Publications (154)386.81 Total impact
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ABSTRACT: Using an improved estimator in the loopcluster algorithm, we investigate the constraint effective potential of the magnetization in the spin $\tfrac{1}{2}$ quantum XY model. The numerical results are in excellent agreement with the predictions of the corresponding lowenergy effective field theory. After its lowenergy parameters have been determined with better than permille precision, the effective theory makes accurate predictions for the constraint effective potential which are in excellent agreement with the Monte Carlo data. This shows that the effective theory indeed describes the physics in the lowenergy regime quantitatively correctly.Journal of Statistical Mechanics Theory and Experiment 02/2011; 6(06). · 1.87 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The logarithmic broadening predicted by the systematic lowenergy effective field theory for the confining string has recently been verified in numerical simulations of (2+1)d SU(2) lattice YangMills theory at zero temperature. The same effective theory predicts linear broadening of the string at low nonzero temperature. In this paper, we verify this prediction by comparison with very precise Monte Carlo data. The comparison involves no additional adjustable parameters, because the lowenergy constants of the effective theory have already been fixed at zero temperature. It yields very good agreement between the underlying YangMills theory and the effective string theory.Journal of High Energy Physics 10/2010; 2011(1). · 5.62 Impact Factor 
Article: Topological Lattice Actions
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ABSTRACT: We consider lattice field theories with topological actions, which are invariant against small deformations of the fields. Some of these actions have infinite barriers separating different topological sectors. Topological actions do not have the correct classical continuum limit and they cannot be treated using perturbation theory, but they still yield the correct quantum continuum limit. To show this, we present analytic studies of the 1d O(2) and O(3) model, as well as Monte Carlo simulations of the 2d O(3) model using topological lattice actions. Some topological actions obey and others violate a lattice Schwarz inequality between the action and the topological charge Q. Irrespective of this, in the 2d O(3) model the topological susceptibility \chi_t = \l< Q^2 >/V is logarithmically divergent in the continuum limit. Still, at nonzero distance the correlator of the topological charge density has a finite continuum limit which is consistent with analytic predictions. Our study shows explicitly that some classically important features of an action are irrelevant for reaching the correct quantum continuum limit. Comment: 37 pages, 11 figuresJournal of High Energy Physics 09/2010; · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The color flux tube connecting a static quarkantiquark pair in YangMills theory supports massless transverse fluctuations, which are the Goldstone bosons of spontaneously broken translation invariance. Just as in chiral perturbation theory, the dynamics of these Goldstone bosons is described by a systematic lowenergy effective field theory. We use the effective theory to calculate the width of the fluctuating string at the 2loop level, using both cylindrical and toroidal boundary conditions. At zero temperature, the string width diverges logarithmically with the quarkantiquark distance r. On the other hand, at low but nonzero temperature T = 1/\beta, for r >> \beta, the string width diverges linearly.Journal of High Energy Physics 06/2010; 2010(11). · 5.62 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The 2d spin 1/2 Heisenberg antiferromagnet with exchange coupling $J$ is investigated on a periodic square lattice of spacing $a$ at very small temperatures using the loopcluster algorithm. Monte Carlo data for the staggered and uniform susceptibilities are compared with analytic results obtained in the systematic lowenergy effective field theory for the staggered magnetization order parameter. The lowenergy parameters of the effective theory, i.e.\ the staggered magnetization density ${\cal M}_s = 0.30743(1)/a^2$, the spin stiffness $\rho_s = 0.18081(11) J$, and the spin wave velocity $c = 1.6586(3) J a$ are determined with very high precision. Our study may serve as a test case for the comparison of lattice QCD Monte Carlo data with analytic predictions of the chiral effective theory for pions and nucleons, which is vital for the quantitative understanding of the strong interaction at low energies.01/2010;  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the stability of strings connecting charges Q in the representation {2Q+1} of SU(2) YangMills theory in (2+1) dimensions. While the fundamental {2}string between two charges Q=1/2 is unbreakable and stable, the string connecting static charges transforming under any other representation Q>1/2 is unstable