This work investigates nanosecond superparamagnetic switching in 50-nm-diameter in-plane magnetized magnetic tunnel junctions (MTJs). Due to the small in-plane uniaxial anisotropy, dwell times below $10\text{ns}$ and autocorrelation times down to $5\text{ns}$ are measured for circular superparamagnetic tunnel junctions (SMTJs). SMTJs exhibit probabilistic switching of the magnetic free layer, which can be used for the generation of true random numbers. The quality of the random bit streams generated by our SMTJ is evaluated with a statistical test suite (NIST STS, sp 800-22) and shows true randomness after three exclusive or (xor) operations of four random SMTJ bit streams. A low-footprint CMOS circuit is proposed for fast and energy-efficient random-number generation. We demonstrate that the probability of a 1 or 0 can be tuned by spin-transfer torque (STT), while the average bit-generation rate is mainly affected by the current density via Joule heating. Although both effects are always present in MTJs, most often Joule heating is neglected. However, with a resistance-area (RA) product of $15\mathrm{\Omega }\text{μ}{\mathrm{m}}^2$ and current densities of the order of $1{\text{MA/cm}}^2$, an increasing temperature at the tunneling site results in a significant increase in the switching rate. As Joule heating and STT scale differently with current density, the device design can be optimized based on our findings.

We present the concept and the results of an investigation of an all-optical vector-magnetic-field sensor scheme developed for biological applications such as nonzero-field magnetoencephalography and magnetocardiography. The scheme differs from the classical two-beam Bell-Bloom scheme in that the detecting laser beam is split into two beams, which are introduced into the cell in orthogonal directions, and the ratio of the amplitudes of the magnetic resonance signals in these beams and their phase difference are measured; strong optical pumping from the lower hyperfine level of the ground state ensures the resonance line narrowing, and detection in two beams is carried out in balanced schemes by measuring the beam-polarization rotation. The proposed sensor is compact, resistant to variations of parameters of laser radiation and highly sensitive to the angle of deflection of the magnetic field vector—with an estimated scalar sensitivity of the order of $16{\mathrm{fT}/\mathrm{Hz}}^{1/2}$ in the $8\times 8\times 8{\mathrm{mm}}^3$ cell, an angular sensitivity of $4\times {10}^{-7}$ rad, or 0.08”, was demonstrated.

Single-shot all-optical helicity-independent switching is established in $\mathrm{Co}/\mathrm{Ho}$ multilayers extending the number of material combinations showing this fascinating property. This ability is studied in wedge-shaped $\mathrm{Co}/\mathrm{Ho}$ multilayers by varying thicknesses and repetition numbers. Surprisingly, even though the spin-orbit coupling is larger than in $\mathrm{Tb}$ and $\mathrm{Dy}$, which should increase the dissipation of angular momentum to the lattice, the pulse duration versus fluence state diagram is close to the one of the $\mathrm{Gd}$-based system.

Laser cooling of a semiconductor material, in which heat is extracted by the emission of photons, requires near-perfect external radiative efficiency. In this theoretical work, we propose a cooling system based on carrier extraction in a large-band-gap reservoir. The electron-hole pairs generated in the material to be cooled are extracted in such a reservoir by absorption of phonons, then carrying a heat flux. With an analytical detailed-balance model, we show that this concept is applicable even in materials with moderate external radiative efficiency. Moreover, by adjustment of the band gap of the reservoir to the laser power, this system can either reach high efficiency or transfer high power with lower efficiency.

Topological insulators are promising candidates for dissipationless electronics and spintronics. For the design and application of small-sized topological transistors, it is vital to suppress the off-state leakage and keep utilizing the edge or surface states to carry the on-state current. Using the nonequilibrium Green’s function method, the transport properties of clean and disordered silicene were studied in the nanoscale regime. The results revealed the following. (1) The low-energy electron transport across the scattering region included two types of paths, namely, helical edge states and interstate tunnels. The choice of electrons for the two transmission paths was related to the length of the scattering region. When there was a band gap in the $k_y$ direction, electrons tended to tunnel between armchair-edge states along the $x$ axis. It was only when the length met $N_x=3n+1$ that the electrons mainly propagated through the helical edge states. (2) The weak electric field could significantly enhance the wave-function overlap between armchair-edge states and can be used to switch electron-transport paths. (3) The origin of the leakage current in the nanoscale transistors was interstate quantum tunneling; this was promoted—instead of suppressed—by weak or strong disorders. (4) The effect of a strong electric field on the electron transport was opposite to that of the weak field. After reaching a critical value of ${\lambda }_E>2{\lambda }_{\mathrm{SO}}$, the vertical electric field decreased the interstate tunneling probability and increasing the staggered potential was an effective method to suppress the off-state leakage.

