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D. W. Snoke

Publications and source records attributed to D. W. Snoke.

35 records · Page 2Linked to original sources
arXiv2014

A Macroscopic Classical System with Entanglement

It is possible to construct a classical, macroscopic system which has a mathematical structure that is exactly the same as that of a quantum mechanical system and which can be put into a state which is identical to quantum mechanical entanglement. This paper presents a simple example, including a way in which the system can be measured to violate Bell's inequalities. This classical simulation of a quantum system allows us to visualize entanglement and also helps us to see what aspects of quantum mechanical systems are truly nonclassical.

quant-ph↗
arXiv2013

True, quasi and unstable Nambu Goldstone modes of the twodimensional Bose Einstein condensed magnetoexcitons

The collective elementary excitations of two dimensional magnetoexcitons in a Bose Einstein condensate with zero wavevector are investigated in the framework of the Bogoliubov theory of quasi averages. The Hamiltonian of the electrons and holes lying in the lowest Landau levels contains supplementary interactions due to virtual quantum transitions of the particles to the excited Landau levels and back. As a result, the interaction between the magnetoexcitons zero wavenumbers does not vanish and their BEC becomes stable. The energy spectrum contains only one gapless,true Nambu Goldstone mode of the second kind, proportional to the square of wavenumber at small values of wavenumber describing the optical plasmon type oscillations. There are two exciton type branches corresponding to normal and abnormal Green functions. Both modes are gapped with roton type segments at intermediary values of the wavevectors and can be named quasi NG modes. The fourth branch is the acoustical plasmon type mode with absolute instability in the region of small and intermediary values of the wavevectors. All branches have a saturation type dependencies at great values of the wave vectors. The number and the kind of the true NG modes are in accordance with the number of the broken symmetry operators.

cond-mat.mes-hall↗
arXiv2012

Dynamics of Phase Coherence Onset in Bose Condensates of Photons by Incoherent Phonon Emission

Recent experiments with photons equilibrating inside a dye medium in a cavity have raised the question of whether Bose condensation can occur in a system with only incoherent interaction with phonons in a bath but without particle-particle interaction. Analytical calculations analogous to those done for a system with particle-particle interactions indicate that a system of bosons interacting only with incoherent phonons can indeed undergo Bose condensation and furthermore can exhibit spontaneous amplification of quantum coherence. We review the basic theory for these calculations.

cond-mat.quant-gas↗
arXiv2010

The Quantum Boltzmann Equation in Semiconductor Physics

The quantum Boltzmann equation, or Fokker-Planck equation, has been used to successfully explain a number of experiments in semiconductor optics in the past two decades. This paper reviews some of the developments of this work, including models of excitons in bulk materials, electron-hole plasmas, and polariton gases.

cond-mat.stat-mech↗
arXiv2010

Coherence and Optical Emission from Bilayer Exciton Condensates

Experiments aimed at demonstrating Bose-Einstein condensation of excitons in two types of experiments with bilayer structures (coupled quantum wells) are reviewed, with an emphasis on the basic effects. Bose-Einstein condensation implies the existence of a macroscopic coherence, also known as off-diagonal long-range order, and proposed tests and past claims for coherence in these excitonic systems are discussed.

cond-mat.mes-hall↗
arXiv2008

The role of the stress trap in polariton quasiequilibrium condensation in GaAs microcavities

Recent experiments have shown several effects indicative of Bose-Einstein condensation in polaritons in GaAs-based microcavity structures when a harmonic potential trap for the two-dimensional polaritons is created by applied stress. These effects include both real-space and momentum-space narrowing, first-order coherence, and onset of linear polarization above a particle density threshold. Similar effects have been seen in systems without traps, raising the question of how important the role of the trap is in these experiments. In this paper we present results for both trapped conditions and resonant, non-trapped conditions in the same sample. We find that the results are qualitatively different, with two distinct types of transitions. At low density in the trap, the polaritons remain in the strong-coupling regime while going through the threshold for onset of coherence; at higher density, there is a different threshold behavior which occurs with weak coupling and can be identified with lasing; this transition occurs both with and without a trap.

cond-mat.other↗
arXiv2008

Stability of a Bose-Einstein condensate revisited for composite bosons

It is known that elementary bosons condense in a unique state, not so much because this state has the lowest free particle energy but because it costs a macroscopic amount of energy to put the particles into different states which can then interact through quantum particle exchanges. Since individual exchanges between the two fermions of a composite boson are ignored when composite particles are replaced by elementary bosons, it is of importance to reconsider the exchange energy argument for the stability of the Bose-Einstein condensate in the case of composite bosons. We do this here in the light of the new many-body theory which allows us to take exactly into account all possible exchanges between the fermionic components of the composite bosons. We confirm that the condensate of composite bosons is made of a unique state, this state being moreover pure: a coherent superposition of states close in energy is shown to be less favorable for both elementary and composite bosons.

