The microscopic quantum interference associated with excitonic condensation in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$ is studied in a BCS-type mean-field approximation. We show that in ultrasonic attenuation the coherence peak appears just below the transition temperature $T_{\mathrm{c}}$, whereas in NMR spin-lattice relaxation the rate rapidly decreases below $T_{\mathrm{c}}$; these observations can offer a crucial experimental test for the validity of the excitonic condensation scenario in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$. We also show that excitonic condensation manifests itself in a jump of the heat capacity at $T_{\mathrm{c}}$ as well as in a softening of the elastic shear constant, in accordance with the second-order phase transition observed in ${\mathrm{Ta}}_2{\mathrm{NiSe}}_5$.

Microwave induced resistance and photovoltage oscillations were investigated in ${\mathrm{Mg}}_x{\mathrm{Zn}}_{1-x}\mathrm{O}/\text{ZnO}$ heterostructures. The physics of these oscillations is controlled significantly by scattering mechanisms, and therefore these experiments were motivated by the recently achieved high quality levels in this material and the apparent dominance of large angle, short-range scattering, which is distinct from the prevailing small angle scattering in state-of-the-art GaAs structures. Within the studied frequency range of 35–120 GHz, up to four oscillations were resolved at 1.4 K temperature, but only in high density samples. This allowed us to extract the value of the effective electron mass $m^{*}=(0.35\pm 0.01)m_0$, which is enhanced over the bare band mass, and estimate a local quantum scattering time of about 5 ps.

We present systematic measurements of the quality factors of surface acoustic wave (SAW) resonators on ST-X quartz in the gigahertz range at a temperature of $10\mathrm{mK}$. We demonstrate an internal quality factor $Q_{\mathrm{i}}$ approaching $0.5$ million at $0.5\text{GHz}$ and show that $Q_{\mathrm{i}}\ge 4.0\times {10}^4$ is achievable up to $4.4\text{GHz}$. We show evidence for a polynomial dependence of propagation loss on frequency, as well as a weak drive power dependence of $Q_{\mathrm{i}}$ that saturates at low power, the latter being consistent with coupling to a bath of two-level systems. Our results indicate that SAW resonators are promising devices for integration with superconducting quantum circuits.

A class of graphene wound into three-dimensional periodic curved surfaces (“graphitic zeolites”) is proposed and their electronic structures are obtained to explore how the massless Dirac fermions behave on periodic surfaces. We find in the tight-binding model that the low-energy band structure around the charge neutrality point is dominated by the topology (cubic or gyroid) of the periodic surface as well as by the spatial period $L$ in modulo 3 in units of the lattice constant. In both cubic and gyroid cases the Dirac electrons become massive around the charge neutrality point, where the band gap is shown to scale as $1/L$ within each mod-3 class. Wave functions around the gap are found to have amplitudes sharply peaked around the topological defects that are required to deform the graphene sheet into a three-dimensional periodic surface, and this is shown to originate from nontrivial Bloch phases at $K$ and $K^{\prime }$ points of the original graphene.

Restructuring of the electronic spectrum in a buckled silicene monolayer under some applied voltage between its two sublattices and in the presence of certain impurity atoms is considered. Special attention is given to formation of localized impurity levels within the band gap and to their collectivization at finite impurity concentration. It is shown that a qualitative restructuring of the quasiparticle spectrum within the initial band gap and then specific metal-insulator phase transitions are possible for such disordered system and can be effectively controlled by variation of the electric field bias at a given impurity perturbation potential and concentration. Since these effects are expected at low impurity concentrations but at not too low temperatures, they can be promising for practical applications in nanoelectronic devices.

The coherent dynamics of the exciton center-of-mass motion in bulk GaN are studied using degenerate four-wave-mixing (FWM) spectroscopy with Laguerre-Gaussian (LG) mode pulses. We evaluate the exciton orbital angular momentum (OAM) dynamics from the degree of OAM, which is derived from the distributions of OAM (topological charge) of the FWM signals. When excitons are excited with two single-mode LG pulses, the exciton OAM decay time significantly exceeds the exciton dephasing time, which can be attributed to high uniformity of the exciton dephasing in our bulk sample because the decoherence of the exciton OAM is governed by the angular variation in the exciton dephasing. We also analyze the topological charge $\left(\ell \right)$ dependence of the OAM decay using a multiple-mode LG pump pulse, which allows us to simultaneously observe the dynamics of the exciton OAM for different $\ell$ values under the same excitation conditions. The OAM decay times of the $\ell =1$ component are usually longer than those of the $\ell =0$ component. The $\ell \text{-dependent}$ OAM decay is supported by a phenomenological model which takes into account the local nonuniformity of the exciton dephasing.