and decays. A charge Q=1 can be completely screened by gluons and so the adjoint {3}string ultimately breaks. A charge Q=3/2 can be only partially screened to a fundamental charge Q=1/2. Thus, stretching a {4}string beyond a critical length, it decays into the stable {2}string by gluon pair creation. The complete breaking of a {5}string happens in two steps, it first decays into a {3}string and then breaks completely. A phenomenological constituent gluon model provides a good quantitative description of the energy of the screened charges at the ends of an unstable string. Comment: 7 pages, 2 figures, contribution to The XXVII International Symposium on Lattice Field Theory, July 2631, 2009, Peking University, Beijing, China10/2009;  [Show abstract] [Hide abstract]
ABSTRACT: We consider a microscopic model for a doped quantum ferromagnet as a test case for the systematic lowenergy effective field theory for magnons and holes, which is constructed in complete analogy to the case of quantum antiferromagnets. In contrast to antiferromagnets, for which the effective field theory approach can be tested only numerically, in the ferromagnetic case both the microscopic and the effective theory can be solved analytically. In this way the lowenergy parameters of the effective theory are determined exactly by matching to the underlying microscopic model. The lowenergy behavior at halffilling as well as in the single and twohole sectors is described exactly by the systematic lowenergy effective field theory. In particular, for weakly bound twohole states the effective field theory even works beyond perturbation theory. This lends strong support to the quantitative success of the systematic lowenergy effective field theory method not only in the ferromagnetic but also in the physically most interesting antiferromagnetic case.Physical review. B, Condensed matter 08/2009; 81(6). · 3.77 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider wave packets of free particles with a general energymomentum dispersion relation $E(p)$. The spreading of the wave packet is determined by the velocity $v = \p_p E$. The positionvelocity uncertainty relation $\Delta x \Delta v \geq {1/2} < \p_p^2 E >$ is saturated by minimal uncertainty wave packets $\Phi(p) = A \exp( \alpha E(p) + \beta p)$. In addition to the standard minimal Gaussian wave packets corresponding to the nonrelativistic dispersion relation $E(p) = p^2/2m$, analytic calculations are presented for the spreading of wave packets with minimal positionvelocity uncertainty product for the lattice dispersion relation $E(p) =  \cos(p a)/m a^2$ as well as for the relativistic dispersion relation $E(p) = \sqrt{p^2 + m^2}$. The boost properties of moving relativistic wave packets as well as the propagation of wave packets in an expanding Universe are also discussed.Annals of Physics 07/2009; 324(12). · 3.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: It is shown that baryon chiral perturbation theory, i.e., the low‐energy effective theory for pions and nucleons in quantum chromodynamics, has its condensed matter analog: A low‐energy effective theory describing magnons as well as holes (or electrons) doped into antiferromagnets. We briefly present a symmetry analysis of the Hubbard and t‐J‐type models, and review the construction of the leading terms in the effective Lagrangian. As a nontrivial application we study different phases of hole‐ and electron‐doped antiferromagnets—in particular, we investigate whether a so‐called spiral phase with an inhomogeneous staggered magnetization (order parameter) may be stable. We would like to emphasize that the effective theory is universal and makes model‐independent predictions for a large class of systems, whereas the material‐specific properties enter the effective theory only through the numerical values of a few low‐energy parameters.AIP Conference Proceedings. 04/2009; 1116(1):356361.  [Show abstract] [Hide abstract]
ABSTRACT: We employ an improved estimator to calculate the constraint effective potential of the staggered magnetization in the spin $\tfrac{1}{2}$ quantum Heisenberg model using a loopcluster algorithm. The first and second moment of the probability distribution of the staggered magnetization are in excellent agreement with the predictions of the systematic lowenergy magnon effective field theory. We also compare the Monte Carlo data with the universal shape of the constraint effective potential of the staggered magnetization and study its approach to the convex effective potential in the infinite volume limit. In this way the higherorder lowenergy parameter $k_0$ is determined from a fit to the numerical data.Journal of Statistical Mechanics Theory and Experiment 01/2009; · 1.87 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Motivated by possible applications to the antiferromagnetic precursor of the hightemperature superconductor Na$_x$CoO$_2\cdot$yH$_2$O, we use a systematic lowenergy effective field theory for magnons and holes to study different phases of doped antiferromagnets on the honeycomb lattice. The effective action contains a leading singlederivative term, similar to the ShraimanSiggia term in the square lattice case, which gives rise to spirals in the staggered magnetization. Depending on the values of the lowenergy parameters, either a homogeneous phase with four or a spiral phase with two filled hole pockets is energetically favored. Unlike in the square lattice case, at leading order the effective action has an accidental continuous spatial rotation symmetry. Consequently, the spiral may point in any direction and is not necessarily aligned with a lattice direction.Physics of Condensed Matter 10/2008; · 1.28 Impact Factor 
