Magnetoelectric memory effect in the Y-type hexaferrite BaSrZnMgFe_(12)O_(22)

We report on the magnetic and magnetoelectric properties of the Y-type hexaferrite BaSrZnMgFe_(12)O_(22),which undergoes transitions from a collinear ferrimagnetic phase to a proper screw phase at 310 K and to a longitudinal conical phase at 45 K.Magnetic and electric measurements revealed that the magnetic structure with spiral spin order can be modified by applying a magnetic field,resulting in magnetically controllable electric polarization.It was observed that BaSrZnMgFe_(12)O_(22)exhibits an anomalous magnetoelectric memory effect:the ferroelectric state can be partially recovered from the paraelectric phase with collinear spin structure by reducing magnetic field at 20 K.We ascribe this memory effect to the pinning of multiferroic domain walls,where spin chirality and structure are preserved even in the nonpolar collinear spin state.     

High-pressure EPR studies of intermolecular interactions in solids

High-pressure technique as applied to studies of different problems in solid state physics by EPR spectroscopy is presented. An influence of hydrostatic pressure up to 500 MPa on a weak superexchange coupling between Cu(II) ions in monomeric and dimeric copper(II) compounds is described. Both decrease and increase in the coupling is observed under pressure depending on the crystal and molecular structure but in all cases a dominant contribution is due to a shortening of intermolecular distances, whereas molecular structure and molecular vibrations are not affected. In dimeric Cu(II) compounds with resolved fine structure of EPR spectra zero-field splitting is weakly or not at all dependent on pressure whereas a strong dependence on temperature exists. It is discussed as a result of vibronic ligand behaviour which is not volume dependent. For all cases complementary temperature dependences are presented and moreover a behaviour of EPR linewidth under pressure and temperature is shown. An influence of high pressure on spin exchange reactions in solutions and on the phase memory time in electron spin echo experiments is briefly described.     

Microwave Spin Frequency Comb Memory Protocol Controlled by Gradient Magnetic Pulses

We have demonstrated a combination of frequency comb spin-echo protocol in a conventional microwave pulsed electron spin resonance spectrometer with gradient pulses of the external magnetic field applied for on-demand retrieval of signal microwave pulses at the required moments of time. A natural high-finesse periodic structure was used as a carrier of stored information. The structure is made out of hyperfine lines of electron spin resonance of tetracyanoethylene anion radicals in toluene at room temperature. Herein, we have also observed that using the pulses of gradient magnetic field can increase the memory capacity. The experimental results demonstrated promising opportunities for controlling electron nuclear spin coherence, which could be useful for implementation of broadband microwave or optical-microwave noise free quantum memory protocols.     

Electron spin relaxation of copper(II) ions in diamagnetic crystals

The electron spin-lattice and spin-spin phase relaxation measurements of Cu²⁺ ions in various crystals are reviewed and discussed. Examples of the Debye temperature determination from a wide temperature range measurements of the spin-lattice relaxation time T₁ are shown. An influence of the Jahn-Teller dynamics on T₁ is presented. The phase relaxation described by the phase memory time T_M is affected by temperature due to the spin packet width modulation by molecular motions. The T_M is anisotropic in crystals and can be different for different hyperfine lines of an EPR spectrum.     

Parameter dependence of resonant spin torque magnetization reversal

We numerically study ultra fast resonant spin torque (ST) magnetization reversal in magnetic tunneling junctions (MTJ) driven by current pulses having a direct current (DC) and a resonant alternating current (AC) component. The precessional ST dynamics of the single domain MTJ free layer cell are modeled in the macro spin approximation. The energy efficiency, reversal time, and reversal reliability are investigated under variation of pulse parameters like direct and AC current amplitude, AC frequency and AC phase. We find a range of AC and direct current amplitudes where robust resonant ST reversal is obtained with faster switching time and reduced energy consumption per pulse compared to purely direct current ST reversal. However, for a certain range of AC and direct current amplitudes a strong dependence of the reversal properties on AC frequency and phase is found. Such regions of unreliable reversal must be avoided for ST memory applications.     

