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ABSTRACT: Quantum confinement of the perpendicular motion of electrons in single-crystalline metallic superconducting nanofilms splits the conduction band into a series of single-electron subbands. A distinctive feature of such a nanoscale multi-band superconductor is that the energetic position of each subband can vary significantly with changing nanofilm thickness, substrate material, protective cover and other details of the fabrication process. It can occur that the bottom of one of the available subbands is situated in the vicinity of the Fermi level. We demonstrate that the character of the superconducting pairing in such a subband changes dramatically and exhibits a clear molecule-like trend, which is very similar to the well-known crossover from the Bardeen-Cooper-Schrieffer regime to Bose-Einstein condensation (BCS-BEC) observed in trapped ultracold fermions. For Pb nanofilms with thicknesses of 4 and 5 monolayers (MLs) this will lead to a spectacular scenario: up to half of all the Cooper pairs nearly collapse, shrinking in the lateral size (parallel to the nanofilm) down to a few nanometers. As a result, the superconducting condensate will be a coherent mixture of almost molecule-like fermionic pairs with ordinary, extended Cooper pairs.
Journal of Physics Condensed Matter 04/2012; 24(18):185701. · 2.55 Impact Factor
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ABSTRACT: Quantum-size oscillations of the basic physical characteristics of a confined
fermionic condensate are a well-known phenomenon. Its conventional
understanding is based on the single-particle physics, whereby the oscillations
follow the size-dependent changes in the single-particle density of states.
Here we present a study of a cigar-shaped ultracold superfluid Fermi gas, which
demonstrates an important many-body aspect of the quantum-size effects,
overlooked previously. The many-body physics is revealed in the atypical
crossover from the Bardeen-Cooper-Schrieffer (BCS) superfluid to the
Bose-Einstein condensate (BEC) induced by the size quantization of the particle
motion. Quantized perpendicular spectrum results in the formation of
single-particle subbands (shells) so that the aggregate fermionic condensate
becomes a coherent mixture of subband condensates. Each time when the lower
edge of a subband crosses the chemical potential, the BCS-BEC crossover is
approached in this subband, and the aggregate condensate contains both the BCS
and BEC-like components.
03/2012;
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ABSTRACT: The dispersions, weights, and widths of the peaks of the single-particle spectral function in the presence of pair correlations, for a Fermi gas with either attractive or repulsive short-range inter-particle interaction, are determined in the normal phase over a wide range of wave vectors, with a twofold purpose. The first one is to determine how these dispersions identify both an energy scale known as the pseudogap near the Fermi wave vector as well as an additional energy scale related to the contact C at large wave vectors. The second one is to differentiate the behaviors of the repulsive gas from the attractive one in terms of crossing versus avoided crossing of the dispersions near the Fermi wave vector. An analogy will also be drawn between the occurrence of the pseudogap physics in a Fermi gas subject to pair fluctuations and the persistence of local spin waves in the normal phase of magnetic materials.
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ABSTRACT: The dispersions, weights, and widths of the peaks of the single-particle spectral function in the presence of pair correlations, for a Fermi gas with either attractive or repulsive short-range inter-particle interaction, are determined in the normal phase over a wide range of wave vectors, with a twofold purpose. The first one is to determine how these dispersions identify both an energy scale known as the pseudogap near the Fermi wave vector as well as an additional energy scale related to the contact C at large wave vectors. The second one is to differentiate the behaviors of the repulsive gas from the attractive one in terms of crossing versus avoided crossing of the dispersions near the Fermi wave vector. An analogy will also be drawn between the occurrence of the pseudogap physics in a Fermi gas subject to pair fluctuations and the persistence of local spin waves in the normal phase of magnetic materials.
