Single-phonon Processes Research Articles

Concept and strategy of SuperSUN: A new ultracold neutron converter

A new source of ultracold neutrons (UCNs), developed at the Institut Laue-Langevin (ILL) and named SuperSUN, is currently being commissioned. Its operational principle is the conversion of cold neutrons, delivered by ILL’s existing beam H523, to UCNs in a vessel filled with superfluid helium-4, wherein the neutron’s energy and momentum are transferred by inelastic scattering to phonons in the superfluid. The inverse Boltzmann-suppressed process is negligible at temperatures below 0.6 K, enabling long storage times and high in-situ UCN densities as demonstrated at the ILL for two prototype sources. These two prototypes are installed at secondary beams behind crystal monochromators, whereas a primary beam with a white cold spectrum illuminates the SuperSUN conversion volume. This provides not only higher intensity around the wavelength 0.89 nm where the dominant single-phonon process for UCN production takes place, but also a contribution to UCN production by multi-phonon processes. In the first phase of the project, material walls will trap the UCNs, while in the second phase an octupole magnet will generate a 2.1 T magnetic field at the edge of the conversion volume. For low-field-seeking UCNs, this field increases the trapping potential and reduces wall losses so that the accumulated UCNs are spin-polarized as a result. SuperSUN aims to deliver the highest possible UCN densities to external storage experiments, the first of which will be the PanEDM experiment measuring the neutron’s permanent electric dipole moment.

High-field transport and hot-electron noise in GaAs from first-principles calculations: Role of two-phonon scattering

The scattering of electrons by phonons in semiconductors has been assumed to be dominated by the lowest-order process involving a single phonon. Using a new numerical approach to incorporate the next-leading-order process involving two phonons into high-field transport calculations from first principles, the authors find here that the two-phonon process contributes nearly as much as the single-phonon process to high-field transport phenomena. This finding resolves a long-standing discrepancy regarding the strength of intervalley scattering in GaAs as determined from different experimental methods.

Multiphonon processes of the inelastic electron transfer in olfaction.

Inelastic electron transfer, regarded as one of the potential mechanisms to explain odorant recognition in atomic-scale processes, is still a matter of intense debate. Here, we study multiphonon processes of electron transfer using the Markvart model and calculate their lifetimes with the values of key parameters widely adopted in olfactory systems. We find that these multiphonon processes are as quick as the single phonon process, which suggests that contributions from different phonon modes of an odorant molecule should be included for electron transfer in olfaction. Meanwhile, the temperature dependence of electron transfer could be analyzed effectively based on the reorganization energy which is expanded into the linewidth of multiphonon processes. Our theoretical results not only enrich the knowledge of the mechanism of olfaction recognition, but also provide insights into quantum processes in biological systems.

Analytical study of static beyond-Fröhlich Bose polarons in one dimension

Grusdt et al. [New J. Phys. 19, 103035 (2017)] recently made a renormalization group study of a one-dimensional Bose polaron in cold atoms. Their study went beyond the usual Frohlich description, which includes only single-phonon processes, by including two-phonon processes in which two phonons are simultaneously absorbed or emitted during impurity scattering [Shchadilova et al. Phys. Rev. Lett. 117, 113002 (2016)]. We study this same beyond-Frohlich model, but in the static impurity limit where the ground state is described by a multimode squeezed state instead of the multimode coherent state in the static Frohlich model. We solve the system exactly by applying the generalized Bogoliubov transformation, an approach that can be straightforwardly adapted to higher dimensions. Using our exact solution, we obtain a polaron energy free of infrared divergences and construct analytically the polaron phase diagram. We find that the repulsive polaron is stable on the positive side of the impurity-boson interaction but is always thermodynamically unstable on the negative side of the impurity-boson interaction, featuring a bound state, whose binding energy we obtain analytically. We find that the attractive polaron is always dynamically unstable, featuring a pair of imaginary energies which we obtain analytically. We expect the multimode squeezed state to help with studies that go not only beyond the Frohlich paradigm but also beyond Bogoliubov theory, just as the multimode coherent state has helped with the study of Frohlich polarons.

