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Recent documents in SJSU ScholarWorksen-usSun, 26 Mar 2017 01:42:25 PDT3600Spartan Daily, March 23, 2017
http://scholarworks.sjsu.edu/spartan_daily_2017/24
http://scholarworks.sjsu.edu/spartan_daily_2017/24Thu, 23 Mar 2017 07:42:31 PDT
Volume 148, Issue 25
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San Jose State University, School of Journalism and Mass CommunicationsSpartan Daily, March 21, 2017
http://scholarworks.sjsu.edu/spartandaily/12016
http://scholarworks.sjsu.edu/spartandaily/12016Wed, 22 Mar 2017 12:19:37 PDT
Volume 148, Issue 23
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San Jose State University, School of Journalism and Mass CommunicationsSpartan Daily, March 22, 2017
http://scholarworks.sjsu.edu/spartan_daily_2017/23
http://scholarworks.sjsu.edu/spartan_daily_2017/23Wed, 22 Mar 2017 07:53:19 PDT
Volume 148, Issue 24
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San Jose State University, School of Journalism and Mass CommunicationsDetection of a Distinct Metal-Poor Stellar Halo in the Early-Type Galaxy NGC 3115*
http://scholarworks.sjsu.edu/physics_astron_pub/159
http://scholarworks.sjsu.edu/physics_astron_pub/159Fri, 17 Mar 2017 14:40:34 PDT
We present the resolved stellar populations in the inner and outer halo of the nearby lenticular galaxy NGC 3115. Using deep Hubble Space Telescope observations, we analyze stars 2 mag fainter than the tip of the red giant branch (TRGB). We study three fields along the minor axis of this galaxy, 19, 37, and 54 kpc from its center—corresponding to 7, 14, and 21 effective radii (re ). Even at these large galactocentric distances, all of the fields are dominated by a relatively enriched population, with the main peak in the metallicity distribution decreasing with radius from [Z/H] ~ –0.5 to –0.65. The fraction of metal-poor stars ([Z/H] < –0.95) increases from 17% at 16-37 kpc to 28% at ~54 kpc. We observe a distinct low-metallicity population (peaked at [Z/H] ~ –1.3 and with total mass 2 × 1010 M ☉ ~ 14% of the galaxy's stellar mass) and argue that this represents the detection of an underlying low-metallicity stellar halo. Such halos are generally predicted by galaxy formation theories and have been observed in several late-type galaxies, including the Milky Way and M31. The metallicity and spatial distribution of the stellar halo of NGC 3115 are consistent with the galaxy's globular cluster system, which has a similar low-metallicity population that becomes dominant at these large radii. This finding supports the use of globular clusters as bright chemodynamical tracers of galaxy halos. These data also allow us to make a precise measurement of the magnitude of the TRGB, from which we derive a distance modulus of NGC 3115 of 30.05 ± 0.05 ± 0.10sys (10.2 ± 0.2 ± 0.5sys Mpc).
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Mark Peacock et al.Discovery of an Ultra-Diffuse Galaxy in the Pisces-Perseus Supercluster
http://scholarworks.sjsu.edu/physics_astron_pub/158
http://scholarworks.sjsu.edu/physics_astron_pub/158Fri, 17 Mar 2017 14:40:29 PDT
We report the discovery of DGSAT I, an ultra-diffuse, quenched galaxy located 10fdg4 in projection from the Andromeda galaxy (M31). This low-surface brightness galaxy (μV = 24.8 mag arcsec−2), found with a small amateur telescope, appears unresolved in sub-arcsecond archival Subaru/Suprime-Cam images, and hence has been missed by optical surveys relying on resolved star counts, in spite of its relatively large effective radius (Re(V) = 12'') and proximity (15') to the well-known dwarf spheroidal galaxy And II. Its red color (V − I = 1.0), shallow Sérsic index (nV = 0.68), and the absence of detectable Hα emission are typical properties of dwarf spheroidal galaxies and suggest that it is mainly composed of old stars. Initially interpreted as an interesting case of an isolated dwarf spheroidal galaxy in the local universe, our radial velocity measurement obtained with the BTA 6 m telescope (Vh = 5450 ± 40 km s−1) shows that this system is an M31-background galaxy associated with the filament of the Pisces-Perseus supercluster. At the distance of this cluster (~78 Mpc), DGSAT I would have an Re ~ 4.7 kpc and MV ~ −16.3. Its properties resemble those of the ultra-diffuse galaxies (UDGs) recently discovered in the Coma cluster. DGSAT I is the first case of these rare UDGs found in this galaxy cluster. Unlike the UDGs associated with the Coma and Virgo clusters, DGSAT I is found in a much lower density environment, which provides a fresh constraint on the formation mechanisms for this intriguing class of galaxy.