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ABSTRACT: Quantum confinement is known to influence fermionic condensates, resulting in
quantum-size oscillations of superfluid/superconducting properties. Here we
show that the impact of quantum-size effects is even more dramatic. Under
realistic conditions, a significant phase-space reconfiguration induced by
quantum-size effects opens a quasi-molecule channel in the fermionic pairing so
that the condensed pairs exhibit features typical of a molecular state. As an
illustration we consider a quasi-one-dimensional fermionic condensate, as
realized, e.g., in cigar-shaped atomic Fermi gases or superconducting quantum
wires. In this case the transverse quantization of the particle motion favors
pairing through a coherent superposition of quantum channels that are formed
due to the grouping of single-particle levels into a series of well
distinguished subbands. Whenever the bottom of a subband approaches the Fermi
level, the longitudinal spatial distribution of fermions in a condensed pair
becomes strongly localized within the corresponding quantum channel. The
fermionic pairs in this channel resemble molecules with bosonic character.
10/2011;
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ABSTRACT: Radio frequency spectra of a trapped unitary Li-6 gas are reported and analyzed in terms of a theoretical approach that includes both final-state and trap effects. The different strength of the final-state interaction across the trap is crucial for evidencing two main peaks associated with two distinct phases residing in different trap regions. These are the pairing-gap and pseudo-gap phases below the critical temperature T-c, which evolve into the pseudo-gap and no-gap phases above T-c. In this way, a long standing puzzle about the interpretation of rf spectra for Li-6 in a trap is solved.
Physical Review A 07/2011; 84(1):11608. · 2.88 Impact Factor
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ABSTRACT: The dispersions, weights, and widths of the peaks of the single-particle
spectral function in the presence of pair correlations, for a Fermi gas with
either attractive or repulsive short-range inter-particle interaction, are
determined in the normal phase over a wide range of wave vectors, with a
twofold purpose. The first one is to determine how these dispersions identify
both an energy scale known as the pseudo-gap near the Fermi wave vector, as
well as an additional energy scale related to the contact C at large wave
vectors. The second one is to differentiate the behaviors of the repulsive gas
from the attractive one in terms of crossing versus avoided crossing of the
dispersions near the Fermi wave vector. An analogy will also be drawn between
the occurrence of the pseudo-gap physics in a Fermi gas subject to pair
fluctuations and the persistence of local spin waves in the normal phase of
magnetic materials.
06/2011;
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ABSTRACT: Radio frequency spectra of a trapped unitary 6Li gas are reported and
analyzed in terms of a theoretical approach that includes both final-state and
trap effects. Final-state effects play a crucial role in evidencing two main
peaks both above and below the critical temperature Tc as being associated with
two distinct phases that reside in different trap regions. These are the
pairing-gap and pseudo-gap phases below Tc, which evolve into the pseudo-gap
and no-gap phases above Tc. In this way, a long standing puzzle about the
interpretation of rf spectra for 6Li in a trap is solved.
02/2011;
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ABSTRACT: Wave-vector resolved radio frequency spectroscopy data for an ultracold trapped Fermi gas are reported for several couplings at T(c), and extensively analyzed in terms of a pairing-fluctuation theory. We map the evolution of a strongly interacting Fermi gas from the pseudogap phase into a fully gapped molecular Bose gas as a function of the interaction strength, which is marked by a rapid disappearance of a remnant Fermi surface in the single-particle dispersion. We also show that our theory of a pseudogap phase is consistent with a recent experimental observation as well as with quantum Monte Carlo data of thermodynamic quantities of a unitary Fermi gas above T(c).
Physical Review Letters 02/2011; 106(6):060402. · 7.37 Impact Factor
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ABSTRACT: Ultracold atomic Fermi gases present an opportunity to study strongly interacting Fermi systems in a controlled and uncomplicated setting. The ability to tune attractive interactions has led to the discovery of superfluidity in these systems with an extremely high transition temperature, near T/T_F = 0.2. This superfluidity is the electrically neutral analog of superconductivity; however, superfluidity in atomic Fermi gases occurs in the limit of strong interactions and defies a conventional BCS description. For these strong interactions, it is predicted that the onset of pairing and superfluidity can occur at different temperatures. This gives rise to a pseudogap region where, for a range of temperatures, the system retains some of the characteristics of the superfluid phase, such as a BCS-like dispersion and a partially gapped density of states, but does not exhibit superfluidity. By making two independent measurements: the direct observation of pair condensation in momentum space and a measurement of the single-particle spectral function using an analog to photoemission spectroscopy, we directly probe the pseudogap phase. Our measurements reveal a BCS-like dispersion with back-bending near the Fermi wave vector k_F that persists well above the transition temperature for pair condensation.