Superconducting radio-frequency (SRF) cavities currently rely on niobium (Nb), and could benefit from a higher-$T_c$ surface, which would enable a higher operating temperature, lower surface resistance, and higher maximum fields. Surface zirconium (Zr) doping is one option for improvement, which has not previously been explored, likely because bulk alloy experiments showed only mild $T_c$ enhancements of 1–2 K relative to Nb. Our ab initio results reveal a more nuanced picture: an ideal bcc Nb-Zr alloy would have $T_c$ over twice that of niobium, but displacements of atoms away from the high-symmetry bcc positions due to the Jahn-Teller-Peierls effect almost completely eliminates this enhancement in typical disordered alloy structures. Ordered Nb-Zr alloy structures, in contrast, are able to avoid these atomic displacements and achieve higher calculated $T_c$ up to a theoretical limit of 17.7 K. Encouraged by this, we tested two deposition methods: a physical-vapor Zr deposition method, which produced Nb-Zr surfaces with $T_c$ values of 13.5 K, and an electrochemical deposition method, which produced surfaces with a possible 16-K $T_c$. An rf test of the highest-$T_c$ surface showed a mild reduction in BCS surface resistance relative to Nb, demonstrating the potential value of this material for RF devices. Finally, our Ginzburg-Landau theory calculations show that realistic surface doping profiles should be able to reach the maximum rf fields necessary for next-generation applications, such as the ground-breaking LCLS-II accelerator. Considering the advantages of Nb-Zr compared to other candidate materials such as ${\mathrm{Nb}}_3\mathrm{Sn}$ and Nb-Ti-N, including a simple phase diagram with relatively little sensitivity to composition, and a stable, insulating ${\mathrm{Zr}\mathrm{O}}_2$ native oxide, we conclude that Nb-Zr alloy is an excellent candidate for next-generation, high-quality-factor superconducting rf devices.

A piezotronic field-effect transistor device based on a topological insulator (TI) is proposed based on wurtzite/zincblende $\mathrm{InAs}$ quantum wells (QWs). The subthreshold swing can reach 5 mV/decade. The subthreshold swing of FETs requires at least a 60 mV gate voltage due to the thermal voltage. Polarization in these QWs can drive a large-gap TI with a bulk band gap of approximately 50 meV. The TI maintains a large band gap characteristic under a compressive stress of 7 GPa.

Many quantum sensing suggestions rely on temporal correlations found in photon pairs generated by parametric down-conversion. In this work, we show that the temporal correlations in light with a thermal photon statistics can be equally useful for such applications. Using a subthreshold laser diode as an ultrabright source of thermal light, we demonstrate optical range finding to a distance of up to 1.8 km.

Resonator measurements are a simple but powerful tool to characterize a material’s microwave response. The losses of a resonant mode are quantified by its internal quality factor $Q_{\mathrm{i}}$, which can be extracted from the scattering coefficient in a microwave reflection or transmission measurement. Here we show that a systematic error on $Q_{\mathrm{i}}$ arises from Fano interference of the signal with a background path. Limited knowledge of the interfering paths in a given setup translates into a range of uncertainty for $Q_{\mathrm{i}}$, which increases with the coupling coefficient. We experimentally illustrate the relevance of Fano interference in typical microwave resonator measurements and the associated pitfalls encountered in extracting $Q_{\mathrm{i}}$. On the other hand, we also show how to characterize and utilize the Fano interference to eliminate the systematic error.

Near-infrared color centers in silicon are emerging candidates for on-chip integrated quantum emitters, optical-access quantum memories, and sensing. We access ensemble G-color-center formation dynamics and radiation-induced atomic disorder in silicon for a series of megaelectronvolt proton-flux conditions. The photoluminescence results reveal that the G centers are formed more efficiently by pulsed-proton irradiation than by continuous-wave proton irradiation. The enhanced transient excitations and dynamic annealing within nanoseconds allows optimization of the ratio of G-center formation to nonradiative defect accumulation. The G centers preserve narrow line widths of about 0.1 nm when they are generated by moderate pulsed-proton fluences, while the line width broadens significantly as the pulsed-proton fluence increases. This implies vacancy or interstitial clustering by overlapping collision cascades. The tracking of G-center properties for a series of irradiation conditions enables sensitive probing of atomic disorder, serving as a complementary analytical method for sensing damage accumulation. Aided by ab initio electronic structure calculations, we provide insight into the atomic disorder induced inhomogeneous broadening by introducing vacancies, silicon interstitials, and oriented strain fields in the vicinity of a G center. A vacancy leads to a tensile strain and can result in either a red shift or a blue shift of the G-center emission, depending on its position relative to the G center. Meanwhile, $\mathrm{Si}$ interstitials lead to compressive strain, which results in a monotonic red shift. High-flux and tunable ion pulses enable the exploration of the fundamental dynamics of radiation-induced defects as well as methods for the optimization of G-center formation and qubit synthesis for quantum information processing.