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David Martínez-Delgado et al.Metallicity and Age of the Stellar Stream Around the Disk Galaxy NGC 5907
http://scholarworks.sjsu.edu/physics_astron_pub/157
http://scholarworks.sjsu.edu/physics_astron_pub/157Fri, 17 Mar 2017 14:40:25 PDT
Stellar streams have become central to studies of the interaction histories of nearby galaxies. To characterize the most prominent parts of the stellar stream around the well-known nearby (d = 17 Mpc) edge-on disk galaxy NGC 5907, we have obtained and analyzed new, deep gri Subaru/Suprime-Cam and 3.6 μm Spitzer/Infrared Array Camera observations. Combining the near-infrared 3.6 μm data with visible-light images allows us to use a long wavelength baseline to estimate the metallicity and age of the stellar population along an ~60 kpc long segment of the stream. We have fitted the stellar spectral energy distribution with a single-burst stellar population synthesis model and we use it to distinguish between the proposed satellite accretion and minor/major merger formation models of the stellar stream around this galaxy. We conclude that a massive minor merger (stellar mass ratio of at least 1:8) can best account for the metallicity of −0.3 inferred along the brightest parts of the stream.
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Seppo Laine et al.Numerical linked-cluster expansion for the distorted kagome lattice Heisenberg model
http://scholarworks.sjsu.edu/physics_astron_pub/156
http://scholarworks.sjsu.edu/physics_astron_pub/156Thu, 16 Mar 2017 14:13:01 PDT
Motivated by experimental results for the thermodynamic properties of the Rb2Cu3SnF12 material and the discovery of its valence-bond solid ground state, we utilize the numerical linked-cluster expansions (NLCEs) and devise an expansion tailored to solve the Heisenberg model on a pinwheel-distorted kagome lattice. Using the exchange interactions that are relevant to Rb2Cu3SnF12, we calculate its uniform spin susceptibility and find a very good agreement with experiment. Next, we focus on the ground state of a simplified model of the distorted kagome lattice and take advantage of a zero-temperature Lanczos-based NLCE to study the approach to the ground state of the uniform kagome lattice Heisenberg model using a tuning parameter. Together with results from exact diagonalization of finite clusters, we find evidence that a phase transition occurs before the uniform limit is reached.
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Ehsan Khatami et al.Fluctuation-dissipation theorem in isolated quantum systems out of equilibrium
http://scholarworks.sjsu.edu/physics_astron_pub/155
http://scholarworks.sjsu.edu/physics_astron_pub/155Thu, 16 Mar 2017 14:12:59 PDT
We study the validity of the fluctuation-dissipation theorem for an isolated quantum system of harmonically trapped dipolar molecules taken out of equilibrium by means of a quench, a sudden change in the Hamiltonian parameters. We find that the integrability of the system plays a crucial role in the validity of the fluctuation-dissipation theorem. Namely, the system thermalizes according to the eigenstate thermalization hypothesis and the theorem holds if the system is nonintegrable after the quench. However, it fails if the system is integrable, unless the initial state is an eigenstate of a nonintegrable Hamiltonian, in which case the system still thermalizes despite the eigenstate thermalization hypothesis failing to describe it.
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Ehsan Khatami et al.QUEST: QUantum Electron Simulation Toolbox
http://scholarworks.sjsu.edu/physics_astron_pub/154
http://scholarworks.sjsu.edu/physics_astron_pub/154Thu, 16 Mar 2017 14:12:55 PDT
QUEST is a part of the SciDAC project on next generation multi-scale quantum simulation software for strongly correlated materials. It is a Fortran 90/95 package that implements the determinant quantum Monte Carlo (DQMC) method for simulation of magnetic, superconducting, and metal-insulator transitions in model Hamiltonians. In this paper, we show how QUEST is capable of treating lattices of unprecedentedly large sizes and how this can be fruitful in the study of the physics of trapped fermionic system, in the development of more efficient solvers for Dynamical Mean Field Theory (DMFT) and as a tool to test and, in the future, improve diagrammatic approaches such as the Parquet approximation. We will also present a range of synergistic activities on the development of stable and robust numerical algorithms and hybrid granularity parallelization scheme that combines algorithmic and implementation techniques to high-performance DQMC simulation. The work reported here is a key step forward in achieving the goals of our SciDAC project.
