We present release 2.0 of the ALPS (Algorithms and Libraries for Physics Simulations) project, an open source software project to develop libraries and application programs for the simulation of strongly correlated quantum lattice models such as quantum magnets, lattice bosons, and strongly correlated fermion systems. The code development is centered on common XML and HDF5 data formats, libraries to simplify and speed up code development, common evaluation and plotting tools, and simulation programs. The programs enable non-experts to start carrying out serial or parallel numerical simulations by providing basic implementations of the important algorithms for quantum lattice models: classical and quantum Monte Carlo (QMC) using non-local updates, extended ensemble simulations, exact and full diagonalization (ED), the density matrix renormalization group (DMRG) both in a static version and a dynamic time-evolving block decimation (TEBD) code, and quantum Monte Carlo solvers for dynamical mean field theory (DMFT). The ALPS libraries provide a powerful framework for programers to develop their own applications, which, for instance, greatly simplify the steps of porting a serial code onto a parallel, distributed memory machine. Major changes in release 2.0 include the use of HDF5 for binary data, evaluation tools in Python, support for the Windows operating system, the use of CMake as build system and binary installation packages for Mac OS X and Windows, and integration with the VisTrails workflow provenance tool. The software is available from our web server at http://alps.comp-phys.org/.

We perform a quantitative simulation of the repulsive Fermi-Hubbard model using an ultracold gas trapped in an optical lattice. The entropy of the system is determined by comparing accurate measurements of the equilibrium double occupancy with theoretical calculations over a wide range of parameters. We demonstrate the applicability of both high-temperature series and dynamical mean-field theory to obtain quantitative agreement with the experimental data. The reliability of the entropy determination is confirmed by a comprehensive analysis of all systematic errors. In the center of the Mott insulating cloud we obtain an entropy per atom as low as 0:77k B which is about twice as large as the entropy at the Néel transition. The corresponding temperature depends on the atom number and for small fillings reaches values on the order of the tunneling energy. Experimental progress in the field of atomic quantum gases has led to a new approach to quantum many-body physics. In particular, the combination of quantum degenerate and strongly interacting Fermi gases [1,2] with optically induced lattice potentials [3] now allows the study of a centerpiece of quantum condensed matter physics, the Fermi-Hubbard model [4]. The high level of control over the atomic systems has led to the concept of quantum simulation, which for the case of the Fermi-Hubbard model is expected to provide answers to intriguing open questions of frustrated magnetism and d-wave superfluidity [5]. Recent experiments [6,7] have indeed demonstrated that the strongly correlated regime of the repulsive FermiHubbard model is experimentally accessible and the emergence of a Mott insulating state has been observed. In this Letter, we succeed in performing a quantitative simulation of the Fermi-Hubbard model using cold atoms. The level of precision of the experiment enables us to determine the entropy and the temperature of the system, and thereby to quantify the approach to the low temperature phases.The main challenge for the quantum simulation of the Fermi-Hubbard model is a further reduction in temperature. Here the lack of a quantitative thermometry method in the lattice is a key obstacle. For strongly correlated bosonic systems thermometry has recently been demonstrated by direct comparison with quantum Monte Carlo simulations [8] or by using the boundary of two spin polarized clouds [9]. In the fermionic case, previous methods to determine the temperature could be used only in limiting regimes of the Hubbard model, namely, the noninteracting [10,11] and zero-tunneling [6,12] regimes. However, intermediate interactions are most interesting for quantum simulation of the Fermi-Hubbard model and no reliable thermometry method has been available up to now.In both the metallic and Mott insulating regimes an accurate measurement of the double occupancy provides direct access to thermal excitations. We analyze the crossover from thermal creation of double occupancies to thermal depletion which is unique to a trapped system (see Fig. 1). The variability of the d...

Cold atom optical lattices typically simulate zero-range Hubbard models. We discuss the theoretical possibility of using excited states of optical lattices to generate extended range Hubbard models. We find that bosons confined to higher bands of optical lattices allow for a rich phase diagram, including the supersolid phase. Using Gutzwiller, mean-field theory we establish the parameter regime necessary to maintain metastable states generated by an extended Bose-Hubbard model.

Bulk Bi2Te3 is known to be a topological insulator. We investigate surface states of Bi2Te3(111) thin films of one to six quintuple layers using density-functional theory including spin-orbit coupling. We construct a method to identify topologically protected surface states of thin film topological insulators. Applying this method to Bi2Te3 thin films, we find that the topological nature of the surface states remains robust with the film thickness and that the films of three or more quintuple layers have topologically nontrivial surface states, which agrees with experiments.

An understanding of the physics of the half-filled lowest Landau level has been achieved in terms of a Fermi sea of composite fermions, but the nature of the state at other even-denominator fractions has remained unclear. We investigate theoretically Landau level fillings of the form nu = (2n + 1)/(4n + 4), which correspond to composite-fermion fillings nu* = n + 1/2. By considering various plausible candidate states with complete spin polarization, we rule out the composite-fermion Fermi sea and paired composite-fermion state at these filling factors, and predict that the system phase separates into stripes of n and n + 1 filled Landau levels of composite fermions.

The rich correlation physics in two-dimensional (2D) electron systems is governed by the dispersion of its excitations. In the fractional quantum Hall regime, excitations involve fractionally charged quasi particles, which exhibit dispersion minima at large momenta referred to as rotons. These rotons are difficult to access with conventional techniques because of the lack of penetration depth or sample volume. Our method overcomes the limitations of conventional methods and traces the dispersion of excitations across momentum space for buried systems involving small material volume. We used surface acoustic waves, launched across the 2D system, to allow incident radiation to trigger these excitations at large momenta. Optics probed their resonant absorption. Our technique unveils the full dispersion of such excitations of several prominent correlated ground states of the 2D electron system, which has so far been inaccessible for experimentation.

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We suggest a technique for the observation of a predicted supersolid phase in extended Bose-Hubbard models which are potentially realizable in cold atom optical lattice systems. In particular, we discuss important subtleties arising from the existence of the trapping potential which leads to an externally imposed (as opposed to spontaneous) breaking of translational invariance. We show, by carefully including the trapping potential in our theoretical formalism, that noise correlations could prove instrumental in identifying the supersolid and density wave phases. We also find that the noise correlation peak width scales inversely with the relative size of trapped Mott domains.