We study simultaneous parametric oscillations in a system composed of two distributed-element-circuit Josephson parametric oscillators in the single-photon Kerr regime coupled via a static capacitance. The energy of the system is described by a two-bit Ising Hamiltonian with an effective coupling whose amplitude and sign depend on the relative phase between parametric pumps. We demonstrate that the binary phases of the parametric oscillations are correlated with each other, and that the parity and strength of the correlation can be controlled by adjusting the pump phase. The observed correlation is reproduced in our simulation taking pure dephasing into account. The present result demonstrates the tunability of the Hamiltonian parameters by the phase of external microwave, which can be used in the Ising machine hardware composed of the Kerr parametric oscillator network.

We theoretically report a remarkable revolving effect of electron flow when applying a lattice temperature gradient across an asymmetric double-barrier heterostructure. Depending on whether the lattice temperature increases or decreases, we demonstrate that electrons respectively absorb or emit a phonon and subsequently go back to the reservoir from which they have been injected. Our simulation code, which self-consistently solves the nonequilibrium Green’s function framework and the heat equation, is capable of calculating the electron temperature and electrochemical potential inside the device. By investigating those nonequilibrium thermodynamic quantities, we show that the revolving effect is due to the sign inversion of the local electron distribution. In particular, simulation results evidenced a variation of the electrochemical potential inside the device to compensate the temperature gradient, and to maintain the electrostatic neutrality in the access regions. Finally, we propose an analytic model which provides an intuitive picture of the effect, and discuss the possibility to use such behavior in the new context of heat management in nanostructures.

We designed and experimentally demonstrated a four-terminal superconducting device, a “quadristor,” that can function as a nonlatching (reversible) superconducting switch from the diode regime to the resistive state by application of a control current much smaller than the main transport current. The device uses a vortex-based superconducting-diode mechanism that is switched back and forth via the injection of flux quanta through auxiliary current leads. Our finding opens a new research area in the field of superconducting electronics.

Recent advances in quantum computing and their increased availability has led to a growing interest in possible applications. Among those is the solution of partial differential equations (PDEs) for, e.g., material or flow simulation. Currently, the most promising route to useful deployment of quantum processors in the short to near term are so-called hybrid variational quantum algorithms (VQAs). Thus, variational methods for PDEs have been proposed as a candidate for quantum advantage in the noisy intermediate-scale quantum (NISQ) era. In this work, we conduct an extensive study of utilizing VQAs on real quantum devices to solve the simplest prototype of a PDE—the Poisson equation. Although results on noiseless simulators for small problem sizes may seem deceivingly promising, the performance on quantum computers is very poor. We argue that direct resolution of PDEs via an amplitude encoding of the solution is not a good use case within reach of today’s quantum devices—especially when considering large system sizes and more complicated nonlinear PDEs that are required in order to be competitive with classical high-end solvers.

Thermoelectric (TE) coolers show considerable promise for replacing aging refrigeration devices, which suffer from the poor low-temperature performance of most TE materials due to the small entropy of carriers and weak lattice vibrational anharmonicity. Here we propose that two-dimensional (2D) semiconductors with multiple bonding layers can be potential TE cooling materials, and the 2D A$\mathrm{Mn}$X (A = $\mathrm{Sr},\mathrm{Ba}$; X = $\mathrm{Sn},\mathrm{Pb}$) family is identified as an example. Owing to the d-orbital inversion of the $\mathrm{Mn}$ element, both the valence and conduction bands possess continuously square-shaped band edges with slight energy divergence, which trigger high mobility and sharply enhanced density of states near the Fermi level, thereby producing large power factors for both p- and n- type materials. Moreover, the coexistence of soft covalent bonds and ionic layers lowers phonon group velocity and strengthens phonon scattering, resulting in extremely low lattice thermal conductivity. At optimal carrier concentrations, both p- and n-type $\mathrm{SrMnPb}$ quintuple layers can reach ultrahigh room-temperature ZT above 2.0, with excellent average ZT beyond 1.8 from 150 to 350 K. This work not only proposes a tellurium-free alternative for TE refrigeration, but also provides general indicators for guiding the search for high-performance TE cooling materials.

Quantum reservoir computing (QRC) exploits the information-processing capabilities of quantum systems to solve nontrivial temporal tasks, improving over their classical counterparts. Recent progress has shown the potential of QRC exploiting the enlarged Hilbert space, but real-time processing and the achievement of a quantum advantage with efficient use of resources are prominent challenges towards viable experimental realizations. In this work, we propose a photonic platform suitable for real-time QRC based on a physical ensemble of reservoirs in the form of identical optical pulses recirculating through a closed loop. While ideal operation achieves maximum capacities, statistical noise is shown to undermine any quantum improvement. We propose a strategy to overcome this limitation and sustain the QRC performance when the size of the system is scaled up. The protocol is conceived for experimental implementations to be viable with current technology.