Field- and temperature-dependent quantum tunnelling of the magnetisation in a large barrier single-molecule magnet

Understanding quantum tunnelling of the magnetisation (QTM) in single-molecule magnets (SMMs) is crucial for improving performance and achieving molecule-based information storage above liquid nitrogen temperatures. Here, through a field- and temperature-dependent study of the magnetisation dynamics of [Dy(tBuO)Cl(THF)5][BPh4]·2THF, we elucidate the different relaxation processes: field-independent Orbach and Raman mechanisms dominate at high temperatures, a single-phonon direct process dominates at low temperatures and fields >1 kOe, and a field- and temperature-dependent QTM process operates near zero field. Accounting for the exponential temperature dependence of the phonon collision rate in the QTM process, we model the magnetisation dynamics over 11 orders of magnitude and find a QTM tunnelling gap on the order of 10−4 to 10−5 cm−1. We show that removal of Dy nuclear spins does not suppress QTM, and argue that while internal dipolar fields and hyperfine coupling support QTM, it is the dynamic crystal field that drives efficient QTM.

113Cd and133Cs NMR Study of Nucleus-Phonon Interactions in Linear-Chain Perovskite-Type CsCdBr3

Resonance frequencies from the <TEX>$^{113}Cd$</TEX> and <TEX>$^{133}Cs$</TEX> nuclear magnetic resonance (NMR) spectra for the <TEX>$CsCdBr_3$</TEX> single crystal were measured at varying temperatures by the static NMR method. The temperature-dependent changes of these frequencies are related to the changing structural geometry of the <TEX>${CdBr_6}^{4-}$</TEX> units, which affects the environment of <TEX>$^{133}Cs$</TEX>. The spin-lattice relaxation rates (<TEX>$1/T_1$</TEX>) for the <TEX>$^{113}Cd$</TEX> and <TEX>$^{133}Cs$</TEX> nuclei were measured in order to obtain detailed information about the dynamics of <TEX>$CsCdBr_3$</TEX> crystals. The dominant relaxation mechanisms for <TEX>$^{113}Cd$</TEX> and <TEX>$^{133}Cs$</TEX> nuclei are direct single-phonon and Raman spin-phonon processes, respectively.

Longitudinal spin relaxation of donor-bound electrons in direct band-gap semiconductors

We measure the donor-bound electron longitudinal spin-relaxation time ($T_1$) as a function of magnetic field ($B$) in three high-purity direct-bandgap semiconductors: GaAs, InP, and CdTe, observing a maximum $T_1$ of $1.4~\text{ms}$, $0.4~\text{ms}$ and $1.2~\text{ms}$, respectively. In GaAs and InP at low magnetic field, up to $\sim2~\text{T}$, the spin-relaxation mechanism is strongly density and temperature dependent and is attributed to the random precession of the electron spin in hyperfine fields caused by the lattice nuclear spins. In all three semiconductors at high magnetic field, we observe a power-law dependence ${T_1 \propto B^{-\nu}}$ with ${3\lesssim \nu \lesssim 4}$. Our theory predicts that the direct spin-phonon interaction is important in all three materials in this regime in contrast to quantum dot structures. In addition, the "admixture" mechanism caused by Dresselhaus spin-orbit coupling combined with single-phonon processes has a comparable contribution in GaAs. We find excellent agreement between high-field theory and experiment for GaAs and CdTe with no free parameters, however a significant discrepancy exists for InP.

Electron Spin Resonance Spectra of ${\hbox{La}}_{0.7}{\hbox{Cd}}_{0.3}({\hbox{Mn}},{\hbox{Co}}){\hbox{O}}_{3}$ Perovskites

We have prepared two perovskite samples La0.7Cd0.3MnO3 (LCMO) and La0.7Cd0.3CoO3 (LCCO) by using a solid-state reaction technique and studied their electron spin resonance (ESR) spectra versus temperature up to ~500 K. Experimental results reveal that as compared to LCMO, the resonant signals of LCCO are weaker and easily extinguished by thermal energy. For both samples, there exists a temperature called Tmin at which the narrowest ESR linewidth (ΔH) is obtained. At temperatures , the resonant lines associated with the ferromagnetic phase are asymmetrical. They become a symmetrical single line in the Lorentzian shape when the samples enter the paramagnetic region with T > Tmin. A detailed analysis of resonant spectra in this paramagnetic region reveals that temperature dependences of ΔH can be described by a linear function of the single-phonon process. The resonant intensity versus temperature obeys an exponent function I = I0 exp(Ea/kBT), where Ea is the activation energy associated with decomposition of ferromagnetic clusters. Temperature dependences of the resonant position (Hr) and the Lande factor (g) indicate the dominance of spin-spin and spin-lattice interactions in both LCMO and LCCO, but with the additional presence of the spin-orbit interaction in LCCO at high temperatures (T > 380 K). We believe that the differences in the electronic structure of eg and tg levels, as well as in the electronic spin configuration of Mn and Co ions, caused the distinct difference in the ESR spectra of LCMO and LCCO.

