Metastable Ground State Research Articles

Microwave assisted efficient non-degenerate four-wave mixing in pulsed regime

We theoretically investigate a N-type 87Rb atomic system for efficient generation and control of a non-degenerate four wave mixing (FWM) signal in pulsed regime. The susceptibility of the atomic medium is customized as a gain profile by a weak probe pulse and two strong continuous wave control fields which allow us to generate the pulsed FWM signal. We study the propagation dynamics of the generated FWM signal inside the nonlinear medium. The FWM signal obtains the exact shape of the probe pulse and travels without changing the shape whereas, the probe pulse is absorbed inside the nonlinear medium. The conversion efficiency of this scheme without a MW field is 5.36%. However, a MW field that couples two metastable ground states enhances the conversion efficiency to 20.6%. The generation and control of such FWM signal in pulsed regime has important applications in signal processing, optical communication and information science.

Determining the diffusion behavior of point defects in zirconium by a multiscale modelling approach

Zr alloys are commonly used materials for in-core components of pressurized water reactors. The evolution of defects in Zr under irradiation conditions will directly contribute to the degradation of reactor components. For a better understanding of the evolution process of defects in Zr, a multiscale simulation was performed to study the diffusion behavior of radiation-induced point defects. Because the existing potentials cannot accurately describe the energy differences of stable states self-interstitial atoms (SIAs) in Zr, an embedded atom method potential that can exactly reproduce the energy difference between metastable states and ground states, and the binding energy of divacancy of Zr, was first developed by fitting a potential to point defect properties calculated by density functional theory and other basic crystal properties. Based on the constructed potential, molecular dynamics (MD) and kinetic Monte Carlo simulations were conducted to investigate the migration of point defects. We determined several frequent migration paths of SIAs by kinetic study, which is rarely reported in previous MD studies. The SIA exhibits obvious anisotropic diffusion characteristics at low temperatures (<600 K). The most frequent migration path for SIAs is the jump between the two nearest basal octahedral (BO) sites in the basal plane, which means that SIAs diffuse faster along the basal plane than along the c axis. It is found that diffusion within the basal plane tends to be two-dimensional (2D) diffusion rather than 1D diffusion, and there is a significant correlation effect for diffusion of SIAs along the basal plane. Additionally, the existence of trivacancy-SIA complex was found in the process of divacancy migration, which can inhibit divacancy migration. Monovacancies and divacancies exhibit anisotropic diffusion characteristics in the considered temperature range Divacancies have a much faster diffusion rate than monovacancies in the present MD simulation and can easily dissociate at high temperatures (>900 K). The rapid migration of the divacancies may contribute to the formation of vacancy dislocation loops. These results are meaningful to understand the evolution process of radiation-induced defects.

Photoinduced intradomain dynamics and nonthermal switching of metastable states in the one-dimensional extended Peierls-Hubbard model

We investigate the microscopic dynamics at the initial stage of photoinduced phase transitions in tetrathiafulvalene-$p$-chloranil by exact diagonalization. We first show that the one-dimensional extended Peierls-Hubbard model exhibits a neutral phase with small ionicity and negligible dimerization and an ionic phase with moderate ionicity and dimerization. Aside from these phases, we find a doubly ionized phase with strong dimerization that we call the "dipole" phase. These ground-state phases are characterized by various order parameters and the Zak phase, which is relevant to electronic polarization. We further explore the microscopic dynamics of the three phases triggered by short monocycle optical pulses. The electronic order parameters and lattice displacement suggest that the neutral-ionic, ionic-neutral, and dipole-ionic transitions are induced. Furthermore, clear spectroscopic changes are observed in the time-dependent spectral density and pump-probe conductivity. A detailed analysis of the spectroscopy demonstrates the generation of coherent charge-transfer strings via multiphoton absorption and the crucial roles of the excited states and the metastable ground state at the new lattice position for the ultrafast dynamics.

