arXiv subjects
Publications and source records attributed to J. Quintanilla.
We present a mean field theory of the Pomeranchuk instability in two dimensions, starting from a generic central interaction potential described in terms of a few microscopic parameters. For a significant range of parameters, the instability is found to be pre-empted by a first-order quantum phase transition. We provide the ground state phase diagram in terms of our generic parameters.
We discuss the emergence of symmetry-breaking {\it via} the Pomeranchuk instability from interactions that respect the underlying point-group symmetry. We use a variational mean-field theory to consider a 2D continuum and a square lattice. We describe two experimental signatures: a symmetry-breaking pattern of Friedel oscillations around an impurity; and a structural transition.
We address at the mean field level the emergence of a Pomeranchuk instability in a uniform Fermi liquid with \emph{central} particle-particle interactions. We find that Pomeranchuk instabilities with all symmetries except $l=1$ can take place if the interaction is repulsive and has a finite range $r_{0}$ of the order of the inter-particle distance. We demonstrate this by solving the mean field equations analytically for an explicit model interaction, as well as numerical results for more realistic potentials. We find in addition to the Pomeranchuk instability other, subtler phase transitions in which the Fermi surface changes topology without rotational symmetry-breaking. We argue that such interaction-driven topological transitions may be as generic to such systems as the Pomeranchuk instability.
We address the problem posed by the inhomogeneous trapping fields when using ultracold fermions to simulate strongly correlated electrons. As a starting point, we calculate the density of states for a single atom. Using semiclassical arguments, we show that this can be made to evolve smoothly towards the desired limit by varying the curvature of the field profile. Implications for mutually interacting atoms in such potentials are briefly discussed.
We consider a single atom in an optical lattice, subject to a harmonic trapping potential. The problem is treated in the tight-binding approximation, with an extra parameter κdenoting the strength of the harmonic trap. It is shown that the κ\to 0 limit of this problem is singular, in the sense that the density of states for a very shallow trap (κ\to 0) is {\it qualitatively different} from that of a translationally invariant lattice (κ= 0). The physics of this difference is discussed, and densities of states and wave functions are exhibited and explained.
We revisit the old problem of exotic superconductivity as Cooper pairing with finite angular momentum emerging from a central potential. Based on some general considerations, we suggest that the phenomenonn is associated with interactions that keep electrons at some particular, finite distance (r_{0}), and occurs at a range of intermediate densities (n\sim 1/r_{0}^{3}). We discuss the ground state and the critical temperature in the framework of a standard functional-integral theory of the BCS to Bose crossover. We find that, due to the lower energy of two-body bound states with (l=0), the rotational symmetry of the ground state is always restored on approaching the Bose limit. Moreover in that limit the critical temperature is always higher for pairs with (l=0.) The breaking of the rotational symmetry of the continuum by the superfluid state thus seems to be a property of weakly-attractive, non-monotonic interaction potentials, at intermediate densities.
In recent photodoping experiments Decca et al. [PRL 85, 3708 (2000)] have observed an unusually long-ranged proximity effect between the superconducting and insulating phases of YBCO. Here we critically examine the claim in that reference that such long range is anomalous. We show that, once the novel nature of the experiments has been properly accounted for, the conventional theory of the proximity effect predicts comparably long ranges.
In the context of a simple model featuring an explicit, central interaction potential, and using a standard functional-integral technique, we study superconductivity with angular momentum quantum number l=2 as an emergent property of the many-body system. Our interaction potential is attractive at a finite distance r_0, and the breaking of the rotational symmetry is the result of an interplay between r_0 and the inter-particle distance r_s which we deem generic to interactions of this type. However such interplay, responsible for the preference of a d-wave state for a range of intermediate densities, takes place only in the BCS limit. In contrast, as the Bose-Einstein (BE) limit is approached the internal energy of the "preformed pairs" becomes the dominant contribution and there is a quantum phase transition in which the s-wave symmetry is restored. We also find that the limiting value of the critical temperature is k_B T_c --> 3.315(\hbar^2/2m*)[n/2(2l+1)]^{2/3}, which coincides with the usual result only for l=0; for l>0, it differs in the degeneracy factor 1/(2l+1), which lowers T_c. Our results thus place constraints on exotic pairing in the BE limit, while at the same time indicating a particularly interesting route to pairing with l>0 in a BCS superconductor.
Recent experiments indicate that the excitation spectrum of the cuprates is characterised, in the superconducting state, by two energy scales: the ``coherence energy'' Δ_c and the ``pseudogap'' Δ_p. Here we consider a simple generalisation of the BCS model that yields exotic pairing and can describe, phenomenologically, the generic trends in the critical temperature T_c of cuprate superconductors. We use the model to predict the gap in the single-particle spectrum arising from the superconductivity and we find evidence that it corresponds to the lower of the two energy scales, Δ_c, seen in the experiments. This supports the view that the origin of the pseudogap is not superconducting fluctuations.
We study a simple BCS-like model which describes unconventional superconductivity on the basis of an electron-electron attraction corresponding to the delta-shell potential: $U(r) = -g δ(r-r_0)$. It predicts a $T_c$ vs. doping behavior similar to that characteristic of the High-$T_c$ cuprates.