Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session N15: Focus Session: Quantum Dimers and Bose Einstein Condensation 
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Sponsoring Units: GMAG Chair: Matthew Stone, Oak Ridge National Laboratory Room: Colorado Convention Center Korbel 4E 
Wednesday, March 7, 2007 8:00AM  8:12AM 
N15.00001: Engineering exotic phases for topologically protected quantum computation: Emulating quantum dimer models A.F. Albuquerque, H.G. Katzgraber, M. Troyer, G. Blatter Motivated by recent interest in engineering topologically ordered phases for achieving faulttolerant quantum computation, we analyze an implementation of a quantum dimer model on the triangular lattice using an array of Josephson junctions [L.~B.~Ioffe {\it et al.}, {\it Nature} {\bf 415}, 503 (2002)]. Using the numerical Contractor Renormalization (CORE) technique, we are able to derive in an unbiased way an effective Hamiltonian describing the lowenergy physics of the underlying Bose Hubbard model on the Josephson junction lattice. Our results show that resonances and interactions with three or more dimers have to be included in order to obtain an optimal set of junction capacitances and currents. We discuss the effects of these higherorder terms on the topological dimer liquid phase which is required for faulttolerant quantum computation. Attempts to suppress higherorder dimer operators can only be attained if the junction's capacitances and currents are far beyond values obtainable with current technology. An alternative implementation based on coldatoms loaded into optical lattices is also considered, but in this case the absence of sizable interactions is a major obstacle. Our results suggest that the emulation of topological phases in quantum devices can only be a viable approach if special attention is paid to the design and engineering limits. [Preview Abstract] 
Wednesday, March 7, 2007 8:12AM  8:24AM 
N15.00002: Devil's staircases, quantum dimer models, and stripe formation in strong coupling models of quantum frustration. Kumar Raman, Stefanos Papanikolaou, Eduardo Fradkin We construct a twodimensional microscopic model of interacting quantum dimers that displays an infinite number of periodic striped phases in its T=0 phase diagram. The phases form an incomplete devil's staircase and the period becomes arbitrarily large as the staircase is traversed. The Hamiltonian has purely shortrange interactions, does not break any symmetries, and is generic in that it does not involve the fine tuning of a large number of parameters. Our model, a quantum mechanical analog of the PokrovskyTalapov model of fluctuating domain walls in two dimensional classical statistical mechanics, provides a mechanism by which striped phases with periods large compared to the lattice spacing can, in principle, form in frustrated quantum magnetic systems with only shortranged interactions and no explicitly broken symmetries. Please see condmat/0611390 for more details. [Preview Abstract] 
Wednesday, March 7, 2007 8:24AM  8:36AM 
N15.00003: Quantum Phase Transition of Heisenberg Antiferromagnet with FourSpin Ring Exchange Daoxin Yao, Valeri N. Kotov, Antonio H. Castro Neto, David K. Campbell We discuss the S=1/2 Heisenberg antiferromagnet model on a square lattice with nearestneighbor (J) and plaquette (K) exchanges, which exhibits a quantum phase transition from a spontaneously dimerized phase to N\'eel order at a critical coupling (K/J). We calculate the triplon spectrum starting from the Valence Bond Solid phase and show that good agreement with recent Monte Carlo data (A. W. Sandvik) can be achieved. The quantum phase transition is signaled by vanishing of the triplon gap at the N\'eel vector. We find that strong quantum fluctuations of the dimer background are present, especially near the critical point, which signals a tendency towards restoration of lattice symmetry as evidenced by the strong reduction of the dimerization. Even though within our method full restoration of symmetry is impossible to achieve, the above features are consistent with a critical point exhibiting ``Deconfined quantum criticality,'' of which the present model is believed to be the first example. [Preview Abstract] 
Wednesday, March 7, 2007 8:36AM  8:48AM 