Disordered Bose–Einstein condensate in hard walls trap

We discuss the effects of quenched disorder in a dilute Bose–Einstein condensate confined in a hard walls trap. Starting from the disordered Gross–Pitaevskii functional, we obtain a representation for the quenched free energy as a series of integer moments of the partition function. Positive and negative disorder-dependent effective coupling constants appear in the integer moments. Going beyond the mean-field approximation, we compute the static two-point correlation functions at first-order in the positive effective coupling constants. We obtain the combined contributions of effects due to boundary conditions and disorder in this weakly disordered condensate. The ground state renormalized density profile of the condensate is presented. We also discuss the appearance of metastable and true ground states for strong disorder, when the effective coupling constants become negative.

Phases of mathcal{N}=1 theories in 2 + 1 dimensions

We study the dynamics of 2+1 dimensional theories with mathcal{N}=1 supersymmetry. In these theories the supersymmetric ground states behave discontinuously at codimension one walls in the space of couplings, with new vacua coming in from infinity in field space. We show that the dynamics near these walls is calculable: the two-loop effective potential yields exact results about the ground states near the walls. Far away from the walls the ground states can be inferred by decoupling arguments. In this way, we are able to follow the ground states of mathcal{N}=1 theories in 2+1 dimensions and construct the infrared phases of these theories. We study two examples in detail: Adjoint SQCD and SQCD with one fundamental quark. In Adjoint QCD we show that for sufficiently small Chern-Simons level the theory has a non-perturbative metastable supersymmetry-breaking ground state. We also briefly discuss the critical points of this theory. For SQCD with one quark we establish an infrared duality between a U(N) gauge theory and an SU(N) gauge theory. The duality crucially involves the vacua that appear from infinity near the walls.

Phase-locked bifrequency Raman lasing in a double- Λ system

We show that it is possible to realize simultaneous Raman lasing at two different frequencies using a double-$\Lambda$ system pumped by a bi-frequency field. The Raman lasers are phase-locked to one another, and the beat-frequency matches the energy difference between the two meta-stable ground states. Akin to a conventional Raman laser, the phase-locked Raman laser pair is expected to be subluminal. As such, it is expected to be highly stable against perturbations in cavity length, and have a quantum noise limited linewidth that is far below that of a conventional laser. Because of these properties, the phase-locked Raman laser pair may find important applications in precision metrology, including atomic interferometry and magnetometry. To elucidate the behavior of this laser pair, we develop an analytical model that describes the stimulated Raman interaction in a double-$\Lambda$ system using an effective 2-level transition. The approximation is valid as long as the excited states adiabatically follow the ground states, as verified by numerical simulations. The effective model is used to identify the optimal operating conditions for the bi-frequency Raman lasing process. This model may also prove useful in other potential applications of the double-$\Lambda$ system, including generation of squeezed light and spatial solitons.

Measurements and kinetic modeling of atomic species in fuel-oxidizer mixtures excited by a repetitive nanosecond pulse discharge

This work presents the results of number density measurements of metastable Ar atoms and ground state H atoms in diluted mixtures of H2 and O2 with Ar, as well as ground state O atoms in diluted H2–O2–Ar, CH4–O2–Ar, C3H8–O2–Ar, and C2H4–O2–Ar mixtures excited by a repetitive nanosecond pulse discharge. The measurements have been made in a nanosecond pulse, double dielectric barrier discharge plasma sustained in a flow reactor between two plane electrodes encapsulated within dielectric material, at an initial temperature of 500 K and pressures ranging from 300 Torr to 700 Torr. Metastable Ar atom number density distribution in the afterglow is measured by tunable diode laser absorption spectroscopy, and used to characterize plasma uniformity. Temperature rise in the reacting flow is measured by Rayleigh scattering. H atom and O atom number densities are measured by two-photon absorption laser induced fluorescence. The results are compared with kinetic model predictions, showing good agreement, with the exception of extremely lean mixtures. O atoms and H atoms in the plasma are produced mainly during quenching of electronically excited Ar atoms generated by electron impact. In H2–Ar and O2–Ar mixtures, the atoms decay by three-body recombination. In H2–O2–Ar, CH4–O2–Ar, and C3H8–O2–Ar mixtures, O atoms decay in a reaction with OH, generated during H atom reaction with HO2, with the latter produced by three-body H atom recombination with O2. The net process of O atom decay is O + H → OH, such that the decay rate is controlled by the amount of H atoms produced in the discharge. In extra lean mixtures of propane and ethylene with O2–Ar the model underpredicts the O atom decay rate. At these conditions, when fuel is completely oxidized by the end of the discharge burst, the net process of O atom decay, O + O → O2, becomes nearly independent of H atom number density. Lack of agreement with the data at these conditions is likely due to diffusion of H atoms from the partially oxidized regions near the side walls of the reactor into the plasma. Although significant fractions of hydrogen and hydrocarbon fuels are oxidized by O atoms produced in the plasma, chain branching remains a minor effect at these relatively low temperature conditions.