Spin anisotropy and slow dynamics in spin glasses

We report on an extensive study of the influence of spin anisotropy on spin glass aging dynamics. New temperature cycle experiments allow us to compare quantitatively the memory effect in four Heisenberg spin glasses with various degrees of random anisotropy and one Ising spin glass. The sharpness of the memory effect appears to decrease continuously with the spin anisotropy. Besides, the spin glass coherence length is determined by magnetic field change experiments for the first time in the Ising sample. For three representative samples, from Heisenberg to Ising spin glasses, we can consistently account for both sets of experiments (temperature cycle and magnetic field change) using a single expression for the growth of the coherence length with time.     

Aging, rejuvenation and memory phenomena in spin glasses

In this paper, we review several important features of the out-of-equilibrium dynamics of spin glasses. Starting with the simplest experiments, we discuss the scaling laws used to describe the isothermal aging observed in spin glasses after a quench down to the low-temperature phase. We report in particular new results on the sub-aging behaviour of spin glasses. We then discuss the rejuvenation and memory effects observed when a spin glass is submitted to temperature variations during aging, from the point of view of both energy landscape pictures and real-space pictures. We highlight the fact that both approaches point out the necessity of hierarchical processes involved in aging. Finally, we report an investigation of the effect of small temperature variations on aging in spin glass samples with various anisotropies which indicates that this hierarchy depends on the spin anisotropy.     

Hybrid quantum processors: molecular ensembles as quantum memory for solid state circuits

We investigate a hybrid quantum circuit where ensembles of cold polar molecules serve as long-lived quantum memories and optical interfaces for solid state quantum processors. The quantum memory realized by collective spin states (ensemble qubit) is coupled to a high-Q stripline cavity via microwave Raman processes. We show that, for convenient trap-surface distances of a few microm, strong coupling between the cavity and ensemble qubit can be achieved. We discuss basic quantum information protocols, including a swap from the cavity photon bus to the molecular quantum memory, and a deterministic two qubit gate. Finally, we investigate coherence properties of molecular ensemble quantum bits.     

圆环结构磁光光子晶体中的拓扑相变

二维圆环结构的三角晶格磁光光子晶体中可以呈现多重拓扑相.在不同的几何参数和磁场下,这些拓扑相包括正常光子带隙相、量子自旋霍尔相和反常量子霍尔相.与文献[1]类似,该结果展现了二维光子晶体丰富的拓扑相变现象.     

A new probe of pore geometry: diffusive MASS NMR

NMR methods are widely used to probe the structure and fluid dynamics of porous materials, as they are uniquely suited to these studies since NMR records the correlation of changing local magnetic fields over a time scale of ns to seconds. The local magnetic fields are established by local variations in the bulk magnetic susceptibility of the sample (and so are directly tied to the sample's local structure). The fluctuation in field that a spin sees is due to molecular transport (including molecular diffusion) through these local fields, and so reports on the length scales of structures and impediments to transport. We have developed a new set of methods DIFFUSIVE-MASS to provide a means of systematically varying the effective time scale of the measurement and thus the effective length scale. This new handle permits a detailed, microscopic picture of the structure and dynamics. Diffusive MASS NMR methods will permit a systematic set of methods and analysis for characterizing the chemistry, structure and fluid dynamics of the mobile phase in porous materials. The approach will be applicable to any diamagnetic material. In particular, the industry of oil discovery depends on understanding heterogeneous porous media.     

Uncovering the features of spin dynamics in hadronic physics

We summarize the program pursued by Michael J. Moravcsik and the authors on spin amplitude analyses and interpretation. We present the recent observations that phase histograms reveal striking peaks corresponding to spin amplitudes that tend to have definite simple phase relations. An interpretation is proposed in which the spin amplitudes receive a coherent and an incoherent contribution over large ranges of energies and angles. Continuing research directions are reported.Dedicated to the memory of Michael J. Moravcsik, whose contribution to this work, our research, and spin physics is immeasurable     

Aging in the ferromagnetic phase of terbium

We report on aging, rejuvenation and memory effects in the ferromagnetic phase of pure terbium. We have applied an experimental method specifically for investigating slow dynamics of spin glasses, because these effects cannot be interpreted as conventional diffusion after-effects. Results show that relaxation times of the magnetic response are widely distributed, and isothermal aging shifted the distribution towards longer durations. If the sample was heated/cooled after such isothermal aging, the relaxation times shortened as if aging was starting anew; the behavior resembles that in spin glasses. Uniform magnetization experiments indicate that, unlike rejuvenation in spin glasses, ferromagnetic correlations are not returned to disorder by thermal perturbations. In contrast with memory effects in spin glasses, the effects of isothermal aging cannot be recovered once these disappear, even if the system is returned to its initial temperature. The observed results can be explained as collective pinning of the domain walls for which the potential is given by a rugged temperature-sensitive energy landscape.     