We study by grazing incidence x-ray diffraction the strain field induced by periodic double steps on a Si(1 1 15) surface that is a vicinal of a Si(001) surface misoriented by $5.4^{\circ }$ towards the $\langle 110\rangle$ direction. The best fit of the experimental structure factors is reached on the basis of the rebonded $D_B$ step edge model and the displacement field is well characterized assuming that steps are described by parallel rows of extended buried elastic dipoles. The dipole characteristics are the dipole position with respect to the step edge, the dipole amplitude ($2.0\pm 0.5$ nN), and the lever arm $\mathrm{\Omega }=5.3^{\circ }$ and force $\mathrm{\Phi }=3.7^{\circ }$ orientations. We show that the dipole is dominated by a large stretch component localized between the lower and the upper corners of the step, which we assign to the presence of the rebonded atom at the step.

We have investigated nonequilibrium properties of proper uniaxial ${\mathrm{Sn}}_2{\mathrm{P}}_2{\left({\mathrm{Se}}_x{\mathrm{S}}_{1-x}\right)}_6$ ferroelectrics with the type II incommensurate phase above Lifshitz point $x_{\mathrm{LP}}\sim 0.28$. We performed measurements of dielectric susceptibility in cooling and heating regimes with the rate ranging 0.002–0.1 K/min, as well as high-resolution ultrasound investigation and hypersound Brillouin scattering experiments. For samples with $x\ge 0.28$ clear anomalies are observed at incommensurate second-order transition $\left(T_i\right)$ and at first-order lock-in transition $\left(T_c\right)$ in the regime of very slow cooling rate, whereas the intermediate incommensurate phase is not observed when the rate is faster than 0.1 K/min. In general, increasing the cooling rate leads to smearing of the anomaly at $T_c$. We relate this effect to cooling rate dependence of domain-wall concentration and their size: domain width decreases when cooling rate increases. At certain conditions, the size of domain is comparable to the incommensurate phase modulation period, which is in the micrometer range in the vicinity of Lifshitz point and leads to pinning of the modulation period by domain walls.

Spin Hall angle, which denotes the conversion efficiency between spin and charge current, is a key parameter in the pure spin current phenomenon. The search for materials with large spin Hall angle is indeed important for scientific interest and potential application in spintronics. Here the large enhanced spin Hall effect (SHE) of Au-Cu alloy is reported by investigating the spin Seebeck effect, spin Hall anomalous Hall effect, and spin Hall magnetoresistance of the ${\mathrm{Y}}_3\mathrm{F}{\mathrm{e}}_5{\mathrm{O}}_{12}$ (YIG)/$\mathrm{A}{\mathrm{u}}_x\mathrm{C}{\mathrm{u}}_{1-x}$ hybrid structure over the full composition. At the near equiatomic Au-Cu composition with maximum atomic disorder scattering, the spin Hall angle of the Au-Cu alloy increases by two to three times together with a moderate spin diffusion length in comparison with Au. The longitudinal spin Seebeck voltage and the spin Hall magnetoresistance ratio also increase by two to three times. More importantly, no evidence of anomalous Hall effect is observed in all YIG/Au-Cu samples, in contrast to the cases of other giant SHE materials Pt(Pd), Ta, and W. This behavior makes Au-Cu free from any suspicion of the magnetic proximity effect involved in the hybrid structure, and thus the Au-Cu alloy can be an ideal material for pure spin current study.

Spin-torque oscillators are strong candidates as nanoscale microwave generators and detectors. However, because of large amplitude-phase coupling (nonlinearity), phase noise is enhanced over other linear autooscillators. One way to reduce nonlinearity is to use ferromagnetic layers as a resonator and excite them at localized spots, making a resonator-excitor pair. We investigated the excitation of oscillations in dipole-coupled ferromagnetic layers, driven by localized current at ferromagnetic nanocontacts. Oscillations possessed properties of optical-mode spin waves and at low field ($\approx 200$ Oe) had high frequency (15 GHz), a moderate precession amplitude $(2^{\circ }–3^{\circ })$, and a narrow spectral linewidth ($<3$ MHz) due to localized excitation at nanocontacts. Micromagnetic simulation showed emission of the resonator's characteristic optical-mode spin waves from disturbances generated by domain-wall oscillations at nanocontacts.

A scaled quantum computer with donor spins in silicon would benefit from a viable semiconductor framework and a strong inherent decoupling of the qubits from the noisy environment. Coupling neighboring spins via the natural exchange interaction according to current designs requires gate control structures with extremely small length scales. We present a silicon architecture where bismuth donors with long coherence times are coupled to electrons that can shuttle between adjacent quantum dots, thus relaxing the pitch requirements and allowing space between donors for classical control devices. An adiabatic SWAP operation within each donor/dot pair solves the scalability issues intrinsic to exchange-based two-qubit gates, as it does not rely on subnanometer precision in donor placement and is robust against noise in the control fields. We use this SWAP together with well established global microwave Rabi pulses and parallel electron shuttling to construct a surface code that needs minimal, feasible local control.