Laser continuum source atomic absorption spectroscopy: Measuring the ground state with nanosecond resolution in laser-induced plasmas

A new dual-beam atomic absorption technique is applied to laser-induced plasmas. The technique uses an optical parametric oscillator pseudocontinuum, producing emission that is both wider than the absorption line profile, but narrow enough to allow the use of an echelle spectrograph without order sorting. The dual-beam-in space implementation makes the technique immune to nonspecific attenuation of the probe beam and the structure of the pseudocontinuum. The potential for plasma diagnostics is demonstrated with spatially and temporally resolved measurements of magnesium metastable and lithium ground state optical depths in a laser-induced plasma under reduced pressure conditions. The lithium measurements further demonstrate the technique's potential for isotope ratio measurements.

Spin dependent inelastic collisions between metastable state two-electron atoms and ground state alkali-atoms

Experimentally the spin dependence of inelastic collisions between ytterbium (Yb) in the metastable state and lithium (Li) in the ground state manifold is investigated at low magnetic fields. Using selective excitation all magnetic sublevels mJ of Yb() are accessed and four of the six lowest lying magnetic sublevels of are prepared by optical pumping. On the one hand, mJ-independence of collisions involving Li() atoms is found. A systematic mJ-dependence in collisions with Li() atoms, in particular suppressed losses for stretched collisional states, is observed on the other hand. Further, mJ-changing processes are found to be of minor relevance. The span of observed inelastic collision rates is between and , and a possible origin of the observed behavior is discussed.

Extra-Dimensional “Metamaterials”: A Model of Inflation Due to a Metric Signature Transition

Lattices of topological defects, such as Abrikosov lattices and domain wall lattices often arise as metastable ground states in higher-dimensional field theoretical models. We demonstrate that such lattice states may be described as extra-dimensional metamaterials via higher-dimensional effective medium theory. A 4+1 dimensional extension of Maxwell electrodynamics with a compactified time-like dimension has been considered as an example. It is demonstrated that from the point of view of macroscopic electrodynamics an Abrikosov lattice state in such a 4+1 dimensional spacetime may be described as a uniaxial hyperbolic metamaterial. Extraordinary photons perceive this medium as a 3+1 dimensional Minkowski spacetime in which one of the original spatial dimensions (the optical axis of the metamaterial) plays the role of a new time-like coordinate. Since metric signature of this effective space-time depends on the Abrikosov lattice periodicity, the described model may be useful in studying metric signature transitions. A particular kind of metric signature transition understood as a macroscopic medium effect may emulate cosmological inflation.

Temperature changes of the Fe8 molecular magnet during its spin reversal process

Tunneling of the spins in the Fe$_8$ molecular magnet from a metastable ground state to an excited state is accompanied by a decay of these spins to the global ground state, and an increase of the crystal temperature. We measured this temperature using two thermometers, one strongly coupled and the other weakly coupled to the thermal bath. We found that the temperature increases to no greater than $2.2$~K. This upper limit agrees with the flame temperature derived from deflagration theory and previous measurements. In light of this temperature increase we re-examine the Landau, Zener and Stuckelberg (LZS) theory of spin tunneling in large Fe$_8$ crystals.

Collisional stability of localized Yb(3P2) atoms immersed in a Fermi sea of Li

We establish an experimental method for a detailed investigation of inelastic collisional properties between ytterbium (Yb) in the metastable state and ground state lithium (Li). By combining an optical lattice and a direct excitation to the state we achieve high selectivity on the collisional partners. Using this method we determine inelastic loss coefficients in collisions between 174Yb() with magnetic sublevels of mJ = 0 and −2 and ground state 6Li to be and , respectively. Absence of spin changing processes in Yb()–Li inelastic collisions at low magnetic fields is confirmed by inelastic loss measurements on the mJ = 0 state. We also demonstrate that our method allows us to look into loss processes in few-body systems separately.