N15.00004: Phase separation and crystallization in doped quantum dimer models Stefanos Papanikolaou, Erik Luijten, Eduardo Fradkin By employing analytical methods and Monte Carlo simulations, we study generalized doped quantum dimer models with exact groundstate wavefunction amplitudes that are given by the weights of generic twodimensional classical partition functions. We derive the phase diagram of these models in the couplingdensity plane. At low doping, liquid and solid phases are separated by continuous transitions, but beyond a tricritical point the transition becomes first order. We focus on the properties of highdoping regions where solidliquid phase coexistence is observed and analyze the fate of these regions under the introduction of additional interactions. 1. S. Papanikolaou, E. Luijten and E. Fradkin, condmat/0607316, 2. S. Papanikolaou, E. Luijten and E. Fradkin (in preparation). [Preview Abstract] 
Wednesday, March 7, 2007 8:48AM  9:00AM 
N15.00005: Exotic Phases of Hardcore Bosons with Correlated Hopping Kai P. Schmidt, Julien Dorier, Andreas Laeuchli, Frederic Mila We investigate the interplay of correlated hopping and repulsive interactions for hardcore bosons on a square lattice using quantum Monte Carlo simulations. The first part of this contribution is on a remarkable lowdensity pairing phase. While for non interacting hardcore bosons the effective attraction induced by the correlated hopping leads to phase separation at low density, we show that a nearestneighbor repulsion suppresses phase separation, leading to a lowdensity pairing phase with no single particle BoseEinstein condensation but longrange twoparticle correlations, signalling a condensation of pairs. The second part of this contribution targets at the physics at higher densities. Here, the most important question is the possible existence of a supersolid phase in such a model with correlated hopping. \newline [1] K.P. Schmidt, J. Dorier, A. Laeuchli, and F. Mila, Phys. Rev. B 74, 174508 (2006). [Preview Abstract] 
Wednesday, March 7, 2007 9:00AM  9:12AM 
N15.00006: BoseEinstein Condensation in quasione dimensional Ladder materials Thierry Giamarchi, Edmond Orignac, Roberta Citro Various magnetic systems are made of assemblies of dimers. In such systems it was proposed [1] that a magnetic field can induce a quantum phase transition in the universality class of a BoseEinstein condensation. Such a transition has been, by now, observed in a variety of dimer systems. Recently, systems such as BPCB [2], where the dimers are organized in quasione dimensional ladders have been investigated. This compounds offers an interesting crossover between one dimensional and three dimensional behavior. We build on [1] and [3] to analyze various physical properties of such a system both in the one and threedimensional regimes. We focus in particular on the NMR relaxation time properties.\newline \newline [1] T. Giamarchi and A. M. Tsvelik Phys. Rev. B {\bf 59} 11398 (1999). \newline [2] B. C. Watson et al. Phys. Rev. Lett {\bf 86} 5168 (2001).\newline [3] R. Chitra and T. Giamarchi Phys. Rev. B {\bf 55} 5816 (1997) [Preview Abstract] 
Wednesday, March 7, 2007 9:12AM  9:24AM 
N15.00007: Excitations from a BoseEinstein condensate of magnons in coupled spin ladders. Andrey Zheludev, Ovidue Garlea, Takatsugu Masuda, Hirotaka Manaka, LouisPierre Regnault, JaeHo Chung, Yiming Qiu, Klaus Habicht, Klaus Kiefer The weakly coupled quasionedimensional spin ladder compound (CH$_3$)$_2$CHNH$_3$CuCl$_3$ is studied by neutron scattering in magnetic fields exceeding the critical field of BoseEinstein condensation of magnons. Commensurate longrange order and the associated Goldstone mode are detected and found to be similar to those in a reference 3D quantum magnet. However, for the upper two massive magnon branches the observed behavior is totally different, culminating in a drastic collapse of excitation bandwidth beyond the transition point. [Preview Abstract] 

N15.00008: ABSTRACT WITHDRAWN 
Wednesday, March 7, 2007 9:36AM  9:48AM 