Dynamics of negatively charged divacancies in neutron irradiated type Ib diamond

Thermal neutron irradiation of synthetic type Ib diamond, followed by annealing, results in a number of paramagnetic point defect centers. One of these is the composite defect designated W29. Previous EPR measurements have shown that the W29 center (S=3/2) is a charged divacancy [VV]− with spin Hamiltonian parameters similar to those of the R4/W6 neutral divacancy center [VV]0 (S=1) found in irradiated high purity type IIa diamond. The present EPR linewidth and spin-lattice relaxation rate measurements, made as a function of temperature on the W29 center, show that an Orbach two-phonon process, with a gap of ~20meV, plays a dominant role in determining the relaxation behavior above 20K. Below 20K a direct single phonon process provides the spin-lattice relaxation mechanism and the relaxation times become long, of the order of milliseconds. The linewidth behavior with temperature is accounted for in terms of changes in the spin dynamics.

Ground-state cooling of a mechanical resonator by single- and two-phonon processes

A scheme for ground-state cooling of a mechanical resonator by single- and two-phonon processes is analyzed. The mechanical resonator is coupled to two coupled quantum dots forming an effective Λ-type three-level structure and connected with two normal metal leads. The quantum dots are driven by two light fields; by choosing appropriate parameters, the electron can be trapped in the dark state of the system, a superposition of the two ground states. When the single-phonon absorption and emission processes are dominant, under the weak (strong) driving field circumstances, the mechanical resonator is cooled through absorbing a phonon when the electron jumps from dark state to bright state (one of the dressed states) and then tunnels out of the two coupled dots. Net cooling of the resonator to its ground state is possible in the absence of the electron-phonon dephasing via single-phonon processes. When the two-phonon processes are tuned to be stronger than the single-phonon processes, the mechanical resonator can be cooled to its nonclassical state.

Dominant channels of exciton spin relaxation in photoexcited self-assembled (In,Ga)As quantum dots

We present a comprehensive theoretical investigation of spin relaxation processes of excitons in photoexcited self-assembled quantum dots. The exciton spin relaxations are considered between dark- and bright-exciton states via the channels created by various spin-admixture mechanisms, including electron Rashba and Dresselhaus spin-orbital couplings (SOCs), hole linear and hole cubic SOCs, and electron hyperfine interactions, incorporated with single- and double-phonon processes. The hole-Dresselhaus SOC is identified as the dominant spin-admixture mechanism, leading to relaxation rates as fast as $~$${10}^{\ensuremath{-}2}$ ns${}^{\ensuremath{-}1}$, consistent with recent observations. Moreover, due to significant electron-hole exchange interactions, single-phonon processes are unusually dominant over two-phonon ones in a photoexcited dot even at temperatures as high as $15 \mathrm{K}$.

Laboratory Astrophysics of Dust

Infrared spectroscopy is the best astronomical tool for studying the composition of cosmic dust. Thanks to the Herschel satellite, dust properties from the FIR to mm wavelength range will be sampled in different astrophysical environments. In the laboratory, the study of the temperature and structural dependence of FIR absorption of cosmic dust analogs including agglomeration is essential to interpret observational spectra. For crystalline materials, FIR single phonon bands are temperature dependent due to the anharmonicity of the vibrational potentials. This strong temperature dependence of the FIR bands’ positions can be used as a thermometer of the dust temperature. In amorphous material, the FIR absorption is dominated by disorder-induced single phonon processes and in the submillimeter and millimeter range by highly temperature-dependent low energy processes, e.g. tunneling transitions in two-level systems. The effect of these processes on the FIR absorptivity in amorphous silicates will be demonstrated.