Spin Squeezing for Superpositions of Dicke States

We investigate spin squeezing for superpositions of Dicke states. We derive the mean spin direction, the length of mean spin, the optimally squeezed angle and numerically calculate the spin squeezing parameter, which is determined by on the superposed coefficients and the relative phase. Approximate same superposed amplitude and the smaller relative phase lead to the larger the length of mean spin, the stronger spin squeezing and spin squeezing maintains in a longer time interval.     

Suppression of electron spin relaxation in Mn-doped GaAs

We report a surprisingly long spin relaxation time of electrons in Mn-doped p-GaAs. The spin relaxation time scales with the optical pumping and increases from 12 ns in the dark to 160 ns upon saturation. This behavior is associated with the difference in spin relaxation rates of electrons precessing in the fluctuating fields of ionized or neutral Mn acceptors, respectively. For the latter, the antiferromagnetic exchange interaction between a Mn ion and a bound hole results in a partial compensation of these fluctuating fields, leading to the enhanced spin memory.     

Entangling strings of neutral atoms in 1D atomic pipeline structures

We study a string of neutral atoms with nearest neighbor interaction in a 1D beam splitter configuration, where the longitudinal motion is controlled by a moving optical lattice potential. The dynamics of the atoms crossing the beam splitter maps to a 1D spin model with controllable time dependent parameters, which allows the creation of maximally entangled states of atoms by crossing a quantum phase transition. Furthermore, we show that this system realizes protected quantum memory, and we discuss the implementation of one- and two-qubit gates in this setup.     

High-fidelity quantum memory realized via Wigner crystals of polar molecules

The collective excitations of spin states of an ensemble of polar molecules are studied as a candidate for high-fidelity quantum memory. To avoid the collisional properties of the molecules, they are arranged in dipolar crystals under one or two dimensional trapping conditions. We calculate the lifetime of the quantum memory by identifying the dominant decoherence mechanisms and estimating their effects on gate operations when a molecular ensemble qubit is transferred to a microwave cavity.     

两分量玻色-爱因斯坦凝聚系统的自旋压缩

在周期性脉冲撞击的两分量玻色－爱因斯坦凝聚系统中研究了自旋压缩动力学性质,结果表明：原子自旋压缩动力学能够揭示相空间的混沌和规则结构,即当初始相干态处在混沌区域时,自旋压缩在很短时间后消失,而当初始相干态处在规则区域时,自旋压缩则存在很长时间.特别是随着时间的演化,系统的平均自旋方向的分布和摆动也与初态所处的空间结构有着密切的联系.最后,研究了相空间的整体自旋压缩动力学,得到了一种较好的量子-经典对应. 关键词： 玻色-爱因斯坦凝聚 混沌 自旋压缩 平均自旋方向     

Spin squeezing of atomic ensembles via nuclear-electronic spin entanglement

We demonstrate spin squeezing in a room temperature ensemble of approximately 10(12) cesium atoms using their internal structure, where the necessary entanglement is created between nuclear and electronic spins of each individual atom. This state provides improvement in measurement sensitivity beyond the standard quantum limit for quantum memory experiments and applications in quantum metrology and is thus a complementary alternative to spin squeezing obtained via interatom entanglement. Squeezing of the collective spin is verified by quantum state tomography.     

Berry phase in a nonisolated system

We investigate the effect of the environment on a Berry phase measurement involving a spin-half. We model the spin + environment using a biased spin-boson Hamiltonian with a time-dependent magnetic field. We find that, contrary to naive expectations, the Berry phase acquired by the spin can be observed, but only on time scales which are neither too short nor very long. However this Berry phase is not the same as for the isolated spin-half. It does not have a simple geometric interpretation in terms of the adiabatic evolution of either bare spin states or the dressed spin resonances. This result is crucial for proposed Berry phase measurements in superconducting nanocircuits.