Surface acoustic waves (SAWs) traveling on the surface of a piezoelectric crystal can, through the magnetoelastic interaction, excite traveling spin-wave resonance in a magnetic film deposited on the substrate. This spin-wave resonance in the magnetic film creates a time-dynamic surface stress of magnetoelastic origin that acts back on the surface of the piezoelectric and modifies the SAW propagation. Unlike previous analyses that treat the excitation as a magnon-phonon polariton, here the magnetoelastic film is treated as a perturbation modifying boundary conditions on the SAW. We use acoustical perturbation theory to find closed-form expressions for the back-action surface stress and strain fields and the resultant SAW velocity shifts and attenuation. We demonstrate that the shear stress fields associated with this spin-wave back-action also generate effective surface currents on the piezoelectric both in phase and out of phase with the driving SAW potential. Characterization of these surface currents and their applications in determination of the magnetoelastic coupling are discussed. The perturbative calculation is carried out explicitly to first order (a regime corresponding to many experimental situations of current interest) and we provide a sketch of the implications of the theory at higher order.

Performing ab initio calculations, we reveal that the magnetic anisotropy (MA) and the spin direction of a single adatom can be manipulated with a combination of electric fields and adsorption of molecules. Choosing the Fe adatom on the ${\mathrm{Cu}}_2\mathrm{N}\text{/}\mathrm{Cu}\left(001\right)$ surface as a typical model system, we show that the MA of the pristine Fe adatom and the Fe adatom with an additional H or F atom adsorption remarkably changes by applying an external electric field. Moreover, we show that the F adsorption leads to the spin-reorientation transition of the Fe adatom from in plane to out of plane. Controlling the magnetization dynamics of a single magnetic adatom by molecule adsorption is demonstrated.

We report time-resolved observations of field-induced domain wall nucleation in asymmetric ferromagnetic rings using single direction field pulses and rotating fields. We show that the asymmetric geometry of a ring allows for controlling the position of nucleation events, when a domain wall is nucleated by a rotating magnetic field. Direct observation by scanning transmission x-ray microscopy (STXM) reveals that the nucleation of domain walls occurs through the creation of transient ripplelike structures. This magnetization state is found to exhibit a surprisingly high reproducibility even at room temperature and we determine the combinations of field strengths and field directions that allow for reliable nucleation of domain walls and directly quantify the stability of the magnetic states. Our analysis of the processes occurring during field induced domain wall nucleation shows how the effective fields determine the nucleation location reproducibly, which is a key prerequisite toward using domain walls for spintronic devices.

We report the low temperature magnetic properties of the pyrochlore ${\mathrm{Pr}}_2{\mathrm{Hf}}_2{\mathrm{O}}_7$. Polycrystalline and single-crystal samples are investigated using time-of-flight neutron spectroscopy and macroscopic measurements, respectively. The crystal-field splitting produces a non-Kramers doublet ground state for ${\mathrm{Pr}}^{3+}$, with Ising-like anisotropy. Below 0.5 K ferromagnetic correlations develop, which suggests that the system enters a spin-ice-like state associated with the metamagnetic behavior observed at ${\mu }_0{H}_c\sim 2.4$ T. In this regime, the development of a discrete inelastic excitation in the neutron spectra indicates the appearance of spin dynamics that are likely related to cooperative quantum fluctuations.

We investigate the electronic properties of the tetragonal FeS superconductor by using scanning tunneling microscopy/spectroscopy. It is found that the typical tunneling spectrum on the top layer of sulfur can be nicely fitted with an anisotropic $s$ wave or a combination of two superconducting components in which one may have a highly anisotropic or nodal-like superconducting gap. The fittings lead to the superconducting gap of about ${\mathrm{\Delta }}_{\max }\approx 0.90\mathrm{meV}$, which yields a ratio of $2{\mathrm{\Delta }}_{\max }/k_B{T}_c\approx$ 4.65. This value is larger than that of the predicted value 3.53 by the BCS theory in the weak-coupling limit, indicating a strong-coupling superconductivity. Two kinds of defects are observed on the surface, which can be assigned to the defects on the S sites (fourfold image) and Fe sites (dumbbell shape). Impurity-induced resonance states are found only for the defects on the S sites and stay at zero-bias energy.