Optical transitions and energy relaxation of hot carriers in Si nanocrystals

Dynamics of hot carriers confined in Si nanocrystals is studied theoretically using atomistic tight binding approach. Radiative, Auger-like, and phonon-assisted processes are considered. The Auger-like energy exchange between electrons and holes is found to be the fastest process in the system. However, the energy relaxation of hot electron-hole pair is governed by the single optical phonon emission. For a considerable number of states in small nanocrystals, single-phonon processes are ruled out by energy conservation law.

Nuclear magnetic resonance study of the relaxation mechanisms of the 9Be, 27Al, and 29Si nuclei in Cr 3+-doped Be 3Al 2Si 6O 18 crystals

The variations with temperature of the relaxation mechanisms of the 9Be, 27Al, and 29Si nuclei in emerald (Be 3Al 2Si 6O 18:Cr 3+) single crystals were investigated by using a pulse NMR spectrometer. The values of the spin-lattice relaxation rates T 1 − 1 for the three nuclei are different, and these differences are attributed to the differences between their Larmor frequencies and their local nuclear environments. The relaxation rates of the 9Be and 27Al nuclei undergo no abrupt changes within the temperature range 180–400 K, which indicates that there are no phase transitions within this temperature range. The spin-lattice relaxation rates of 9Be and 27Al were found to be proportional to temperature. Therefore, the nuclear spin-lattice relaxation processes of these two nuclei proceed via single-phonon processes.

Ultra cold neutron production by multiphonon processes in superfluid helium under pressure

Cold neutrons are converted to ultra cold neutrons (UCN) by the excitation of a single phonon or multiphonons in superfluid helium. The dynamic scattering function S ( q , ℏ ω ) of the superfluid helium strongly depends on pressure, leading to a pressure-dependent differential UCN production rate. A phenomenological expression for the multiphonon part of the scattering function s ( λ ) describing UCN production has been derived from inelastic neutron scattering data. When combined with the production rate from single phonon processes this allows us to calculate the UCN production for any incident neutron flux. For calculations of the UCN production from single phonon processes we propose to use the values S * = 0.118 ( 8 ) at saturated vapour pressure and S * = 0.066 ( 6 ) at 20 bar. As an example we will calculate the expected UCN production rate at the cold neutron beam for fundamental physics PF1b at the Institut Laue Langevin. We conclude that UCN production in superfluid helium under pressure is not attractive.

Nuclear-spin relaxation ofPb207in ferroelectric powders

Motivated by a recent proposal by O. P. Sushkov and co-workers to search for a P,T-violating Schiff moment of the $^{207}$Pb nucleus in a ferroelectric solid, we have carried out a high-field nuclear magnetic resonance study of the longitudinal and transverse spin relaxation of the lead nuclei from room temperature down to 10 K for powder samples of lead titanate (PT), lead zirconium titanate (PZT), and a PT monocrystal. For all powder samples and independently of temperature, transverse relaxation times were found to be $T_2\approx 1.5 $ms, while the longitudinal relaxation times exhibited a temperature dependence, with $T_1$ of over an hour at the lowest temperatures, decreasing to $T_1\approx 7 $s at room temperature. At high temperatures, the observed behavior is consistent with a two-phonon Raman process, while in the low temperature limit, the relaxation appears to be dominated by a single-phonon (direct) process involving magnetic impurities. This is the first study of temperature-dependent nuclear-spin relaxation in PT and PZT ferroelectrics at such low temperatures. We discuss the implications of the results for the Schiff-moment search.

Rb 87 NMR relaxation of RbCdCl3 and Rb4CdCl6 single crystals with the electric-quadrupole-type interactions

The Rb87 spin-lattice relaxation time T1 and spin-spin relaxation time T2 of RbCdCl3 and Rb4CdCl6 single crystals grown by the slow evaporation method were measured over the temperature range of 160–410K. In the case of the RbCdCl3 crystal, changes were observed in the Rb87 T1−1 near 340, 363, and 395K, which correspond to phase transitions. The abrupt decrease in T1−1 observed at 395K is due to a shortening in the c direction as a result of a phase transition from a tetragonal to a cubic structure. We suggest that the cubic Rb environment is favored above 395K, which causes relaxation and averages out the quadrupolar splittings. On the other hand, the Rb87 T1 in Rb4CdCl6 decreased with increasing temperature, and no phase transitions were observed in the temperature range considered. For both RbCdCl3 and Rb4CdCl6, the relaxation rate of Rb was found to be proportional to the square of the temperature, indicating that the relaxation proceeds via a Raman process. This behavior for the electric-quadrupolar-type RbCdCl3 and Rb4CdCl6 crystals stands in contrast to a previous report that the relaxations in Cs2CuCl4 and Cs2CoCl4 crystals with the magnetic relaxation type proceed via single-phonon processes. Therefore, the relaxation mechanism in crystals with an electric quadrupolar NMR interaction mechanism is completely different from that in crystals with magnetic-type NMR behavior.