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C.-R. Lee et al.Spartan Daily, March 16, 2017
http://scholarworks.sjsu.edu/spartan_daily_2017/22
http://scholarworks.sjsu.edu/spartan_daily_2017/22Thu, 16 Mar 2017 08:08:48 PDT
Volume 148, Issue 22
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San Jose State University, School of Journalism and Mass CommunicationsA short introduction to numerical linked-cluster expansions
http://scholarworks.sjsu.edu/physics_astron_pub/153
http://scholarworks.sjsu.edu/physics_astron_pub/153Wed, 15 Mar 2017 17:22:19 PDT
We provide a pedagogical introduction to numerical linked-cluster expansions (NLCEs). We sketch the algorithm for generic Hamiltonians that only connect nearest-neighbor sites in a finite cluster with open boundary conditions. We then compare results for a specific model, the Heisenberg model, in each order of the NLCE with the ones for the finite cluster calculated directly by means of full exact diagonalization. We discuss how to reduce the computational cost of the NLCE calculations by taking into account symmetries and topologies of the linked clusters. Finally, we generalize the algorithm to the thermodynamic limit, and discuss several numerical resummation techniques that can be used to accelerate the convergence of the series.
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Baoming Tang et al.Observation of antiferromagnetic correlations in the Hubbard model with ultracold atoms
http://scholarworks.sjsu.edu/physics_astron_pub/152
http://scholarworks.sjsu.edu/physics_astron_pub/152Wed, 15 Mar 2017 17:22:16 PDT
Ultracold atoms in optical lattices have great potential to contribute to a better understanding of some of the most important issues in many-body physics, such as high-temperature superconductivity. The Hubbard model—a simplified representation of fermions moving on a periodic lattice—is thought to describe the essential details of copper oxide superconductivity. This model describes many of the features shared by the copper oxides, including an interaction-driven Mott insulating state and an antiferromagnetic (AFM) state. Optical lattices filled with a two-spin-component Fermi gas of ultracold atoms can faithfully realize the Hubbard model with readily tunable parameters, and thus provide a platform for the systematic exploration of its phase diagram. Realization of strongly correlated phases, however, has been hindered by the need to cool the atoms to temperatures as low as the magnetic exchange energy, and also by the lack of reliable thermometry. Here we demonstrate spin-sensitive Bragg scattering of light to measure AFM spin correlations in a realization of the three-dimensional Hubbard model at temperatures down to 1.4 times that of the AFM phase transition. This temperature regime is beyond the range of validity of a simple high-temperature series expansion, which brings our experiment close to the limit of the capabilities of current numerical techniques, particularly at metallic densities. We reach these low temperatures using a compensated optical lattice technique, in which the confinement of each lattice beam is compensated by a blue-detuned laser beam. The temperature of the atoms in the lattice is deduced by comparing the light scattering to determinant quantum Monte Carlo simulations and numerical linked-cluster expansion calculations. Further refinement of the compensated lattice may produce even lower temperatures which, along with light scattering thermometry, would open avenues for producing and characterizing other novel quantum states of matter, such as the pseudogap regime and correlated metallic states of the two-dimensional Hubbard model.
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Russell Hart et al.Observation of spatial charge and spin correlations in the 2D Fermi-Hubbard model
http://scholarworks.sjsu.edu/physics_astron_pub/151
http://scholarworks.sjsu.edu/physics_astron_pub/151Wed, 15 Mar 2017 17:22:13 PDT
Strong electron correlations lie at the origin of high-temperature superconductivity. Its essence is believed to be captured by the Fermi-Hubbard model of repulsively interacting fermions on a lattice. Here we report on the site-resolved observation of charge and spin correlations in the two-dimensional (2D) Fermi-Hubbard model realized with ultracold atoms. Antiferromagnetic spin correlations are maximal at half-filling and weaken monotonically upon doping. At large doping, nearest-neighbor correlations between singly charged sites are negative, revealing the formation of a correlation hole, the suppressed probability of finding two fermions near each other. As the doping is reduced, the correlations become positive, signaling strong bunching of doublons and holes, in agreement with numerical calculations. The dynamics of the doublon-hole correlations should play an important role for transport in the Fermi-Hubbard model.
