Quantum Kinetic Effects Research Articles

Quantum fluctuation of ferroelectric order in polar metals

The polar metallic phase is an unusual metallic phase of matter containing long-range ferroelectric (FE) order in the electronic and atomic structure. Distinct from the typical FE insulating phase, this phase spontaneously breaks the inversion symmetry without global polarization. Unexpectedly, the FE order is found to be dramatically suppressed and destroyed at moderate ~ 10% carrier density. Here, we propose a general mechanism based on carrier-induced quantum fluctuations to explain this puzzling phenomenon. The quantum kinetic effect would drive the formation of polaronic quasi-particles made of the carriers and their surrounding dipoles. The disruption in dipolar directions can therefore weaken or even destroy the FE order. We demonstrate such polaron formation and the associated FE suppression via a concise model using exact diagonalization, perturbation, and quantum Monte Carlo approaches. This quantum mechanism also provides an intuitive picture for many puzzling experimental findings, thereby facilitating new designs of multifunctional FE electronic devices augmented with quantum effects.

H2/D2 Separation Using UTSA-16@CAU-10-H@γ-AlOOH Composites as the Stationary Phase in Gas Chromatography via the Additive Effects of Kinetic Sieving and Chemical Affinity Quantum Sieving.

In this work, CAU-10-H@γ-AlOOH is prepared, and then UTSA-16 is loaded on CAU-10-H@γ-AlOOH to obtain UTSA-16@CAU-10-H@γ-AlOOH. Using the as-prepared composites as stationary materials by cryogenic gas chromatography at 77 K, while CAU-10-H@γ-AlOOH achieves the complete separation of ortho-H2 (o-H2) and D2 with a resolution R of 1.66 and a separation time t of 9.52 min, UTSA-16@CAU-10-H@γ-AlOOH achieves higher efficiency separation of hydrogen isotopes in a shorter separation time (4.56 min) with R = 1.7. Molecular simulation results show that CAU-10-H has both chemical affinity quantum sieving and kinetic sieving effects for H2/D2 at 77 K, and UTSA-16 can only exert the kinetic sieving effect. UTSA-16's load on CAU-10-H@γ-AlOOH weakens the adsorption of hydrogen isotopes, and the presence of Co2+ in UTSA-16 promotes the conversion of para-H2 to ortho-H2. In gas chromatography, H2 was preferentially desorbed from the system due to strong D2 adsorption caused by the chemical affinity quantum sieving effect and faster H2 diffusion caused by the kinetic sieving effect. These additive effects achieved efficient hydrogen isotope separation at 77 K.

Rapid diffusion of H2 and strong adsorption of D2 in Ni-4PyC realized the efficient separation of H2/D2 by gas chromatography.

In this work, Ni-4PyC was selected as the material for the separation of hydrogen isotopes H2/D2, and the mechanism of hydrogen isotope H2/D2 separation was investigated by molecular simulation. The results showed that the adsorption selectivity of Ni-4PyC for D2/H2 increased from 1.26 to 1.46 when the temperature was decreased from 77 K to 20 K, indicating that Ni-4PyC effected chemical affinity quantum sieving on D2/H2. Also, the diffusion rates of H2/D2 were different at different temperatures. At 20 K, the kinetic selectivity of D2/H2 reached 1.30, indicating that Ni-4PyC had a kinetic quantum sieving effect on D2/H2 at low temperatures. However, when the temperature was higher than 30 K, the diffusion rate of H2 was faster than that of D2, and when the temperature was 77 K, Ni-4PyC exhibited the kinetic sieving effect with a kinetic selectivity of 1.59 for H2/D2. Due to the chemical affinity quantum sieving and kinetic sieving effects of Ni-4PyC on H2/D2, the adsorption capacity of Ni-4PyC for D2 was better than that for H2 and the diffusion rate of H2 was faster than that of D2 at 77 K. Therefore, Ni-4PyC was expected to achieve the separation of H2/D2. In order to verify the accuracy of the theoretical calculation results, an Ni-4PyC@γ-Al2O3 composite material was synthesized by a liquid phase epitaxy method and was used to separate H2/D2 at 77 K in a 0.6 m × 2 mm chromatographic column. Under optimal separation conditions, the resolution R reached 1.84 with a separation time t = 7.47 min. In addition, Ni-4PyC@γ-Al2O3 showed excellent separation performances for different ratios of H2/D2 mixtures. The stationary phase was repeatable and reproducible.

Kinetic Isotope Effect in Low-Energy Collisions between Hydrogen Isotopologues and Metastable Helium Atoms: Theoretical Calculations Including the Vibrational Excitation of the Molecule.