Mechanism of hopping conduction in new CeO 2–B 2O 3 semiconducting glasses

CeO 2–B 2O 3 glasses having a broad range of compositions, prepared by a press-quenching method from glass melts, were studied for DC conductivity. The conductivity at 583 K (1000/ T=1.715 K −1), ln σ 583, increases by about 16% in the composition regime 20–50 mol% CeO 2 while in the composition range 50–60 mol% CeO 2, ln σ 583 decreases by about 28%. The behavior of ln σ 583 with composition can be interpreted in terms of the changes in the high-temperature activation energy W and the hopping distance, R. Between 20 and 50 mol% CeO 2, increasing the mechanical strength and cross-linking of the glass network results in decreasing the polaron hopping energy, W H. At higher concentrations, the breaking of the 3D network observed to be probable for these glasses may increase the disorder energy, W D. The conduction mechanism is found to be non-adiabatic in nature for glasses investigated. Mott's model for the small polaron hopping conduction between nearest-neighbors gives unreasonable values for the Ce–Ce distance and the phonon frequency of the lattice vibrations. The increase in the conductivity with temperature can be interpreted in terms of the Schnakenberg model for optical multiphonon assisted hopping process at high temperatures and single optical phonon process at the lower ones. This model gives reasonable values for the phonon frequency of the lattice vibrations, the polaron hopping energy, and slightly higher values of W D.

Hole spin relaxation in Ge quantum dots

Hole spin relaxation in an isolated Ge quantum dot due to interaction with phonons is investigated. Spin relaxation in this case occurs through the mechanism of the modulation of the spin-orbit interaction by lattice vibrations. According to the calculations performed, the spin relaxation time due to direct single-phonon processes for the hole ground state equals 1.4 ms in the magnetic field H = 1 T at the temperature T = 4 K. The dependence of the relaxation time on the magnetic field is described by the power function H−5. At higher temperatures, a substantial contribution to spin relaxation is made by two-phonon (Raman) processes. Because of this, the spin relaxation time decreases to nanoseconds as the temperature is raised to T = 20 K. Analysis of transition probabilities shows that the third and twelfth excited hole states, which are intermediate in two-step relaxation processes, play the main part in Raman processes.

A study of the NMR relaxation mechanisms of ABCl 3 ( A=Rb, Cs, B=Cd, Mn) single crystals with the electric quadrupole and the electric magnetic types

The 87Rb and 133Cs spin-lattice relaxation rates of RbCdCl 3 and CsCdCl 3 single crystals grown using the slow evaporation method were measured over the temperature range 160–400 K. The changes in the 87Rb spin-lattice relaxation rate near 340, 363, and 395 K correspond to phase transitions of the RbCdCl 3 crystal. The jump in T 1 −1 at 395 K is due to a shortening in the c-direction as a result of a phase transition from a cubic to a tetragonal structure. We suggest that the cubic Rb environment is favored above 395 K due to the fast motions and soft modes, which cause relaxation and average out the quadrupolar splittings. The temperature dependence of the relaxation rate below 340 K in RbCdCl 3 can be represented by T 1 - 1 = α T 2 and is thus in accordance with a Raman process. The 133Cs nuclei in the CsCdCl 3 crystal produce only one resonance line, which indicates that the local structure around the Cs atoms is cubic. The temperature dependence of the relaxation rate of the Cs nuclei can also be described with the quadratic equation T 1 - 1 = α T 2 . In the case of the RbCdCl 3 and CsCdCl 3 crystals, which are of electric quadrupolar type, their relaxations proceed via Raman processes, whereas in RbMnCl 3 and CsMnCl 3 crystals, which are of magnetic relaxation type, the relaxations proceed via single phonon processes. Therefore, the relaxation mechanisms of these different types of ABCl 3 crystals (quadrupolar and magnetic) are completely different NMR behavior.