N15.00009: Magnetic Excitation Spectrum of Doped and Undoped Spin Ladders B. Lake, S. Notbohm, D. A. Tennant, R. I. Bewley, C. D. Frost, P. Manuel, R. S. Eccleston, K. P. Schmidt, G. S. Uhrig, P. Ribeiro, C. Hess, C. Sekar, R. Klingeler, G. Krabbes, G. Behr, B. Buchner We present inelastic neutron scattering measurements of three spinladder compounds. All are based on twodimensional copper oxide layers where the copper ions form twoleg, spin1/2, spinladders. Strong antiferromagnetic interactions couple the spin moments along the legs and rungs of the ladder, weaker higher order fourspin ring exchange terms exist, while interladder coupling is weak and frustrated. Twoleg spin1/2 spinladders are characterized by gapped, welldefined onemagnon excitations and multimagnon continuum excitations. CaCu$_{2}$O$_{3}$ has a weak rung interaction (compared to the leg) and a strong ring interaction which act to drive the system gapless and quantum critical. La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ has a stronger rung and is gapped. Both compounds are undoped and their magnetic excitation spectrum is compared to theoretical models. The third material, Sr$_{2.5}$Ca$_{11.5}$Cu$_{24}$O$_{41}$, is similar to La$_{4}$Sr$_{10}$Cu$_{24}$O$_{41}$ but with holes on the ladder. The holes give rise to significant changes in the excitations which are discussed and compared to theory and the excitation spectrum of highT$_{c}$ superconductors. [Preview Abstract] 
Wednesday, March 7, 2007 9:48AM  10:00AM 
N15.00010: A lowfield disorderedfreemoment phase in sitediluted spingap antiferromagnets Rong Yu, Tommaso Roscilde, Stephan Haas Site dilution of spingapped antiferromagnets leads to localized free moments, which can order antiferromagnetically in two and higher dimensions. A very important question of high experimental relevance is: what is the response of the diluted system to an applied magnetic field? This is a very complicated problem since the exponentially decaying interactions between the free moments introduce a large variety of energy scales, which respond differently to the field. Here we show how a weak magnetic field drives this orderbydisorder state into a novel \emph{disorderedfreemoment} phase, characterized by the formation of local singlets between neighboring moments and by localized moments aligned antiparallel to the field. This disordered phase is characterized by the absence of a gap, as it is the case in a Bose glass. The associated fielddriven quantum phase transition is consistent with the universality of a superfluidtoBoseglass transition. The robustness of the disorderedfreemoment phase and its prominent features, in particular a series of \emph{pseudo}plateaus in the magnetization curve, makes it accessible and relevant to experiments. [Preview Abstract] 
Wednesday, March 7, 2007 10:00AM  10:12AM 
N15.00011: SP (N) Treatment of Frustrated Spin Dimer Systems in Magnetic Field Marianna Maltseva, Rebecca Flint, Piers Coleman We present a Schwinger boson treatment of a frustrated bilayer dimer spin system using a reformulation of the SP (N) approach to frustrated spin systems. Unlike previous SP (N) approaches[1], our starting model is composed uniquely of SP (N) spin generators, which permits a more symmetric treatment of antiferromagnetic and ferromagnetic bonds. We apply our methods to model the spin condensation process that occurs in $Ba CuSi_{2}O_{6}$. One of the issues of particular interest is the dependence of the interlayer orderfromdisorder effects[4] on the applied magnetic field, and the interesting possibility that these couplings vanish at the critical field[2,3]. [1] S. Sachdev, N. Read, International Journal of Modern Physics B 5, 219 (1991). [2] S. E. Sebastian, N. Harrison, C. D. Batista, L. Balicas, M. Jaime, P. A. Sharma, N. Kawashima, I. R. Fisher, Nature 441, pp 617620 (2006). [3] C. D. Batista, J. Schmalian, N. Kawashima, S. E. Sebastian, N. Harrison, M. Jaime, I. R. Fisher, condmat/0608703. [4] M. Maltseva, P. Coleman, Phys. Rev. B 72, 1744159 (2005). [Preview Abstract] 
Wednesday, March 7, 2007 10:12AM  10:24AM 
