A single down spin Fermion with an attractive, zero range interaction with a Fermi sea of up-spin Fermions forms a polaronic quasiparticle. The associated quasiparticle weight vanishes beyond a critical strength of the attractive interaction, where a many-body bound state is formed. From a variational wavefunction in the molecular limit, we determine the critical value for the polaron to molecule transition. The value agrees well with the diagrammatic Monte Carlo results of Prokof'ev and Svistunov and is consistent with recent rf-spectroscopy measurements of the quasiparticle weight by Schirotzek et. al. In addition, we calculate the contact coefficient of the strongly imbalanced gas, using the adiabatic theorem of Tan and discuss the implications of the polaron to molecule transition for the phase diagram of the attractive Fermi gas at finite imbalance.

We present a calculation of the spectral functions and the associated rf response of ultracold fermionic atoms near a Feshbach resonance. The single particle spectra are peaked at energies that can be modeled by a modified BCS dispersion. However, even at very low temperatures their width is comparable to their energy, except for a small region around the dispersion minimum. The structure of the excitation spectrum of the unitary gas at infinite scattering length agrees with recent momentum-resolved rf spectra near the critical temperature. A detailed comparison is made with momentum integrated, locally resolved rf spectra of the unitary gas at arbitrary temperatures and shows very good agreement between theory and experiment. The pair size defined from the width of these spectra is found to coincide with that obtained from the leading gradient corrections to the effective field theory of the superfluid.

We study the unitary time evolution of antiferromagnetic order in anisotropic Heisenberg chains that are initially prepared in a pure quantum state far from equilibrium. Our analysis indicates that the antiferromagnetic order imprinted in the initial state vanishes exponentially. Depending on the anisotropy parameter, oscillatory or non-oscillatory relaxation dynamics is observed. Furthermore, the corresponding relaxation time exhibits a minimum at the critical point, in contrast to the usual notion of critical slowing down, from which a maximum is expected.Introduction. Experiments with ultracold atoms offer a highly controlled environment for investigating open questions of quantum magnetism. In particular, coherent spin dynamics in a lattice of double wells has been observed in recent experiments, which have demonstrated remarkable precision in tuning magnetic exchange interactions [1]. The ability to observe quantum dynamics over long time intervals allows one to study strongly correlated states from a new perspective. The idea is to prepare the system in a simple quantum state which, in general, is not an eigenstate of the Hamiltonian, and investigate the dynamics that follows. In the two-spin system, studied in [1], the dynamics is completely tractable and describes simple oscillations between a singlet and a triplet states.In the present paper we investigate how the nature of the dynamics changes in the case of a macroscopic number of spins interacting via nearest neighbor magnetic exchange. Are there new effects, and in particular new time scales, dynamically generated by the complex many-body evolution? Our starting point for investigating this question is the spin-

We consider two-dimensional metals near a Pomeranchuk instability which breaks 90 • lattice rotation symmetry. Such metals realize strongly-coupled non-Fermi liquids with critical fluctuations of an Isingnematic order. At low temperatures, impurity scattering provides the dominant source of momentum relaxation, and hence a non-zero electrical resistivity. We use the memory matrix method to compute the resistivity of this non-Fermi liquid to second order in the impurity potential, without assuming the existence of quasiparticles. Impurity scattering in the d-wave channel acts as a random "field" on the Ising-nematic order. We find contributions to the resistivity with a nearly linear temperature dependence, along with more singular terms; the most singular is the random-field contribution which diverges in the limit of zero temperature.