Photoionization spectrum of 85RbCs molecules produced by short range photoassociation

We present photoionization (PI) spectrum of ultracold ground state 85RbCs molecules formed by spontaneous decay from J=1, v=8 rovibrational level of (2)Π0−3 short range excited state molecules, which are produced from a space-adjustable dark spontaneous force optical trap. PI spectrum is obtained by scanning PI laser when photoassociation laser is locked by utilizing transfer cavity technique based on an ultrastable He–Ne laser as a reference laser. We calculated vibrational distribution of RbCs molecules in metastable ground state and corresponding transition probabilities to RbCs+ ion in (1)Σ+2 state. An enlarged PI spectrum with relative clear peaks is assigned to v=24–33 vibrational levels of molecular ion state. The method we used in PI spectrum analysis is also adoptable for other states of RbCs molecules or other kinds of molecules.

Gauged R-symmetry and its anomalies in 4D N=1 supergravity and phenomenological implications

We consider a class of models with gauged U(1)_R symmetry in 4D N=1 supergravity that have, at the classical level, a metastable ground state, an infinitesimally small (tunable) positive cosmological constant and a TeV gravitino mass. We analyse if these properties are maintained under the addition of visible sector (MSSM-like) and hidden sector state(s), where the latter may be needed for quantum consistency. We then discuss the anomaly cancellation conditions in supergravity as derived by Freedman, Elvang and K\"ors and apply their results to the special case of a U(1)_R symmetry, in the presence of the Fayet-Iliopoulos term ($\xi$) and Green-Schwarz mechanism(s). We investigate the relation of these anomaly cancellation conditions to the "naive" field theory approach in global SUSY, in which case U(1)_R cannot even be gauged. We show the two approaches give similar conditions. Their induced constraints at the phenomenological level, on the above models, remain strong even if one lifted the GUT-like conditions for the MSSM gauge couplings. In an anomaly-free model, a tunable, TeV-scale gravitino mass may remain possible provided that the U(1)_R charges of additional hidden sector fermions (constrained by the cubic anomaly alone) do not conflict with the related values of U(1)_R charges of their scalar superpartners, constrained by existence of a stable ground state. This issue may be bypassed by tuning instead the coefficients of the Kahler connection anomalies (b_K, b_{CK}).

Combined optical emission and resonant absorption diagnostics of an Ar-O2-Ce-reactive magnetron sputtering discharge

We report the results on combined optical characterization of Ar-O2-Ce magnetron sputtering discharges by optical emission and resonant absorption spectroscopy. In this study, a DC magnetron sputtering system equipped with a movable planar magnetron source with a Ce target is used. The intensities of Ar, O, and Ce emission lines, as well as the absolute densities of Ar metastable and Ce ground state atoms are analyzed as a function of the distance from the magnetron target, applied DC power, O2 content, etc. The absolute number density of the Arm is found to decrease exponentially as a function of the target-to-substrate distance. The rate of this decrease is dependent on the sputtering regime, which should be due to the different collisional quenching rates of Arm by O2 molecules at different oxygen contents. Quantitatively, the absolute number density of Arm is found to be equal to ≈3×108cm−3 in the metallic, and ≈5×107cm−3 in the oxidized regime of sputtering, whereas Ce ground state densities at the similar conditions are found to be few times lower. The absolute densities of species are consistent with the corresponding deposition rates, which decrease sharply during the transition from metallic to poisoned sputtering regime.