N15.00012: Complex 2D Oxide BaCuSi$_2$O$_6$: A NMR Study R. Stern, S. Kr\"{a}mer, M. Horvatic, C. Berthier, I. Heinmaa, E. Joon, T. Kimura, S.E. Sebastian, I.R. Fisher BaCuSi$_2$O$_6$ is a quasi2D oxide composed of Cu$_2$Si$_4$O$_{12}$ layers where Cu$^{2+}$ ions are arranged in wellseparated dimers perpendicular to 2D layers. It has a singlet ground state in zero magnetic field, with a large gap to the lowest excited triplet states. Magnetic fields in excess of $H_{c1}$ $\sim$ 23.5 T close the gap, cooling in $H \leq H_{c1}$ results in a state characterized by longrange magnetic order, the nature of which has not been determined yet. We present nuclear magnetic resonance (NMR) measurements of $^{29}$Si and $^{63,65}$Cu on single crystals of BaCuSi$_2$O$_6$ below as well as above $H_{c1}$. Our results prove that the system is less symmetric and more complicated than initially supposed. In the ``normal'' phase we confirmed an IC character of the phase below 100 K. Unexpectedly, two copper sites having strongly different spin polarizations have been observed, which can be associated by the presence of two different gaps and J (J$_1$ and J$_2$) values in the system. Analysis of these data provides a quantitative measure for the size of the perturbation of the ``ideal'' Hamiltonian, helping to decide whether a BECtype description is still possible. [Preview Abstract] 
Wednesday, March 7, 2007 10:24AM  10:36AM 
N15.00013: Magnons dispersion and anisotropies in SrCu$_2$(BO$_3$)$_2$ Olivier Cepas, Y. F. Cheng, P.W. Leung, T. Ziman We study the dispersion of the lowest excited states in the 2d ShastrySutherland system, SrCu$_2$(BO$_3$)$_2$, including all relevant DzyaloshinskiiMoriya interactions. We can reduce the complexity of the general Hamiltonian to a new simpler model at zerofield that is obtained by transformations of the spin operators. The resulting Hamiltonian is studied by means of exact numerical diagonalization on a 32site cluster and the couplings are extracted. The DzyaloshinskiiMoriya interactions affect the dispersion of the magnons (triplet states) to linearorder because they partially lift the frustration of the lattice. We argue that earlier perturbative techniques have overestimated the dispersion and missed the dominant interactions responsible for the dispersion. [Preview Abstract] 
Wednesday, March 7, 2007 10:36AM  10:48AM 
N15.00014: Perturbation Effects in the SpinSinglet State of the TwoDimensional System SrCu$_2$(BO$_3$)$_2$ Adam Aczel, Graeme Luke, Greg MacDougall, Jose Rodriguez, Chris Wiebe, Hanna Dabkowska, Yasutomo Uemura, Peter Russo, Andrei Savici, Hiroshi Kageyama SrCu$_2$(BO$_3$)$_2$ is a quasitwo dimensional spin system with a spinsinglet ground state. This system has attracted much interest recently due to its relevance to the twodimensional ShastrySutherland model. We have performed $\mu$SR studies on single crystals of SrCu$_2$(BO$_3$)$_2$. We observe two different muon sites which we associate with muons located adjacent to the two inequivalent O sites in the system. One site, presumed to be located in the CuOCu superexchange path, exhibits a large increase in the Knight shift with decreasing temperature which is unaffected by the singlet formation, indicating that the muon has locally broken at least one spin singlet bond and created some quasifree spins. Further evidence of this phenomenon is provided by examining our ZF$\mu$SR data, as we observe a large increase in relaxation for the site with the large Knight shift. This is in contrast to the weak, practically temperatureindependent relaxation expected in a singlet state. In this talk, I will present TF, ZF, and LF$\mu$SR data describing this system and explain this perturbation effect in more detail. In addition, results for single crystals doped on the Cu and Sr sites will be presented. [Preview Abstract] 
Wednesday, March 7, 2007 10:48AM  11:00AM 
N15.00015: Signatures of phonon splitting in the infrared spectra of a quantum magnet SrCu$_2$(BO$_3$)$_2$ S.V. Dordevic, C.C. Homes, T. R{\~o}{\~o}m, D. H{\"u}vonen, U. Nagel, A. Gozar, G. Blumberg, A. LaForge, D.N. Basov, N. Drichko, M. Dressel, H. Kageyama Infrared spectroscopy studies of SrCu$_2$(BO$_3$)$_2$ have been performed along both the inplane and caxis crystallographic directions. The reflectance will be reported over a broad range of frequencies (from about 30 cm$^{1}$ to 20,000 cm$^{1}$) and temperatures (from 4.2 K to 300 K). In the inplane spectra we observe a new feature developing at 443 cm$^{1}$ (55 meV) below about 20 K. Detail temperature, magnetic field and polarization dependence of this feature will be reported. All the results point toward close relation of 443 cm$^{1}$ mode with the development of singlet ground state in SrCu$_2$(BO$_3$)$_2$. [Preview Abstract] 
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