Nature Physics 07/2010; 6(8):569-573. · 18.97 Impact Factor
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ABSTRACT: Physical properties of an ultracold Fermi gas in the temperature-coupling phase diagram can be characterized by the contact intensity C, which enters the pair-correlation function at short distances and describes how the two-body problem merges into its surrounding. We show that the local order established by pairing fluctuations about the critical temperature Tc of the superfluid transition considerably enhances the contact C in a temperature range where pseudogap phenomena are maximal. Our ab initio results for C in a trap compare well with recently available experimental data over a wide coupling range. An analysis is also provided for the effects of trap averaging on C. Comment: 5 pages, 5 figures
05/2010;
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ABSTRACT: The radio-frequency spectra of ultracold Fermi atoms are calculated by including final-state interactions affecting the excited level of the transition and compared with the experimental data. A competition is revealed between pairing-gap effects which tend to push the oscillator strength toward high frequencies away from threshold and final-state effects which tend instead to pull the oscillator strength toward threshold. As a result of this competition, the position of the peak of the spectra cannot be simply related to the value of the pairing gap, whose extraction thus requires support from theoretical calculations.
Physical Review Letters 02/2008; 100(1):010402. · 7.37 Impact Factor
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ABSTRACT: A debated issue in the physics of the BCS-BEC crossover with trapped Fermi atoms is to identify characteristic properties of the superfluid phase. Recently, a condensate fraction was measured on the BCS side of the crossover by sweeping the system in a fast (nonadiabatic) way from the BCS to the Bose-Einstein condensation (BEC) sides, thus "projecting" the initial many-body state onto a molecular condensate. We analyze here the theoretical implications of these projection experiments, by identifying the appropriate quantum-mechanical operator associated with the measured quantities and relating them to the many-body correlations occurring in the BCS-BEC crossover. Calculations are presented over wide temperature and coupling ranges, by including pairing fluctuations on top of the mean field.
Physical Review Letters 08/2005; 95(1):010407. · 7.37 Impact Factor
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ABSTRACT: Theoretical predictions for the Bardeen-Cooper-Schrieffer-Bose-Einstein condensation crossover of trapped Fermi atoms are compared with recent experimental results for the density profiles of 6Li. The calculations rest on a single theoretical approach that includes pairing fluctuations beyond mean-field. Excellent agreement with experimental results is obtained. Theoretical predictions for the zero-temperature chemical potential and gap at the unitarity limit are also found to compare extremely well with Quantum Monte Carlo simulations and with recent experimental results.
Physical Review Letters 10/2004; 93(10):100404. · 7.37 Impact Factor
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ABSTRACT: We consider the BCS-BEC (Bose-Einstein-condensate) crossover for a system of trapped Fermi atoms at finite temperature, both below and above the superfluid critical temperature, by including fluctuations beyond mean field. We determine the superfluid critical temperature and the pair-breaking temperature as functions of the attractive interaction between Fermi atoms, from the weak- to the strong-coupling limit (where bosonic molecules form as bound-fermion pairs). Density profiles in the trap are also obtained for all temperatures and couplings.
