques. In summary, our results suggest that $p{\text{-H}}_2$ within carbon nanotubes could be a practical and stable realization of the long sought-after, elusive superfluid phase of parahydrogen.

Memory effects have been observed across a wide range of geometrically frustrated magnetic materials, possibly including ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$ where a spontaneous Hall effect has been observed. Frustrated magnets are also famous for the emergence of topological defects. Here we explore how the interaction between these defects can be responsible for a rich diversity of out-of-equilibrium dynamics, dominated by topological bottlenecks and multiscale energy barriers. Our model is an extension of the spinice model on the pyrochlore lattice, where farther-neighbor spin interactions give rise to a nearest-neighbor coupling between topological defects. This coupling can be chosen to be “unnatural” or not, i.e., attractive or repulsive between defects carrying the same topological charge. After applying a field quench, our model supports, for example, long-lived magnetization plateaux, and allows for the metastability of a “fragmented” spin liquid, an unconventional phase of matter where long-range order co-exists with a spin liquid. Perhaps most strikingly, the attraction between same-sign charges produces clusters of these defects in equilibrium, whose stability is due to a combination of energy and topological barriers. These clusters may take the form of a “jellyfish” spin texture, centered on a hexagonal ring with branches of arbitrary length. The ring carries a clockwise or counterclockwise circular flow of magnetization. This emergent toroidal degrees of freedom provide a possibility for time-reversal symmetry breaking with potential relevance to the spontaneous Hall effect observed in ${\mathrm{Pr}}_2{\mathrm{Ir}}_2{\mathrm{O}}_7$.

Spin-transfer torques, both reactive and dissipative, induced by temperature gradients in conducting ferromagnets are calculated microscopically for smooth magnetization textures. Temperature gradients are treated à la Luttinger by introducing a fictitious gravitational field that couples to the energy density. The thermal torque coefficients obtained by the Kubo formula contain unphysical terms that diverge towards zero temperature. Such terms are caused by equilibrium components and should be subtracted before applying the Einstein-Luttinger relation. Only by following this procedure a familiar Mott-like formula is obtained for the dissipative spin-transfer torque. The result indicates that a fictitious field that couples to the entropy rather than energy would solve the issue from the outset.