We study the ground-state phase diagram of the quantum $\text{spin-}1/2$ Heisenberg model on the kagome lattice with first- $(J_1<0)$, second- $(J_2<0)$, and third-neighbor interactions $(J_d>0)$ by means of analytical low-energy field theory and numerical density-matrix renormalization group (DMRG) studies. The results offer a consistent picture of the $J_d\text{-dominant}$ regime in terms of three sets of spin chains weakly coupled by the ferromagnetic interchain interactions $J_{1,2}$. When either $J_1$ or $J_2$ is much stronger than the other one, the model is found to support one of two cuboctohedral phases, cuboc1, and cuboc2. These cuboc states host noncoplanar long-ranged magnetic order and possess finite scalar spin chirality. However, in the compensated regime $J_1\simeq J_2$, a valence bond crystal phase emerges between the two cuboc phases. We find excellent agreement between an analytical theory based on coupled spin chains and unbiased DMRG calculations, including at a very detailed level of comparison of the structure of the valence bond crystal state. To our knowledge, this is the first such comprehensive understanding of a highly frustrated two-dimensional quantum antiferromagnet. We find no evidence of either the one-dimensional gapless spin liquid or the chiral spin liquids, which were previously suggested by parton mean-field theories.

The magnetic and electrical-transport properties of $\mathrm{CuFe}{\mathrm{O}}_2$ have been studied by temperature-dependent ${}^{57}\mathrm{Fe}$ Mössbauer spectroscopy and resistance measurements to near megabar (∼100 GPa) pressures. Previous studies show that $\mathrm{CuFe}{\mathrm{O}}_2$ comprises the following sublattices at $P>23\mathrm{GPa}:\frac{1}{3}\left[{\mathrm{Cu}}_{(S=1/2)}^{2+}{\mathrm{Fe}}_{(S=2)}^{2+}{\mathrm{O}}_2\right]+\frac{2}{3}\left[{\mathrm{Cu}}_{(S=0)}^{1+}{\mathrm{Fe}}_{(S=5/2)}^{3+}{\mathrm{O}}_2\right]$. The magnetic ordering temperatures of the sublattices, with both Fe valences in the high-spin state, reach a maximum at $\sim 50\mathrm{GPa}$. At higher pressures two different magnetic components with collapsed magnetic hyperfine fields of 0 and $\sim 10\mathrm{T}$ have been detected by the Mössbauer probe. The pressure evolution and temperature dependence of the associated hyperfine interaction parameters to ∼95 GPa identifies these as low-spin ${\mathrm{Fe}}_{(S=0)}^{2+}$ and ${\mathrm{Fe}}_{(S=1/2)}^{3+}$ states in the very-high-pressure phase of $\mathrm{CuFe}{\mathrm{O}}_2$. The $\mathrm{F}{\mathrm{e}}^{3+}$ low-spin state exhibits an unusually high onset temperature for magnetic ordering, $T_M\sim 50\mathrm{K}$. Resistance-temperature dependences show $\mathrm{CuFe}{\mathrm{O}}_2$ in the low-spin state at very high pressure to be a narrow-gap semiconductor with variable range hopping of charge carriers. The derived Mott temperature from this conductivity mechanism, $T_0$, undergoes an appreciable decrease from $\sim 50\mathrm{GPa}$ onwards where the spin-crossover regions nucleate and grow with increasing abundance in the lattice. An extrapolated $T_0$ value of $\sim {10}^5\mathrm{K}$ at 100 GPa is interpreted as persistent charge carrier confinement in the low-spin phase. At these high densities a mixed-valence low-spin magnetic semiconducting ground state has been stabilized, in which strong electron correlations are persistent.

Two prevailing formalisms are currently used to model charge transport in organic semiconductor devices. Drift-diffusion calculations, on the one hand, are time effective but assume local thermodynamic equilibrium, which is not always realistic. Kinetic Monte Carlo models, on the other hand, do not require this assumption but are computationally expensive. Here, we present a nonequilibrium drift-diffusion model that bridges this gap by fusing the established multiple trap and release formalism with the drift-diffusion transport equation. For a prototypical photovoltaic system the model is shown to quantitatively describe, with a single set of parameters, experiments probing (1) temperature-dependent steady-state charge transport—space-charge limited currents, and (2) time-resolved charge transport and relaxation of nonequilibrated photocreated charges. Moreover, the outputs of the developed kinetic drift-diffusion model are an order of magnitude, or more, faster to compute and in good agreement with kinetic Monte Carlo calculations.

Carrier dynamics in monolayer $\mathrm{Mo}{\mathrm{S}}_2$ have been investigated using broadband femtosecond transient absorption spectroscopy (FTAS). A tunable pump pulse was used while a broadband probe pulse revealed ground and excited state carrier dynamics. Interestingly, for pump wavelengths both resonant and nonresonant with the A and B excitons, we observe a broad ground state bleach around 2.9 eV, with decay components similar to A and B. Associating this bleach with the band nesting region between $K$ and Γ in the band structure indicates significant k-space delocalization and overlap among excitonic wave functions identified as A, B, C, and D. Comparison of time dynamics for all features in resonance and nonresonance excitation is consistent with this finding.

