Biomedical subjects

P W Brouwer

Publications and source records attributed to P W Brouwer.

8 recordsLinked to original sources

PubMed2006

Interference as a probe of spin incoherence in strongly interacting quantum wires.

We show that interference experiments can be used to identify the spin-incoherent regime of strongly interacting one-dimensional conductors. Two qualitative signatures of spin incoherence are found: a strong magnetic field dependence of the interference contrast and an anomalous scaling of the interference contrast with the applied voltage, with a temperature and magnetic field dependent scaling exponent. The experiments distinguish the spin-incoherent from the spin-polarized regime, and so may be useful in deciding between alternative explanations proposed for the anomalous conductance quantization observed in quantum point contacts and quantum wires at low density.

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PubMed2005

Measurement of long-range wave-function correlations in an open microwave billiard.

We investigate the statistical properties of wave functions in an open chaotic cavity. When the number of channels in the openings of the billiard is increased by varying the frequency, wave functions cross over from real to complex. The distribution of the phase rigidity, which characterizes the degree to which a wave function is complex, and long-range correlations of intensity and current density are studied as a function of the number of channels in the openings. All measured quantities are in perfect agreement with theoretical predictions.

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PubMed2004

Exponential sensitivity to dephasing of electrical conduction through a quantum dot.

According to random-matrix theory, interference effects in the conductance of a ballistic chaotic quantum dot should vanish proportional to (tau(phi)/tau(D))(p) when the dephasing time tau(phi) becomes small compared to the mean dwell time tau(D). Aleiner and Larkin have predicted that the power law crosses over to an exponential suppression proportional to exp((-tau(E)/tau(phi)) when tau(phi) drops below the Ehrenfest time tau(E). We report the first observation of this crossover in a computer simulation of universal conductance fluctuations. Their theory also predicts an exponential suppression proportional to exp((-tau(E)/tau(D)) in the absence of dephasing--which is not observed. We show that the effective random-matrix theory proposed previously for quantum dots without dephasing explains both observations.

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PubMed2004

Current-induced transverse spin-wave instability in a thin nanomagnet.

We show that an unpolarized electric current incident perpendicular to the plane of a thin ferromagnet can excite a spin-wave instability transverse to the current direction if source and drain contacts are not symmetric. The instability, which is driven by the current-induced "spin-transfer torque," exists for one current direction only.

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PubMed2002

Density of proper delay times in chaotic and integrable quantum billiards.

We calculate the density P(tau) of the eigenvalues of the Wigner-Smith time delay matrix for two-dimensional rectangular and circular billiards with one opening. For long times, the density of these so-called "proper delay times" decays algebraically, in contradistinction to chaotic quantum billiards for which P(tau) exhibits a long-time cutoff.

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PubMed2001

Fano resonances as a probe of phase coherence in quantum dots.

In the presence of direct trajectories connecting source and drain contacts, the conductance of a quantum dot may exhibit resonances of the Fano type. Since Fano resonances result from the interference of two transmission pathways, their line shape (as described by the Fano parameter q) is sensitive to dephasing in the quantum dot. We show that under certain circumstances the dephasing time can be extracted from a measurement of q for a single resonance. We also show that q fluctuates from level to level, and we calculate its probability distribution for a chaotic quantum dot. Our results are relevant to recent experiments by Göres et al. [Phys. Rev. B 62, 2188 (2000)].

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PubMed2001

Universal gap fluctuations in the superconductor proximity effect.

Random-matrix theory is used to study the mesoscopic fluctuations of the excitation gap in a metal grain or quantum dot induced by the proximity to a superconductor. We propose that the probability distribution of the gap is a universal function in rescaled units. Our analytical prediction for the gap distribution agrees well with exact diagonalization of a model Hamiltonian.

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PubMed1999

High-frequency dynamics of wave localization.

We study the effect of localization on the propagation of a pulse through a multimode disordered waveguide. The correlator of the transmitted wave amplitude u at two frequencies differing by delta omega has for large delta omega the stretched exponential tail proportional to (- square root[tau(D)delta omega/2]). The time constant tau(D)=L(2)/D is given by the diffusion coefficient D, even if the length L of the waveguide is much greater than the localization length xi. Localization has the effect of multiplying the correlator by a frequency-independent factor exp(-L/2 xi), which disappears upon breaking time-reversal symmetry.

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