## High-Temperature Superﬂuidity in Double-Bilayer Graphene

A. Perali,^{1} D. Neilson,^{1,2} and A.R. Hamilton^{3 }

^{1}Universita ` di Camerino, 62032 Camerino, Italy

^{2}NEST CNR-INFM, 56127 Pisa, Italy

^{3}School of Physics, University of New South Wales, 2052 Sydney, Australia

(Received 6 September 2012; published 3 April 2013)

Exciton bound states in solids between electrons and holes are predicted to form a superﬂuid at high temperatures. We show that by employing atomically thin crystals such as a pair of adjacent bilayer graphene sheets, equilibrium superﬂuidity of electron-hole pairs should be achievable for the ﬁrst time. The transition temperatures are well above liquid helium temperatures. Because the sample parameters needed for the device have already been attained in similar graphene devices, our work suggests a new route toward realizing high-temperature superﬂuidity in existing quality graphene samples.

DOI: 10.1103/PhysRevLett.110.146803 PACS numbers: 73.21. b, 71.35. y, 73.22.Gk, 74.78.Fk

## Extracting the Condensate Density from Projection Experiments with Fermi Gases

A. Perali, P. Pieri, and G. C. Strinati*Dipartimento di Fisica, Universita` di Camerino, I-62032 Camerino, Italy*

(Received 26 January 2005; published 30 June 2005)

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 **95**, 010407 (2005).

## Quantitative Comparison between Theoretical Predictions and Experimental Results

A. Perali, P. Pieri, and G. C. Strinati*Dipartimento di Fisica, Universita` di Camerino, I-62032 Camerino, Italy*

(Received 5 May 2004; published 3 September 2004)

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 ^{6}Li. 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.

[6] See, e.g., V. M. Loktev, R. M. Quick, and S. G. Sharapov, Phys. Rep.349, 1 (2001).

Physical Review Letters **93**, 100404 (2004).

## BCS-BEC Crossover at Finite Temperature for Superfluid Trapped Fermi Atoms

A. Perali, P. Pieri, L. Pisani, and G. C. Strinati*Dipartimento di Fisica, UdR INFM, Universita` di Camerino, I-62032 Camerino, Italy*

(Received 13 November 2003; published 4 June 2004)

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 **92**, 220404 (2004).

## Pseudogap and spectral function from superconducting fluctuations to the bosonic limit

A. Perali,^{1,2} P. Pieri,^{1} G. C. Strinati,^{1} and C. Castellani^{2}^{1}Dipartimento di Matematica e Fisica, Sezione INFM, Universita` di Camerino, Via Madonna delle Carceri, I-62032 Camerino, Italy^{2}Dipartimento di Fisica, Sezione INFM, Universita` di Roma ‘‘La Sapienza,’’ P.le A. Moro, 2, I-00185 Roma, Italy

[Received 6 February 2002; published 3 July 2002]

The crossover from weak to strong coupling for a three-dimensional continuum model of fermions interact-ing via an attractive contact potential is studied above the superconducting critical temperature T_{c}. 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 at a given wave vector, a broad one at negative frequencies and a narrow one at positive frequencies. The broad peak is asymmetric about its maximum, with its spectral weight decreasing by increasing coupling and temperature. In this regime, two crossover temperatures T_{1}^{*} (at which the two peaks in the spectral function merge in one peak) and T_{0}^{*} (at which the maximum of the lower peak crosses zero frequency) can be identified, with Tc«T_{0}^{*}<T_{1}^{*}. 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 and the two crossover temperatures approach T_{c}. The 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 super-conductors is also attempted.

Physical Review B **66**, 024510 (2002).