We report the hydrostatic pressure-induced electronic and magnetic phase transitions in a Mott insulator, a bilayer ruthenate $\mathrm{C}{\mathrm{a}}_3{\left(\mathrm{R}{\mathrm{u}}_{0.97}\mathrm{T}{\mathrm{i}}_{0.03}\right)}_2{\mathrm{O}}_7$, via electronic transport and single crystal neutron diffraction measurements. The system undergoes an insulator-metal transition at a very small hydrostatic pressure ≈0.04 GPa, followed by a magnetic phase transition around 0.3 GPa, suggesting that the low energy charge fluctuation and magnetic ordering couple to the pressure separately in this compound. The $ab$ initio calculations show that the suppressed $\mathrm{Ru}{\mathrm{O}}_6$ flattening induced by the pressure reduces the orbital polarization and gives rise to an insulator-metal transition preceding the magnetic phase transition.

We investigate the phase diagram of a quantum spin-1 chain whose Hamiltonian is invariant under a global on-site $A_4$, translation, and lattice inversion symmetries. We detect different gapped phases characterized by a symmetry protected topological (SPT) order and symmetry breaking using matrix product state order parameters. We observe a rich variety of phases of matter characterized by a combination of symmetry breaking and symmetry fractionalization and also the interplay between the on-site and spatial symmetries. Examples of continuous phase transitions directly between topologically nontrivial SPT phases are also observed.

The quantum spin Hall effect has been predicted theoretically and observed experimentally in InAs/GaSb quantum wells as a result of inverted band structures, for which electron bands in InAs layers are below heavy-hole bands in GaSb layers in energy. The hybridization between electron bands and heavy-hole bands leads to a hybridization gap away from $k=0$. A recent puzzling observation in experiments is that when the system is tuned to more inverted regime by a gate voltage (a larger inverted gap at $k=0)$, the hybridization gap decreases. Motivated by this experiment, we explore the dependence of the hybridization gap as a function of external electric fields based on the eight-band Kane model. We identify two regimes when varying the electric fields: (1) Both inverted and hybridization gaps increase and (2) the inverted gap increases while the hybridization gap decreases. Based on the effective model, we find that light-hole bands in GaSb layers play an important role in determining the hybridization gap. In addition, a large external electric field can induce a strong Rashba splitting and also influence the hybridization gap.

We theoretically consider the temporal dynamics of two coupled spin qubits (e.g., semiconductor quantum dots) driven by the interqubit spin-spin coupling. The presence of environmental noise (e.g., charge traps, nuclear spins, random magnetic impurities) is accounted for by including random magnetic field and random interqubit coupling terms in the Hamiltonian. Both Heisenberg coupling and Ising coupling between the spin qubits are considered, corresponding respectively to exchange and capacitive gates as appropriate for single spin and singlet-triplet semiconductor qubit systems, respectively. Both exchange (Heisenberg) and capacitive (Ising) coupling situations can be solved numerically exactly even in the presence of noise, leading to the key findings that (i) the steady-state return probability to the initial state remains close to unity in the presence of strong noise for many, but not all, starting spin configurations, and (ii) the return probability as a function of time is oscillatory with a characteristic noise-controlled decay toward the steady-state value. We also provide results for the magnetization dynamics of the coupled two-qubit system. Our predicted dynamics can be directly tested in the already existing semiconductor spin qubit setups providing insight into their coherent interaction dynamics. Retention of the initial state spin memory even in the presence of strong environmental noise has important implications for quantum computation using spin qubits.

An external electric field changes the physical properties of polar liquids due to the reorientation of their permanent dipoles. Using molecular dynamics simulations, we predict that an in-plane electric field applied parallel to the channel polarizes water molecules which are confined between two graphene layers, resulting in distinct ferroelectricity and electrical hysteresis. We found that electric fields alter the in-plane order of the hydrogen bonds: Reversing the electric field does not restore the system to the nonpolar initial state, instead a residual dipole moment remains in the system. The square-rhombic structure of 2D ice is transformed into two rhombic-rhombic structures. Our study provides insights into the ferroelectric state of water when confined in nanochannels and shows how this can be tuned by an electric field.

The time evolution of the magnetization normal component change in a garnet film with a labyrinthine domain structure under the action of circularly and linearly polarized laser pump pulses (the pulse duration is 5 ns; the wavelength is 527 nm) has been studied. The dynamic state of the magnetic film was registered using an induction method with a time resolution of 1 ns. It was found that for the initial state of the magnetic film with an equilibrium domain structure, the form of the photomagnetization pulse reflects the time evolution of a domain-wall motion. The domain-wall motion initiated by the circularly polarized laser pump pulse continues in the same direction for a time more than an order of magnitude exceeding the laser pulse duration. In general, the time evolution of the domain-wall movement occurs in three stages. The separation of the contributions to the photomagnetization from the polarization-dependent and polarization-independent effects was carried out. The photomagnetization pulses that reflect the contributions by the aforementioned effects differ by form, and more than two orders of magnitude by duration. Their form doesn't change under a magnetic bias field change, only the photomagnetization pulse amplitude does: for the polarization-dependent contribution, it's an even function of the field, and for the polarization-independent contribution, it's an odd function. The interconnection between the polarization-dependent and polarization-independent effects, on the one hand, and the domain-wall displacement and the change of the film's saturation magnetization, on the other hand, was identified and described.