We present very accurate theoretical results of Penning ionization rate coefficients of the excited metastable helium atoms (4He(23S) and 3He(23S)) colliding with the hydrogen isotopologues (H2, HD, D2) in the ground and first excited rotational and vibrational states at subkelvin regime. The calculations are performed using the current best ab initio interaction energy surface, which takes into account the nonrigidity effects of the molecule. The results confirm a recently observed substantial quantum kinetic isotope effect (Nat. Chem. 2014, 6, 332–335) and reveal that the change of the rotational or vibrational state of the molecule can strongly enhance or suppress the reaction. Moreover, we demonstrate the mechanism of the appearance and disappearance of resonances in Penning ionization. The additional model computations, with the morphed interaction energy surface and mass, give better insight into the behavior of the resonances and thereby the reaction dynamics under study. Our theoretical findings are compared with all available measurements, and comprehensive data for prospective experiments are provided.

High Temperature Quantum Kinetic Effects in Silicon Nanosandwiches

The negative-U impurity stripes confining the edge channels of semiconductor quantum wells are shown to allow the effective cooling inside in the process of the spin-dependent transport, with the reduction of the electron-electron interaction. The aforesaid promotes also the creation of composite bosons and fermions by the capture of single magnetic flux quanta on the edge channels under the conditions of low sheet density of carriers, thus opening new opportunities for the registration of the high temperature de Haas-van Alphen, 300 K, quantum Hall, 77 K, effects as well as quantum conductance staircase in the silicon sandwich structure.

Trend reversal in the magnetic-field dependence of exciton spin-transfer rates in diluted magnetic semiconductors due to non-Markovian dynamics

We investigate theoretically the influence of an external magnetic field on the spin dynamics of excitons in diluted magnetic semiconductor quantum wells. To this end, we apply a quantum kinetic theory beyond the Markov approximation which reveals that non-Markovian effects can significantly influence the exciton spin dynamics. If the magnetic field is oriented parallel to the growth direction of the well, the Markovian spin-transfer rate decreases monotonically with increasing field as predicted by Fermi's golden rule. The quantum kinetic theory follows this result qualitatively but predicts pronounced quantitative differences in the spin-transfer rate as well as in the long-time spin polarization. However, for an in-plane magnetic field, where the Markovian spin-transfer rate first drops and then increases again, quantum kinetic effects become so pronounced that the Markovian trend is completely reversed. This is made evident by a distinct maximum of the rate followed by a monotonic decrease. The deviations can be traced back to a redistribution of carriers in energy space caused by correlations between excitons and magnetic dopants. The same effect leads to a finite electron-spin polarization at long times in longitudinal as well as transverse fields which is much larger than the corresponding Markovian prediction.

High Temperature Quantum Kinetic Effects in Silicon Nanosandwiches

AbstractThe negative- U impurity stripes confining the edge channels of semiconductor quantum wells are shown to allow the effective cooling inside in the process of the spin-dependent transport, with the reduction of the electron-electron interaction. The aforesaid promotes also the creation of composite bosons and fermions by the capture of single magnetic flux quanta on the edge channels under the conditions of low sheet density of carriers, thus opening new opportunities for the registration of the high temperature de Haas-van Alphen, 300 K, quantum Hall, 77 K, effects as well as quantum conductance staircase in the silicon sandwich structure.

High-temperature quantum kinetic effect in silicon nanosandwiches

The negative-U impurity stripes confining the edge channels of semiconductor quantum wells are shown to allow the effective cooling inside in the process of the spin-dependent transport, with the reduction of the electron-electron interaction. The aforesaid promotes also the creation of composite bosons and fermions by the capture of single magnetic flux quanta on the edge channels under the conditions of low sheet density of carriers, thus opening new opportunities for the registration of the quantum kinetic phenomena in weak magnetic fields at high-temperatures up to the room temperature. As a certain version noted above we present the first findings of the high temperature de Haas-van Alphen, 300 K, quantum Hall, 77 K, effects as well as quantum conductance staircase in the silicon sandwich structure that represents the ultra-narrow, 2 nm, p-type quantum well (Si-QW) confined by the delta barriers heavily doped with boron on the n-type Si (100) surface.

Carrier relaxation in (In,Ga)As quantum dots with magnetic field-induced anharmonic level structure

Sophisticated models have been worked out to explain the fast relaxation of carriers into quantum dot ground states after non-resonant excitation, overcoming the originally proposed phonon bottleneck. We apply a magnetic field along the quantum dot heterostructure growth direction to transform the confined level structure, which can be approximated by a Fock–Darwin spectrum, from a nearly equidistant level spacing at zero field to strong anharmonicity in finite fields. This changeover leaves the ground state carrier population rise time unchanged suggesting that fast relaxation is maintained upon considerable changes of the level spacing. This corroborates recent models explaining the relaxation by polaron formation in combination with quantum kinetic effects.