## Multi-patch model for transport properties of cuprate superconductors

A. Perali, M. Sindel, and G. Kotliar

Department of Physics and Astronomy, Rutgers University, 136 Frelinghuysen Rd., Piscataway, New Jersey, 08854-8019, USA

Received 3 August 2001 and Received in ﬁnal form 1st November 2001

**Abstract**. A number of normal state transport properties of cuprate superconductors are analyzed in detail using the Boltzmann equation. The momentum dependence of the electronic structure and the strong momentum anisotropy of the electronic scattering are included in a phenomenological way via a multi-patch model. The Brillouin zone and the Fermi surface are divided in regions where scattering between the electrons is strong and the Fermi velocity is low (hot patches) and in regions where the scattering is weak and the Fermi velocity is large (cold patches). We present several motivations for this phenomenology starting from various microscopic approaches. A solution of the Boltzmann equation in the case of N patches is obtained and an expression for the distribution function away from equilibrium is given. Within this framework, and limiting our analysis to the two patches case, the temperature dependence of resistivity, thermoelectric power, Hall angle, magnetoresistance and thermal Hall conductivity are studied in a systematic way analyzing the role of the patch geometry and the temperature dependence of the scattering rates. In the case of Bi-based cuprates, using ARPES data for the electronic structure, and assuming an inter-patch scattering between hot and cold states with a linear temperature dependence, a reasonable agreement with the available experiments is obtained.

European Physical Journal B **24**, 487 (2001).

## d-wave superconductivity near charge instabilities

A. Perali, C. Castellani, C. Di Castro, and M. Grilli

*Istituto di Fisica della Materia e Dipartimento di Fisica, Universita ` di Roma ‘‘La Sapienza,’’ Piazzale A. Moro 2, 00185 Roma, Italy*

[Received 30 May 1996]

We investigate the symmetry of the superconducting order parameter in the proximity of a phase separation or of an incommensurate charge-density-wave instability. The attractive effective interaction at small or inter- mediate transferred momenta is singular near the instability. This strongly *q*-dependent interaction, together with a residual local repulsion between the quasiparticles and an enhanced density of states for band structures appropriate for the high-temperature superconducting oxides, strongly favors the formation of *d*-wave super- conductivity. The relative stability with respect to superconductivity in the *s*-wave channel is discussed in detail, ﬁnding this latter hardly realized in the above conditions. The superconducting temperature is mostly determined by the closeness to the quantum critical point associated with the charge instability and displays a stronger dependence on doping with respect to a simple proximity to a van Hove singularity. The relevance of this scenario and the generic agreement of the resulting phase diagram with the properties displayed by high-temperature superconducting oxides is discussed. [S0163-1829(96)02645-8]

Physical Review B **54**, 16216 (1996).

## THE GAP AMPLIFICATION AT A SHAPE RESONANCE IN A SUPERLATTICE OF QUANTUM STRIPES: A MECHANISM FOR HIGH Tc

A. Perali,^{a} A. Biancoma^{a} A. Lanzara^{a} and N.L. Saini^{b} ^{a}Università di Roma “La Sapienza”, Dipartimento di Fisica, 00185 Roma, Italy ^{b}Istituto Nazionale di Fisica Nucleare, Dipartimento di Fisica, P. Aldo Moro 2, 00185 Roma, Italy

(*Received 16 April 1996; accepted 24 May 1996 by C.N.R. Rao*)

The amplification of the superconducting critical temperature T_{c} from the low temperature range in homogeneous 2D planes (T_{c }< 23 K) to the high temperature range (23 K < T_{c }< 150 K) in an artificial heterostructure of quantum stripes is calculated. The high T_{c} is obtained by tuning the chemical potential µ near the bottom of the nth subband (E_{n}), at a “shape resonance”, in a range µ-E_{n} ~ hω_{D} where hω_{D} is the energy cutoff for the pairing interaction. The resonance for the gap A_{n} at the nth “shape resonance” is studied for a free electron gas in the BCS approximation as a function of the stripe width L, and of the number of electrons p per unit surface. An amplification factor 650 > Δ_{3}/Δ_{∞ }> 6 for coupling 0.1 < λ < 0.3 is obtained at the third shape resonance raising the critical temperature in the high T_{c} range. Copyright © 1996 Elsevier Science Ltd

Solid State Communication. **100**, 181 (1996).