cond-mat.other↗
arXiv2006

Pouring Excitons into a Two-Dimensional Bucket: Equilibration of Indirect Excitons in an In-Plane Harmonic Potential

We have trapped a gas of long-lifetime, high-mobility excitons in an in-plane harmonic potential. Trapping is an important step toward the goal of a controlled Bose-Einstein condensate of excitons. We show that the repulsive interaction between the excitons plays a dominant role in the behavior of the excitons, in contrast to the weak interactions in atomic gases. We show that under proper conditions the excitons thermalize in the trap to a well-defined equilibrium spatial distribution.

cond-mat.str-el↗
arXiv2005

Actively Tuned and Spatially Trapped Polaritons

We report active tuning of the polariton resonance of quantum well excitons in a semiconductor microcavity using applied stress. Starting with the quantum well exciton energy higher than the cavity photon mode, we use stress to reduce the exciton energy and bring it into resonance with the photon mode. At the point of zero detuning, line narrowing and strong increase of the photoluminescence are seen. By the same means, we create an in-plane harmonic potential for the polaritons, which allows trapping, potentially making Bose-Einstein condensation of polaritons analogous to trapped atoms possible. We demonstrate drift of the polaritons into this trap.

cond-mat.other↗
arXiv2005

Long-Distance Diffusion of Excitons in Double Quantum Well Structures

In this Letter we report on lateral diffusion measurements of excitons at low temperature in double quantum wells of various widths. The structure is designed so that excitons live up to 30 $μ$s and diffuse up to 500 $μ$m. Particular attention is given to establishing that the transport occurs by exciton motion. The deduced exciton diffusion coefficients have a very strong well-width dependence, and obey the same power law as the diffusion coefficient for electrons.

cond-mat.other↗
arXiv2004

Trapping Long-Lifetime Excitons in a Two-Dimensional Harmonic Potential

We report an important step forward for the goal of unambiguous observation of Bose-Einstein condensation of excitons in semiconductors. We have demonstrated a system in which excitons live for microseconds, much longer than their thermalization time, move over distances of hundreds of microns, and can be trapped in a harmonic potential exactly analous to the traps for atomic condensates. We also report recent results of a new method for observing evidence of Bose-Einstein condensation, by angular resolution of the emitted luminescence.

cond-mat.str-el↗
arXiv2003

Spin flip from dark to bright states in InP quantum dots

We report measurements of the time for spin flip from dark (non-light emitting) exciton states in quantum dots to bright (light emitting) exciton states in InP quantum dots. Dark excitons are created by two-photon excitation by an ultrafast laser. The time for spin flip between dark and bright states is found to be approximately 200 ps, independent of density and temperature below 70 K. This is much shorter than observed in other quantum dot systems. The rate of decay of the luminescence intensity, approximately 300 ps, is not simply equal to the radiative decay rate from the bright states, because the rate of decay is limited by the rate of conversion from dark excitons into bright excitons. The dependence of the luminescence decay time on the spin flip time is a general effect that applies to many experiments.

cond-mat↗
arXiv2001

Stress Dependence of Exciton Relaxation Processes in Cu2O

A comprehensive study of the exciton relaxation processes in Cu2O has led to some surprises. We find that the ortho-para conversion rate becomes slower at high stress, and that the Auger nonradiative recombination rate increases with stress, with apparently no Auger recombination at zero stress. These results have important consequences for the pursuit of Bose-Einstein condensation of excitons in a harmonic potential.

cond-mat.mtrl-sci↗
arXiv1999

Controlled Drift of Indirect Excitons in Coupled Quantum Wells: Toward Bose Condensation in a Two-Dimensional Trap

We have succeeded in trapping indirect excitons in coupled quantum wells in a harmonic potential minimum via inhomogeneous applied stress and electric field. These excitons exhibit a strong Stark shift (over 60 meV), long lifetime (100 ns), and high diffusivity (1000 cm$^2$/s). This approach is very promising for obtaining the high exciton density needed for Bose condensation of excitons in two dimensions.

cond-mat.str-el↗
arXiv1996

Comparison of Bond Character in Hydrocarbons and Fullerenes

We present a comparison of the bond polarizabilities for carbon-carbon bonds in hydrocarbons and fullerenes, using two different models for the fullerene Raman spectrum and the results of Raman measurements on ethane and ethylene. We find that the polarizabilities for single bonds in fullerenes and hydrocarbons compare well, while the double bonds in fullerenes have greater polarizability than in ethylene.

cond-mat↗
arXiv1995

Hysteresis in the Mott Transition between Plasma and Insulating Gas

We show that hysteresis can occur in the transition between a neutral plasma and the insulating gas consisting of neutral pairs bound by Coulomb attraction. Since the transition depends sensitively on the screening length in the plasma, regions of bistability occur in density--temperature phase space. We present numerical results which indicate where these regions occur for systems such as spin-polarized hydrogen, positronium gas, and excitons in a semiconductor.

cond-mat↗