Accelerating quantum instanton calculations of the kinetic isotope effects.

Path integral implementation of the quantum instanton approximation currently belongs among the most accurate methods for computing quantum rate constants and kinetic isotope effects, but its use has been limited due to the rather high computational cost. Here, we demonstrate that the efficiency of quantum instanton calculations of the kinetic isotope effects can be increased by orders of magnitude by combining two approaches: The convergence to the quantum limit is accelerated by employing high-order path integral factorizations of the Boltzmann operator, while the statistical convergence is improved by implementing virial estimators for relevant quantities. After deriving several new virial estimators for the high-order factorization and evaluating the resulting increase in efficiency, using ⋅Hα + HβHγ → HαHβ + ⋅ Hγ reaction as an example, we apply the proposed method to obtain several kinetic isotope effects on CH4 + ⋅ H ⇌ ⋅ CH3 + H2 forward and backward reactions.

Observation of the isotope effect in sub-kelvin reactions

Quantum phenomena in the translational motion of reactants, which are usually negligible at room temperature, can dominate reaction dynamics at low temperatures. In such cold conditions, even the weak centrifugal force is enough to create a potential barrier that keeps reactants separated. However, reactions may still proceed through tunnelling because, at low temperatures, wave-like properties become important. At certain de Broglie wavelengths, the colliding particles can become trapped in long-lived metastable scattering states, leading to sharp increases in the total reaction rate. Here, we show that these metastable states are responsible for a dramatic, order-of-magnitude-strong, quantum kinetic isotope effect by measuring the absolute Penning ionization reaction rates between hydrogen isotopologues and metastable helium down to 0.01 K. We demonstrate that measurements of a single isotope are insufficient to constrain ab initio calculations, making the kinetic isotope effect in the cold regime necessary to remove ambiguity among possible potential energy surfaces.

Communication: Full dimensional quantum rate coefficients and kinetic isotope effects from ring polymer molecular dynamics for a seven-atom reaction OH + CH4 → CH3 + H2O

The kinetic isotope effect (KIE) of the seven-atom reactions OH + CH4 → CH3 + H2O and OH + CD4 → CD3 + HDO over the temperature range 200-1000 K is investigated using ring polymer molecular dynamics (RPMD) on a full-dimensional potential energy surface. A comparison of RPMD with previous theoretical results obtained using transition state theory shows that RPMD is a more reliable theoretical approach for systems with more than 6 atoms, which provides a predictable level of accuracy. We show that the success of RPMD is a direct result of its independence of the choice of transition state dividing surface, a feature that is not shared by any of the transition state theory-based methods. Our results demonstrate that RPMD is a prospective method for studies of KIEs for polyatomic reactions for which rigorous quantum mechanical calculations are currently impossible.

H/D kinetic isotope effect in HCOOH cis–trans conversion of formic acid in noble gas matrices

Intramolecular tunneling of hydrogen and deuterium atoms in noble gas matrices (Ar, Kr and Xe) has been studied theoretically for cis–trans isomerization of formic acid at low temperatures. The temperature dependence of the tunneling rate constant and quantum kinetic isotope effect taking into account the media reorganization upon the H/D atomic transfer is described in terms of the modified theory of radiationless transitions. The theoretical results are found to be in a reasonable agreement with the literature experimental data on the rate constants and the anomalous quantum kinetic H/D isotope effect.

Transition from excitonic to plasma emission from localized electron‐hole pair states in GaAs‐AlGaAs quantum wells

AbstractWe present a theoretical analysis of the emission of localized excitons in GaAs‐AlGaAs quantum wells, which shows a strong nonlinear behavior with increasing excitation. Considering the influence of dynamical screening both on the one‐particle properties of carriers and on the whole spectrum of electron‐hole pair states, we are able to explain the nonlinearity as a transition of the emission from excitonic to electron‐hole pair states in the continuum (electron‐hole plasma). Moreover, our theoretical approach based on the quasi‐particle approximation for the carriers states and quantum kinetic effects in the screening describes the observed changes of the shift of the exciton energy from higher to lower energies at a temperature of T = 10 K (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Mott transition of excitons in ZnSe studied by phase resolved reflection