In this paper we present an experiment where we measured the quantum coherence of a quasiparticle injected at a well-defined energy above the Fermi sea into the edge states of the integer quantum Hall regime. Electrons are introduced in an electronic Mach-Zehnder interferometer after passing through a quantum dot that plays the role of an energy filter. Measurements show that above a threshold injection energy, the visibility of the quantum interferences is almost independent of the energy. This is true even for high energies, up to $130 \mu \mathrm{eV}$, well above the thermal energy of the measured sample. This result is in contradiction with our theoretical predictions, which instead predict a continuous decrease of the interference visibility with increasing energy.

We address the recent experiment (S. Tewari et al., Phys. Rev. B 93, 035420 (2016)) where the suppression of phase coherence of a single-electron wave packet created at the edge of a quantum Hall (QH) system at filling factor 2 has been investigated with the help of an electronic Mach-Zehnder (MZ) interferometer. The authors of the experiment have observed an unexpected behavior of phase coherence that saturates at high energies instead of vanishing, presumably suggesting the relaxation of a wave packet to the ground state before it arrives to the MZ interferometer. Here, we theoretically investigate this situation using the model of edge states [I. P. Levkivskyi and E. V. Sukhorukov, Phys. Rev. B 78, 045322 (2008)], which accounts for the strong Coulomb interaction between the two electron channels at the edge of a QH system. We conclude that the observed phenomenon cannot be explained within this model for the reason that under an assumption of linearity of the electron spectrum at low energies, the system remains integrable in terms of the collective charge excitations and therefore full relaxation to the ground state is not possible, despite strong interactions. As a result, the degree of the phase coherence decreases with the energy of the initial state in a power-law manner. Since this does not happen in the experiment, a different physical phenomenon may take place at the edge of a QH state, which deserves further investigation. We support our findings by calculating the energy distribution and the Wigner function of the outgoing nonequilibrium state of the single-electron wave packet.

We use quantum optimal control theory algorithms to design external electric fields that drive the coupled spin and orbital dynamics of an electron in a double quantum dot, subject to the spin-orbit coupling and Zeeman magnetic fields. We obtain time profiles of multifrequency electric field pulses which increase the rate of spin-flip transitions by several orders of magnitude in comparison with monochromatic fields, where the spin Rabi oscillations were predicted to be very slow. This precise (with fidelity higher than $1\times {10}^{-4}$) and fast (at the time scale of the order of 0.1 ns, comparable with the Zeeman spin rotation and the interdot tunneling time) simultaneous control of the spin and position is achieved while keeping the electron in the four lowest tunneling- and Zeeman-split levels through the duration of the pulse. The proposed algorithms suggest effective applications in spintronics and quantum information devices.

We present an experimental and theoretical study on how thermal energy alters van der Waals adhesion forces in nanoscale contacts stretched by mechanical probes. The force follows a distribution whose density function is an asymmetric bell-shaped curve presenting a temperature-dependent negative skewness. With increasing temperature the asymmetry increases whereas the most probable force value decreases. Using a 2–8 Lennard-Jones interaction potential within the reaction rate theory, we offer a theoretical framework permitting an evaluation of the microscopic parameters governing adhesion in a van der Waals nanocontact subjected to mechanical fluctuations.

An infinite projected entangled-pair state (iPEPS) is a variational tensor network ansatz for two-dimensional wave functions in the thermodynamic limit where the accuracy can be systematically controlled by the bond dimension $D$. We show that for the doped Hubbard model in the strongly correlated regime $\left(U/t=8,n=0.875\right)$ the iPEPS yields lower variational energies than state-of-the-art variational methods in the large two-dimensional limit, which demonstrates the competitiveness of the method. In order to obtain an accurate estimate of the energy in the exact infinite-$D$ limit we introduce and test an extrapolation technique based on a truncation error computed in the iPEPS imaginary time evolution algorithm. The extrapolated energies are compared with accurate quantum Monte Carlo results at half-filling and with various other methods in the doped, strongly correlated regime.

Longitudinal charged carrier spin dynamics is studied for ZnO quantum wells and epilayers using the optical transition of the negatively charged exciton ${\mathrm{X}}^{-}$ and the neutral donor bound exciton ${\mathrm{D}}^0\mathrm{X}$, respectively. The hole spin relaxation is derived from the optical orientation of ${\mathrm{X}}^{-}$ and ${\mathrm{D}}^0\mathrm{X}$ photoluminescence, whereas the spin relaxation of the resident electrons and donor electrons is accessed via the bleaching of the spin selective excitation process. Hole spin relaxation times of ${\tau }_1^{s,h}$ of 80 and 140 ps are found for ${\mathrm{D}}^0\mathrm{X}$ and ${\mathrm{X}}^{-}$, respectively, which are practically independent of a magnetic field $B_{\parallel }$ applied along the ZnO $\vec{c}$ axis. Much longer longitudinal electron spin relaxation times in the $1 \mu \mathrm{s}$ range are uncovered if the hyperfine interaction is suppressed by a proper $B_{\parallel }$. A field strength of $\approx 2$ mT is large enough proving the extremely small value of the Overhauser field in ZnO. This is related to the very restricted number of magnetic nuclei interacting with the electron inside the volume of the exciton complex.