We propose a quantum dimer model for the metallic state of the hole-doped cuprates at low hole density, p. The Hilbert space is spanned by spinless, neutral, bosonic dimers and spin S = 1=2, charge +e fermionic dimers. The model realizes a "fractionalized Fermi liquid" with no symmetry breaking and small hole pocket Fermi surfaces enclosing a total area determined by p. Exact diagonalization, on lattices of sizes up to 8 × 8, shows anisotropic quasiparticle residue around the pocket Fermi surfaces. We discuss the relationship to experiments.T he recent experimental progress in determining the phase diagram of the hole-doped Cu-based high-temperature superconductors has highlighted the unusual and remarkable properties of the pseudogap (PG) metal (Fig. 1). A characterizing feature of this phase is a depletion of the electronic density of states at the Fermi energy (1, 2), anisotropically distributed in momentum space, that persists up to room temperature.Attempts have been made to explain the pseudogap metal using thermally fluctuating order parameters; we argue below that such approaches are difficult to reconcile with recent transport experiments. Instead, we introduce a new microscopic model that realizes an alternative perspective (3), in which the pseudogap metal is a finite temperature (T) realization of an interesting quantum state: the fractionalized Fermi liquid (FL*). We show that our model is consistent with key features of the pseudogap metal observed by both transport and spectroscopic probes.The crucial observation that motivates our work is the tension between photoemission and transport experiments. In the cuprates, the hole density p is conventionally measured relative to that of the insulating antiferromagnet (AF), which has one electron per site in the Cu d band. Therefore, the hole density relative to a filled Cu band, with two electrons per site, is actually 1 + p. In fact, photoemission experiments at large hole doping observe a Fermi surface enclosing an area determined by the hole density 1 + p (4), in agreement with the Luttinger relation. In contrast, in the pseudogap metal, a mysterious "Fermi arc" spectrum is observed (5-7), with no clear evidence of closed Fermi surfaces. However, despite this unusual spectroscopic feature, transport measurements report vanilla Fermi liquid properties, but associated with carrier density p, rather than 1 + p. The carrier density of p was indicated directly in Hall measurements (8), whereas other early experiments indicated suppression of the Drude weight (9-11). Although the latter could be compatible with a carrier density of 1 + p but with a suppressed kinetic term, the Hall measurements indicate the suppression of the Drude weight is more likely due to a small carrier density. Two recent experiments displaying Fermi liquid behavior at low p are especially notable: (i) the quasiparticle lifetime τðω, TÞ determined from optical conductivity experiments (12) has the Fermi liquidlike dependence 1=τ ∝ ðZωÞ 2 + ðcπk B TÞ 2 , with c an order unity constan...

We show that strong pairing correlations in Fermi gases lead to the appearance of a gaplike structure in the rf spectrum, both in the balanced superfluid and in the normal phase above the Clogston-Chandrasekhar limit. The average rf shift of a unitary gas is proportional to the ratio of the Fermi velocity and the scattering length with the final state. In the strongly imbalanced case, the rf spectrum measures the binding energy of a minority atom to the Fermi sea of majority atoms. Our results provide a qualitative understanding of recent experiments by Schunck et al.

Recent experimental achievements in controlling ultracold gases in optical lattices open a new perspective on quantum many-body physics. In these experimental setups it is possible to study coherent time evolution of isolated quantum systems. These dynamics reveal new physics beyond the low-energy properties usually relevant in solid-state many-body systems. In this paper we study the time evolution of antiferromagnetic order in the Heisenberg chain after a sudden change of the anisotropy parameter, using various numerical and analytical methods. As a generic result we find that the order parameter, which can show oscillatory or nonoscillatory dynamics, decays exponentially except for the effectively non-interacting case of the XX limit. For weakly ordered initial states we also find evidence for an algebraic correction to the exponential law. The study is based on numerical simulations using a numerical matrix product method for infinite system sizes (iMPS), for which we provide a detailed description and an error analysis. Additionally, we investigate in detail the exactly solvable XX limit. These results are compared to approximative analytical approaches including an effective description by the XZ-model as well as by mean-field, Luttinger-liquid and sine-Gordon theories. This reveals which aspects of non-equilibrium dynamics can as in equilibrium be described by low-energy theories and which are the novel phenomena specific to quantum quench dynamics. The relevance of the energetically high part of the spectrum is illustrated by means of a full numerical diagonalization of the Hamiltonian. Contents

We present a projective symmetry group (PSG) analysis of the spinless excitations of Z 2 spin liquids on the kagome lattice. In the simplest case, vortices carrying Z 2 magnetic flux ('visons') are shown to transform under the 48 element group GL(2, Z 3 ). Alternative exchange couplings can also lead to a second case with visons transforming under 288 element group GL(2, Z 3 ) × D 3 . We study the quantum phase transition in which visons condense into confining states with valence bond solid order. The critical field theories and confining states are classified using the vison PSGs.