We report on the electrical manipulation of coherent acoustic phonon waves in GaN-based nanoscale piezoelectric heterostructures which are strained both from the pseudomorphic growth at the interfaces as well as through external electric fields. In such structures, transverse symmetry within the $c$ plane hinders both the generation and detection of the transverse acoustic (TA) modes, and usually only longitudinal acoustic phonons are generated by ultrafast displacive screening of potential gradients. We show that even for $c$-GaN, the combined application of lateral and vertical electric fields can not only switch on the normally forbidden TA mode, but they can also modulate the amplitudes and frequencies of both modes. By comparing the transient differential reflectivity spectra in structures with and without an asymmetric potential distribution, the role of the electrical controllability of phonons was demonstrated as changes to the propagation velocities, the optical birefringence, the electrically polarized TA waves, and the geometrically varying optical sensitivities of phonons.

Chiral $p$-wave superconductors in applied magnetic field can exhibit more complex topological defects than just conventional superconducting vortices, due to the two-component order parameter (OP) and the broken time-reversal symmetry. We investigate the electronic properties of those exotic states, some of which contain clusters of one-component vortices in chiral components of the OP and/or exhibit skyrmionic character in the relative OP space, all obtained as a self-consistent solution of the microscopic Bogoliubov–de Gennes equations. We reveal the link between the local density of states (LDOS) of the novel topological states and the behavior of the chiral domain wall between the OP components, enabling direct identification of those states in scanning tunneling microscopy. For example, a skyrmion always contains a closed chiral domain wall, which is found to be mapped exactly by zero-bias peaks in LDOS. Moreover, the LDOS exhibits electron-hole asymmetry, which is different from the LDOS of conventional vortex states with same vorticity. Finally, we present the magnetic field and temperature dependence of the properties of a skyrmion, indicating that this topological defect can be surprisingly large in size, and can be pinned by an artificially indented nonsuperconducting closed path in the sample. These features are expected to facilitate the experimental observation of skyrmionic states, thereby enabling experimental verification of chirality in emerging superconducting materials.

We investigate the transport and single particle relaxation times of silicene in the presence of neutral and charged impurities. The static charge conductivity is studied using the semiclassical Boltzmann formalism when the spin-orbit interaction is taken into account. The screening is modeled within Thomas-Fermi and random-phase approximations. We show that the transport relaxation time is always longer than the single particle one. Easy electrical controllability of both carrier density and band gap in this buckled two-dimensional structure makes it a suitable candidate for several electronic and optoelectronic applications. In particular, we observe that the dc charge conductivity could be easily controlled through an external electric field, a very promising feature for applications as electrical switches and transistors. Our findings would be qualitatively valid for other buckled honeycomb lattices of the same family, such as germanine and stanine.

We present results of our research on two-field (two-frequency) microwave spectroscopy in nitrogen-vacancy ($\mathrm{N}{\mathrm{V}}^{-}$) color centers in a diamond. Both fields are tuned to transitions between the spin sublevels of the $\mathrm{N}{\mathrm{V}}^{-}$ ensemble in the ${}^3{A}_2$ ground state (one field has a fixed frequency while the second one is scanned). Particular attention is focused on the case where two microwave fields drive the same transition between two $\mathrm{N}{\mathrm{V}}^{-}$ ground state sublevels ($m_{\mathrm{s}}=0\leftrightarrow m_{\mathrm{s}}=+1$). In this case, the observed spectra exhibit a complex narrow structure composed of three Lorentzian resonances positioned at the pump-field frequency. The resonance widths and amplitudes depend on the lifetimes of the levels involved in the transition. We attribute the spectra to coherent population oscillations induced by the two nearly degenerate microwave fields, which we have also observed in real time. The observations agree well with a theoretical model and can be useful for investigation of the NV relaxation mechanisms.

Electronic transport through a two-path triple-quantum-dot system with two source leads and one drain is studied and the interaction between the two paths is analyzed. We observe a channel blockade as a result of interchannel Coulombic interaction. The experimental results are understood with the help of a theoretical model which allows one to obtain the parameters of the system, the stability regions of each state, and the full dynamical transport in the triple-dot resonances.