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Lawrence Cheuk et al.Quantum criticality and incipient phase separation in the thermodynamic properties of the Hubbard model
http://scholarworks.sjsu.edu/physics_astron_pub/150
http://scholarworks.sjsu.edu/physics_astron_pub/150Wed, 15 Mar 2017 17:22:10 PDT
Transport measurements on the cuprates suggest the presence of a quantum critical point (QCP) hiding underneath the superconducting dome near optimal hole doping. We provide numerical evidence in support of this scenario via a dynamical cluster quantum Monte Carlo study of the extended two-dimensional Hubbard model. Single-particle quantities, such as the spectral function, the quasi-particle weight and the entropy, display a crossover between two distinct ground states: a Fermi liquid at low filling and a non-Fermi liquid with a pseudo-gap at high filling. Both states are found to cross over to a marginal Fermi-liquid state at higher temperatures. For finite next-nearest-neighbour hopping t′, we find a classical critical point at temperature Tc. This classical critical point is found to be associated with a phase-separation transition between a compressible Mott gas and an incompressible Mott liquid corresponding to the Fermi liquid and the pseudo-gap state, respectively. Since the critical temperature Tc extrapolates to zero as t′ vanishes, we conclude that a QCP connects the Fermi liquid to the pseudo-gap region, and that the marginal Fermi-liquid behaviour in its vicinity is the analogue of the supercritical region in the liquid–gas transition.
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D. Galanakis et al.Three-dimensional Hubbard model in the thermodynamic limit
http://scholarworks.sjsu.edu/physics_astron_pub/149
http://scholarworks.sjsu.edu/physics_astron_pub/149Wed, 15 Mar 2017 16:25:30 PDT
We employ the numerical linked-cluster expansion to study finite-temperature properties of the uniform cubic lattice Hubbard model in the thermodynamic limit for a wide range of interaction strengths and densities. We carry out the expansion to the 9th order and find that the convergence of the series extends to lower temperatures as the strength of the interaction increases, giving us access to regions of the parameter space that are difficult to reach by most other numerical methods. We study the precise trends in the specific heat, the double occupancy, and magnetic correlations at temperatures as low as 0.2 of the hopping amplitude in the strong-coupling regime. We show that in this regime, accurate estimates for transition temperatures to the Néel ordered phase, in agreement with the predicted asymptotic behavior, can be deduced from the low-temperature magnetic structure factor. We also find evidence for possible instability to the magnetically ordered phase away from, but close to, half filling. Our results have important implications for parametrizing fermionic systems in optical lattice experiments and for benchmarking other numerical methods.
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Ehsan KhatamiFinite-temperature superconducting correlations of the Hubbard model
http://scholarworks.sjsu.edu/physics_astron_pub/148
http://scholarworks.sjsu.edu/physics_astron_pub/148Wed, 15 Mar 2017 16:25:27 PDT
We utilize numerical linked-cluster expansions (NLCEs) and the determinantal quantum Monte Carlo algorithm to study pairing correlations in the square-lattice Hubbard model. To benchmark the NLCE, we first locate the finite-temperature phase transition of the attractive model to a superconducting state away from half filling. We then explore the superconducting properties of the repulsive model for the d-wave and extended s-wave pairing symmetries. The pairing structure factor shows a strong tendency to d-wave pairing and peaks at an interaction strength comparable to the bandwidth. The extended s-wave structure factor and correlation length are larger at higher temperatures but clearly saturate as temperature is lowered, whereas the d-wave counterparts, which start off lower at high temperatures, continue to rise near half filling. This rise is even more dramatic in the d-wave susceptibility. The convergence of NLCEs breaks down as the susceptibilities and correlation lengths become large, so we are unable to determine the onset of long-range order. However, our results extend the conclusion, previously restricted to only magnetic and charge correlations, that NLCEs offer a unique window into pairing in the Hubbard model at strong coupling.