Article: Loopcluster simulation of the zeroand onehole sectors of the tJ model on the honeycomb lattice
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ABSTRACT: Inspired by the unhydrated variant of the superconducting material NaxCoO2⋅yH2O at x=1/3, we study the tJ model on a honeycomb lattice by using an efficient loopcluster algorithm. The lowenergy physics of the undoped system and of the singlehole sector is described by a systematic lowenergy effective field theory. The staggered magnetization per spin M̃s=0.2688(3), the spin stiffness ρs=0.102(2)J, the spinwave velocity c=1.297(16)Ja, and the kinetic mass M′ of a hole are obtained by fitting the numerical Monte Carlo data to the effective field theory predictions.Physical Review B 07/2008; 78(21). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A classical particle in a constant magnetic field undergoes cyclotron motion on a circular orbit. At the quantum level, the fact that all classical orbits are closed gives rise to degeneracies in the spectrum. It is wellknown that the spectrum of a charged particle in a constant magnetic field consists of infinitely degenerate Landau levels. Just as for the $1/r$ and $r^2$ potentials, one thus expects some hidden accidental symmetry, in this case with infinitedimensional representations. Indeed, the position of the center of the cyclotron circle plays the role of a RungeLenz vector. After identifying the corresponding accidental symmetry algebra, we reanalyze the system in a finite periodic volume. Interestingly, similar to the quantum mechanical breaking of CP invariance due to the $\theta$vacuum angle in nonAbelian gauge theories, quantum effects due to two selfadjoint extension parameters $\theta_x$ and $\theta_y$ explicitly break the continuous translation invariance of the classical theory. This reduces the symmetry to a discrete magnetic translation group and leads to finite degeneracy. Similar to a particle moving on a cone, a particle in a constant magnetic field shows a very peculiar realization of accidental symmetry in quantum mechanics.Annals of Physics 07/2008; 324(2). · 3.32 Impact Factor 
Article: From an Antiferromagnet to a Valence Bond Solid: Evidence for a First Order Phase Transition
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ABSTRACT: Using a loopcluster algorithm we investigate the spin 1/2 Heisenberg antiferromagnet on a square lattice with exchange coupling $J$ and an additional fourspin interaction of strength $Q$. We confirm the existence of a phase transition separating antiferromagnetism at $J/Q > J_c/Q$ from a valence bond solid (VBS) state at $J/Q < J_c/Q$. Although our Monte Carlo data are consistent with those of previous studies, we do not confirm the existence of a deconfined quantum critical point. Instead, using a flowgram method on lattices as large as $80^2$, we find evidence for a weak first order phase transition. We also present a detailed study of the antiferromagnetic phase. For $J/Q > J_c/Q$ the staggered magnetization, the spin stiffness, and the spinwave velocity of the antiferromagnet are determined by fitting Monte Carlo data to analytic results from the systematic lowenergy effective field theory for magnons. Finally, we also investigate the physics of the VBS state at $J/Q < J_c/Q$, and we show that long but finite antiferromagnetic correlations are still present. Comment: 21 pages, 10 figuresJournal of Statistical Mechanics Theory and Experiment 10/2007; · 1.87 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We consider a particle moving on a cone and bound to its tip by 1/r or harmonic oscillator potentials. When the deficit angle of the cone divided by 2π is a rational number, all bound classical orbits are closed. Correspondingly, the quantum system has accidental degeneracies in the discrete energy spectrum. An accidental SU(2) symmetry is generated by the rotations around the tip of the cone as well as by a Runge–Lenz vector. Remarkably, some of the corresponding multiplets have fractional “spin” and unusual degeneracies.Annals of Physics 07/2007; · 3.32 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We have constructed a systematic lowenergy effective theory for hole and electrondoped antiferromagnets, where holes reside in momentum space pockets centered at $(\pm\frac{\pi}{2a},\pm\frac{\pi}{2a})$ and where electrons live in pockets centered at $(\frac{\pi}{a},0)$ or $(0,\frac{\pi}{a})$. The effective theory is used to investigate the magnonmediated binding between two holes or two electrons in an otherwise undoped system. We derive the onemagnon exchange potential from the effective theory and then solve the corresponding twoquasiparticle Schr\"odinger equation. As a result, we find bound state wave functions that resemble $d_{x^2y^2}$like or $d_{xy}$like symmetry. We also study possible ground states of lightly doped antiferromagnets.Physica B Condensed Matter 07/2007; · 1.28 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: In contrast to holedoped systems which have hole pockets centered at $(\pm \frac{\pi}{2a},\pm \frac{\pi}{2a})$, in lightly electrondoped antiferromagnets the charged quasiparticles reside in momentum space pockets centered at $(\frac{\pi}{a},0)$ or $(0,\frac{\pi}{a})$. This has important consequences for the corresponding lowenergy effective field theory of magnons and electrons which is constructed in this paper. In particular, in contrast to the holedoped case, the magnonmediated forces between two electrons depend on the total momentum $\vec P$ of the pair. For $\vec P = 0$ the onemagnon exchange potential between two electrons at distance $r$ is proportional to $1/r^4$, while in the hole case it has a $1/r^2$ dependence. The effective theory predicts that spiral phases are absent in electrondoped antiferromagnets.Physical Review B 01/2007; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The order parameter of a finite system with a spontaneously broken continuous global symmetry acts as a quantum mechanical rotor. Both antiferromagnets with a spontaneously broken $SU(2)_s$ spin symmetry and massless QCD with a broken $SU(2)_L \times SU(2)_R$ chiral symmetry have rotor spectra when considered in a finite volume. When an electron or hole is doped into an antiferromagnet or when a nucleon is propagating through the QCD vacuum, a Berry phase arises from a monopole field and the angular momentum of the rotor is quantized in halfinteger units.Physical review D: Particles and fields 01/2007;  [Show abstract] [Hide abstract]
ABSTRACT: The Z(N) center symmetry plays an important role in the deconfinement phase transition of SU(N) Yang–Mills theory at finite temperature. The exceptional group G(2) is the smallest simply connected gauge group with a trivial center. Hence, there is no symmetry reason why the low and hightemperature regimes in G(2) Yang–Mills theory should be separated by a phase transition. Still, we present numerical evidence for the presence of a first order deconfinement phase transition at finite temperature. Via the Higgs mechanism, G(2) breaks to its SU(3) subgroup when a scalar field in the fundamental {7} representation acquires a vacuum expectation value v. Varying v we investigate how the G(2) deconfinement transition is related to the one in SU(3) Yang–Mills theory. Interestingly, the two transitions seem to be disconnected. We also discuss a potential dynamical mechanism that may explain this behavior.Nuclear Physics B 01/2007; · 4.33 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Using the lowenergy effective field theory for magnons and holes  the condensed matter analog of baryon chiral perturbation theory for pions and nucleons in QCD  we study different phases of doped antiferromagnets. We systematically investigate configurations of the staggered magnetization that provide a constant background field for doped holes. The most general configuration of this type is either constant itself or it represents a spiral in the staggered magnetization. Depending on the values of the lowenergy parameters, a homogeneous phase, a spiral phase, or an inhomogeneous phase is energetically favored. The reduction of the staggered magnetization upon doping is also investigated.Physical Review B 10/2006; · 3.66 Impact Factor
Publication Stats
3k  Citations  
386.81  Total Impact Points  
Top Journals
Institutions

1992–2014

Universität Bern
 Institute for Theoretical Physics
Berna, Bern, Switzerland


1995–2013

Massachusetts Institute of Technology
 Center for Theoretical Physics
Cambridge, Massachusetts, United States


2007

Università degli Studi di MilanoBicocca
Milano, Lombardy, Italy


2006

Christian Brothers University
Memphis, Tennessee, United States


2004

HumboldtUniversität zu Berlin
Berlín, Berlin, Germany


2002–2004

University of California, San Diego
 Department of Physics
San Diego, CA, United States


2000–2004

Duke University
 Department of Physics
Durham, NC, United States


2003

University of Tsukuba
Tsukuba, Ibaraki, Japan


1996

Boston University
 Department of Physics
Boston, MA, United States


1991

Carl von Ossietzky Universität Oldenburg
Oldenburg, Lower Saxony, Germany


1987–1991

Deutsches ElektronenSynchrotron
 DESY  T Theory Group
Hamburg, Hamburg, Germany 
Universität Heidelberg
 Institute of Theoretical Physics
Heidelburg, BadenWürttemberg, Germany


1987–1989

Universität Hamburg
 II. Institut für Theoretische Physik
Hamburg, Hamburg, Germany