Gibbs Density Surface of Fluid Argon: Revised Critical Parameters

A phase diagram of simple fluids, based upon percolation transition loci determined from literature experimental density-pressure isotherms of argon, shows three fluid phases. The liquid phase spans all temperatures from a metastable amorphous ground state to supercritical temperatures. There is a supercritical mesophase of a colloidal nature between two percolation transition loci, PA and PB, that bound the existences of liquid and gas phases, respectively. There is no ‘critical point’ on the Gibbs density surface. When two percolation transitions have the same pressure, i.e., on intersection of the loci in the \(p\)–\(T\) plane, an equilibrium ‘line of critical states’ is thermodynamically defined. This description of criticality yields slightly different values of critical parameters than those hitherto obtained by interpolation of experimental \(p\)–\(\rho \)–\(T\) data from the coexistence region. Revised values of the critical temperature (\(T_\mathrm{c})\) and critical pressure (\(p_\mathrm{c})\) of argon can be obtained from the original high precision thermodynamic measurements for fluid argon, using only data from one-phase supercritical isotherms. Values of coexisting densities at \(T_\mathrm{c}\) and the density loci of the percolation transitions that bound the existence of supercritical gas and liquid phases for \(T>T_\mathrm{c}\) are also reported. Percolation loci, for \(T<T_\mathrm{c}\), extend into the gas–liquid coexistence region to become the spinodal lines that limit the existence of metastable gas and liquid phases.

Comment on “The diatomic dication CuZn2+ in the gas phase” [J. Chem. Phys. 135, 034306 (2011)

In this Comment, the density functional theory (DFT) calculations carried out by Diez et al. [J. Chem. Phys. 135, 034306 (2011)] are revised within the framework of the coupled-cluster single double triple method. These more sophisticated calculations allow us to show that the (2)Σ(+) electronic ground state of CuZn(2+), characterized as the metastable ground state by DFT calculations, is a repulsive state instead. The (2)Δ and (2)Π metastable states of CuZn(2+), on the other hand, should be responsible for the formation mechanism of the dication through the near-resonant electron transfer CuZn(+) + Ar(+) → CuZn(2+) + Ar reaction.

Fluid phases of argon: A debate on the absence of van der Waals’ “critical point”

A phase diagram of argon based upon percolation transition loci determined from literature experimental p-V isotherms, and simulation values using a Lennard-Jones model shows three fluid phases. The liquid phase spans all temperatures, from a metastable amorphous ground state at 0K, to ultra-high T. There is a supercritical mesophase bounded by percolation transition loci, and a gas phase. Intersection of two percolation loci in the p-T plane thermodynamically defines a critical line between two coexisting gas and liquid critical states at T = Tc, and the single mesophase for T > Tc. A debate on the absence of a van der Waals critical point in the Gibbs p-T density surface is appended.

Quantum deflagration and supersonic fronts of tunneling in molecular magnets

The theory of magnetic deflagration taking into account dipolar-controlled spin tunneling has been applied to the realistic model of molecular magnet Mn${}_{12}$Ac. At small transverse field, the front speed $v$ has tunneling maxima on the bias field ${B}_{z}$, reflecting those of the molecular spin's relaxation rate calculated from the density-matrix equation. At high transverse field, spin tunneling directly out of the metastable ground state leads to front speeds that can exceed the speed of sound. Both for the weak and strong transverse fields, the spatial profile of the deflagration front near tunneling resonances shows a front of tunneling that triggers a burning front behind it.

Photoassociative formation of ultracold RbCs molecules in the(2)3Πstate

We have investigated photoassociative formation of RbCs molecules in the ${(2)}^{\phantom{\rule{0.16em}{0ex}}3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ excited state correlated to $v=8$, $(5){0}^{+}$ vibrational level in detail. The metastable ground-state RbCs molecules formed by spontaneous decay are ionized by pulsed dye laser through resonance-enhanced two-photon ionization. A rate equation of the photoionization process is introduced to explain the dependence of RbCs${}^{+}$ molecular ion intensity on the ionization laser intensity. The saturation effect of molecular ion intensity appears as the photoassociation laser intensity increases. The rotational constant and centrifugal distortion constant are derived to be 0.01304 cm${}^{\ensuremath{-}1}$ and 0.000015 cm${}^{\ensuremath{-}1}$ from the photoassociation spectrum with a high sensitivity, respectively. The measured electric dipole moment of the observed $(2){\phantom{\rule{0.16em}{0ex}}}^{3}\phantom{\rule{-0.16em}{0ex}}\ensuremath{\Pi}$ state RbCs molecules is 4.7(6) D by Stark effect in static electric field.