Physical Review Letters 07/2004; 92(22):220404. · 7.37 Impact Factor
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ABSTRACT: We determine the zero-temperature density profile of a cloud of fermionic atoms in a trap subject to a mutual attractive interaction, as the strength of the interaction is progressively increased. We find a significant decrease of the size of the atomic cloud as it evolves from the weak-coupling (BCS) regime of overlapping Cooper pairs to the strong-coupling (Bose-Einstein) regime of non-overlapping bound-fermion pairs. Most significantly, we find a pronounced increase of the value of the density at the center of the trap (even by an order of magnitude) when evolving between the two regimes. Our results are based on a generalized Thomas-Fermi approximation for the superfluid state, that covers continuously all coupling regimes. Comment: 5 pages, 3 postscript figures
12/2002;
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ABSTRACT: The work by Soares et al. [Phys. Rev. B 65, 174506 (2002)] investigates the BCS-BE crossover for d-wave pairing in the 2-dimensional attractive Hubbard model. Contrary to their claims, we found that a non-pairing region does not exist in the density vs coupling phase diagram. The gap parameter at T=0, as obtained by solving analytically as well as numerically the BCS equations, is in fact finite for any non-zero density and coupling, even in the weak-coupling regime. Comment: 7 pages, 1 figure
11/2002;
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ABSTRACT: The crossover from weak to strong coupling for a three dimensional continuum model of fermions interacting via an attractive contact potential is studied above the superconducting critical temperature. The pair-fluctuation propagator, the one-loop self-energy, and the spectral function are investigated in a systematic way from the superconducting fluctuation regime (weak coupling) to the bosonic regime (strong coupling). Analytic and numerical results are reported. In the strong-coupling regime, where the pair fluctuation propagator has bosonic character, two quite different peaks appear in the spectral function, a broad one at negative frequencies and a narrow one at positive frequencies. By decreasing coupling, the two-peak structure evolves smoothly. In the weak-coupling regime, where the fluctuation propagator has diffusive Ginzburg-Landau character, the overall line-shape of the spectral function is more symmetric. The systematic analysis of the spectral function identifies specific features which allow one to distinguish by ARPES whether a system is in the weak- or strong-coupling regime. Connection of the results of our analysis with the phenomenology of cuprate superconductors is also attempted and rests on the recently introduced two-gap model. Comment: 19 pages, 18 figures
02/2002;
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ABSTRACT: A summary is given of the main outcomes of the quantum-critical-point scenario for high-Tc superconductors, developed in the last few years by the Rome group. Phase separation, which commonly occurs in strongly correlated
electronic systems, turns into a stripe instability when Coulomb interaction is taken into account. The stripe phase continuously
connects the high-doping regime, dominated by charge degrees of freedom to the low-doping regime, where spin degrees of freedom
are most relevant. Dynamical stripe fluctuations enslave antiferromagnetic fluctuations at high doping. Critical fluctuations
near the stripe instability mediate a singular interaction between quasi-particles, which is responsible for the non-Fermi
liquid behavior in the metallic phase and for the Cooper pairing with d-wave symmetry in the superconducting phase.
12/2001: pages 45-53;
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A. Malinowski,
Marta Z. Cieplak,
S. Guha,
Q. Wu,
B. Kim,
A. Krickser, A. Perali,
K. Karpińska,
M. Berkowski,
C. H. Shang,
P. Lindenfeld
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ABSTRACT: We have studied the magnetotransport properties in the normal state for a series of La1.85Sr0.15Cu1-yZnyO4 films with values of y between 0 and 0.12. A variable degree of compressive or tensile strain results from the lattice mismatch between the substrate and the film, and affects the transport properties differently from the influence of the zinc impurities. In particular, the orbital magnetoresistance (OMR) varies with y but is strain-independent. The relations for the resistivity (ρ=ρ0+AT) and the Hall angle (cotΘH=αT2+C), and the proportionality between the OMR and tan2ΘH (Δρ/ρ=ζtan2ΘH) are followed above about 70 K. We have been able to separate the strain and impurity effects by rewriting the last two of these relations as cotΘH/α=T2+C/α and Δρ/ρ=(ζ/α2)(α2tan2ΘH), where each term is strain-independent and depends on y only. We also find that changes in the lattice constants give rise to approximately the same fractional changes in A, C, and α, while ρ0 is, in addition, increased by changes in the microstructure. The OMR is more strongly suppressed by the addition of impurities than tan2ΘH, so that ζ decreases as y increases. We conclude that the relaxation rate that governs the Hall effect is not the same as for the magnetoresistance. We also suggest a correspondence between the transport properties and the opening of the pseudogap at a temperature which changes when the La-Sr ratio changes, but does not change with the addition of the zinc impurities. Several theoretical models seem to be in conflict with our results. Some recent ones may be more compatible, but have not been carried sufficiently far for a detailed comparison.
Phys. Rev. B. 08/2001; 66(10).