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Ehsan Khatami et al.Thermodynamics of the antiferromagnetic Heisenberg model on the checkerboard lattice
http://scholarworks.sjsu.edu/physics_astron_pub/147
http://scholarworks.sjsu.edu/physics_astron_pub/147Wed, 15 Mar 2017 16:25:24 PDT
Employing numerical linked-cluster expansions (NLCEs) along with exact diagonalizations of finite clusters with periodic boundary condition, we study the energy, specific heat, entropy, and various susceptibilities of the antiferromagnetic Heisenberg model on the checkerboard lattice. NLCEs, combined with extrapolation techniques, allow us to access temperatures much lower than those accessible to exact diagonalization and other series expansions. We show that the high-temperature peak in specific heat decreases as the frustration increases, consistent with the large amount of unquenched entropy in the region around maximum classical frustration, where the nearest-neighbor and next-nearest-neighbor exchange interactions (J and J′, respectively) have the same strength, and with the formation of a second peak at lower temperatures. The staggered susceptibility shows a change of character when J′ increases beyond 0.75J, implying the disappearance of the antiferromagnetic order at low temperatures. For J′=4J, in the limit of weakly coupled crossed chains, we find large susceptibilities for stripe and Néel order with Q=(π/2,π/2) at intermediate temperatures. Other magnetic and bond orderings, such as a plaquette valence-bond solid and a crossed-dimer order suggested by previous studies, are also investigated.
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Ehsan Khatami et al.Thermodynamics of strongly interacting fermions in two-dimensional optical lattices
http://scholarworks.sjsu.edu/physics_astron_pub/146
http://scholarworks.sjsu.edu/physics_astron_pub/146Wed, 15 Mar 2017 16:25:22 PDT
We study finite-temperature properties of strongly correlated fermions in two-dimensional optical lattices by means of numerical linked cluster expansions, a computational technique that allows one to obtain exact results in the thermodynamic limit. We focus our analysis on the strongly interacting regime, where the on-site repulsion is of the order of or greater than the band width. We compute the equation of state, double occupancy, entropy, uniform susceptibility, and spin correlations for temperatures that are similar to or below the ones achieved in current optical lattice experiments. We provide a quantitative analysis of adiabatic cooling of trapped fermions in two dimensions, by means of both flattening the trapping potential and increasing the interaction strength.
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Ehsan Khatami et al.Effect of particle statistics in strongly correlated two-dimensional Hubbard models
http://scholarworks.sjsu.edu/physics_astron_pub/145
http://scholarworks.sjsu.edu/physics_astron_pub/145Wed, 15 Mar 2017 16:25:20 PDT
We study the onset of particle statistics effects as the temperature is lowered in strongly correlated two-dimensional Hubbard models. We utilize numerical linked-cluster expansions and focus on the properties of interacting lattice fermions and two-component hard-core bosons. In the weak-coupling regime, where the ground state of the bosonic system is a superfluid, the thermodynamic properties of the two systems at half filling exhibit very large differences even at high temperatures. In the strong-coupling regime, where the low-temperature behavior is governed by a Mott insulator for either particle statistics, the agreement between the thermodynamic properties of both systems extends to regions where the antiferromagnetic (iso)spin correlations are exponentially large. We analyze how particle statistics affects adiabatic cooling in those systems.
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Ehsan Khatami et al.Thermodynamics and phase transitions for the Heisenberg model on the pinwheel distorted kagome lattice
http://scholarworks.sjsu.edu/physics_astron_pub/144
http://scholarworks.sjsu.edu/physics_astron_pub/144Wed, 15 Mar 2017 16:25:17 PDT
We study the Heisenberg model on the pinwheel distorted kagome lattice as observed in the material Rb2Cu3SnF12. Experimentally relevant thermodynamic properties at finite temperatures are computed utilizing numerical linked-cluster expansions. We also develop a Lanczos-based, zero-temperature, numerical linked-cluster expansion to study the approach of the pinwheel distorted lattice to the uniform kagome-lattice Heisenberg model. We find strong evidence for a phase transition before the uniform limit is reached, implying that the ground state of the kagome-lattice Heisenberg model is likely not pinwheel dimerized and is stable to finite pinwheel-dimerizing perturbations.
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Ehsan Khatami et al.