AbstractThe Mott transition of excitons in a semiconductor with increasing carrier density is in principle well understood as a consequence of screening of the Coulomb interaction between carriers. While the position of the exciton stays widely unchanged the exciton peak disappears due to band gap shrinkage. However, a more quantitative check of applied theoretical models for the screening and of the role of quantum kinetic effects in this process is still open.We demonstrate that the phase‐resolved reflection in shallow‐confined ZnSSe heterostructures opens the possibility of a detailed study of the Mott transition due to the appearance of pronounced interferences effects of propagating polariton modes.Our theoretical approach for the investigation of the influence of excited carriers on the dielectric susceptibility is based (i) on a quasi‐particle approximation for the carrier energies and damping, and (ii) on the semiconductor Bloch equations including dynamical screening and a quantum kinetic treatment of scattering. These manybody effects lead to drastic changes of both amplitude and phase of the reflected light: Jumps of the phase are steepened, show abrupt changes from +π to –π and are smoothed out, and interference peaks in the amplitude decrease and disappear completely with increasing excitation. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Viability of Dirac phase leptogenesis

We discuss the conditions for a non-vanishing Dirac phaseδ and mixingangle θ13,sources of CP violation in neutrino oscillations, to be uniquely responsible for the observed matter–antimatterasymmetry of the Universe through leptogenesis. We show that this scenario, that we callδ-leptogenesis, is viable when the degenerate limit for the heavy right-handed(RH) neutrino spectrum is considered. We derive an interesting joint condition onsinθ13 and the absolute neutrino mass scale that can be tested in future neutrino oscillation experiments.In the limit of the hierarchical heavy RH neutrino spectrum, we strengthen the previous result thatδ-leptogenesis is only very marginally allowed, even when the production from the twoheavier RH neutrinos is taken into account. An improved experimental upper bound onsinθ13 and/oran account of quantum kinetic effects could completely rule out this option in the future. Therefore,δ-leptogenesis can be also regarded as motivation for models with degenerate heavy neutrinospectrum.

Quantum kinetic effects in the optical absorption of semiconductor quantum-dot systems

A microscopic theory is used to study the optical properties of semiconductor quantum dots. The dephasing of the coherent polarization due to carrier-carrier Coulomb interaction and carrier-phonon interaction is determined from quantum kinetic equations. We investigate the density dependence of the dephasing mechanisms, and compare the relevance of various interaction processes. The failure of frequently used approximations based on the GKBA with free single-particle energies is demonstrated for pure dephasing processes involving only the localized quantum-dot states.

Ultrafast electron-phonon interaction of intersubband transitions: Quantum kinetics from adiabatic following to Rabi-oscillations

The interaction of electrons with LO phonons provides an important mechanism of optical dephasing and carrier scattering for the two-dimensional electron gas in semiconductor quantum wells. In this paper, the corresponding ultrafast nonlinearities for off-resonant and resonant intersubband excitations are investigated. Quantum kinetic effects of the electron-phonon interaction and the corresponding violation of the microscopic energy conservation yield a qualitative different picture compared to the standard Markovian theory, if the phonon energy is larger than the intersubband-gap energy.

Quantum kinetics and linear optical response of semiconductors

We investigate the influence of quantum-kinetic effects on the linear optical properties of semiconductors near the band edge. These effects arise from a non-Markovian treatment of the scattering terms in the semiconductor Bloch equations and result in a strong dependence of many-body effects on the energy. We compare with results using a Markovian approximation of scattering, where the many-body effects do not depend on energy and correspond to those in a quasi-particle approximation. We demonstrate, that within such a treatment both linewidth and shift of the exciton are strongly overestimated. Finally, the Mott density of excitons differs by a factor of two. The importance of the quantum-kinetic approach for the explanation of polariton-pulse propagation experiments is shown.

Influence of electron–phonon scattering on optical absorption in ZnSe

We investigate the influence of quantum kinetic effects connected with the scattering of carriers with plasmons and LO-phonons on the excitonic absorption in ZnSe within a linear response theory. Within our approach the quantum-kinetic effects are reflected in a dependence of the many-body effects on the energy. In contrast to earlier treatments we use a unified description of the longitudinal dielectric function, including coupled plasmon and LO-phonon modes. We present a way to handle the well known polaron contributions in order to provide the correct excitonic features at low excitation. We find, that after elimination of the polaron contributions the phonons only weakly influence the interband self-energy for small densities at about 1014 − 1015 cm−3 in comparison to the plasmons. With increasing density structures around the LO-phonon-energy ± ωLO appear. These are more pronounced inside the band, but increasingly affect the region of excitonic absorption too. Surprisingly there is a stronger coupling of plasmon and phonon modes as expected from the pure dispersions.