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Michael Thies

Publications and source records attributed to Michael Thies.

At least 37 records · Page 2Linked to original sources
arXiv2014

Semiclassical time crystal in the chiral Gross-Neveu model

In the limit of a large number of flavors, the ground state of the chiral Gross-Neveu model at finite fermion density exhibits a spatially periodic mean field in the form of the chiral spiral, thereby breaking translational invariance. Here we show that the ground state of the same model at finite fermion current density gives rise to a mean field which is periodic in time, a temporal chiral spiral. Since the current density is the same as axial charge density in two dimensions and axial charge is conserved, this may serve as an example of a time crystal. More specifically, as mean field theory is invoked in the large N limit, we are dealing with a semiclassical time crystal.

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arXiv2014

Integrable Gross-Neveu models with fermion-fermion and fermion-antifermion pairing

The massless Gross-Neveu and chiral Gross-Neveu models are well known examples of integrable quantum field theories in 1+1 dimensions. We address the question whether integrability is preserved if one either replaces the four-fermion interaction in fermion-antifermion channels by a dual interaction in fermion-fermion channels, or if one adds such a dual interaction to an existing integrable model. The relativistic Hartree-Fock-Bogoliubov approach is adequate to deal with the large N limit of such models. In this way, we construct and solve three integrable models with Cooper pairing. We also identify a candidate for a fourth integrable model with maximal kinematic symmetry, the "perfect" Gross-Neveu model. This type of field theories can serve as exactly solvable toy models for color superconductivity in quantum chromodynamics.

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arXiv2013

Full time-dependent Hartree-Fock solution of large N Gross-Neveu models

We find the general solution to the time-dependent Hartree-Fock problem for scattering solutions of the Gross-Neveu models, with both discrete (GN_2) and continuous (NJL_2) chiral symmetry. We find new multi-breather solutions both for the GN_2 model, generalizing the Dashen-Hasslacher-Neveu breather solution, and also new twisted breathers for the NJL_2 model. These solutions satisfy the full TDHF consistency conditions, and only in the special cases of GN_2 kink scattering do these conditions reduce to the integrable Sinh-Gordon equation. We also show that all baryons and breathers are composed of constituent twisted kinks of the NJL_2 model. Our solution depends crucially on a general class of transparent, time dependent Dirac potentials found recently by algebraic methods.

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arXiv2013

Transparent Dirac potentials in one dimension: the time-dependent case

We generalize the original derivation of transparent, static Schroedinger potentials by Kay and Moses, to obtain a large class of time-dependent transparent Dirac potentials in one spatial dimension. They contain all known transparent potentials as special cases and play a key role in the semi-classical solution of 1+1 dimensional, fermionic quantum field theories of Gross-Neveu and Nambu-Jona-Lasinio type.

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arXiv2013

Breathers and their interaction in the massless Gross-Neveu model

The breather is a vibrating multifermion bound state of the massless Gross-Neveu model, originally found by Dashen, Hasslacher and Neveu in the large N limit. We exhibit the salient features of this state and confirm that it solves the relativistic time-dependent Hartree-Fock equations. We then solve the scattering problem of two breathers with arbitrary internal parameters and velocities, generalizing an ansatz recently developed for the baryon-baryon scattering problem in the same model. The exact analytical solution is given and illustrated with a few examples.

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arXiv2012

Evidence for factorized scattering of composite states in the Gross-Neveu model

Scattering of two baryons in the large-N Gross-Neveu model via the time-dependent Dirac-Hartree-Fock approach has recently been solved in closed analytical form. Here, we generalize this result to scattering processes involving any number and complexity of the scatterers. The result is extrapolated from the solution of few baryon problems, found via a joint ansatz for the scalar mean field and the Dirac spinors, and presented in analytical form. It has been verified numerically for up to 8-baryon problems so far, but a full mathematical proof is still missing. Examples shown include the analogue of proton-nucleus and nucleus-nucleus scattering in this toy model. All the parameters of the general result can be fixed by one- and two-baryon input only. We take this finding as evidence for factorized scattering, but on the level of composite multi-fermion states rather than elementary fermions.

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arXiv2011

Baryon-baryon scattering in the Gross-Neveu model: the large N solution

The scattering of two Dashen-Hasslacher-Neveu (DHN) baryons in the large N Gross-Neveu model is solved exactly using the relativistic, time-dependent Hartree-Fock approach. Unlike the special case of kink-antikink scattering, the scattering of DHN baryons is sensitive to the back-reaction of the fermions bound inside the baryons. Correspondingly the solution is much more complicated than the kink-antikink scattering solutions, which can be expressed in terms of sinh-Gordon solitons. Nevertheless, we present a simple ansatz form that gives closed analytic expressions for both the space-time dependent mean fields and the Dirac spinors for all continuum and bound states. The solution can also be applied to the scattering of polarons and solitons in conducting polymers.

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arXiv2011

Exact solution of N baryon problem in the Gross-Neveu model

Recently it was shown that kink baryons and kink-antikink scattering in the massless Gross-Neveu model are closely related to one- and two-soliton solutions of the sinh-Gordon equation. Here we generalize these findings to the case of n kinks and antikinks. Using the known n soliton solution of the sinh-Gordon equation, we solve the general n kink-antikink scattering problem in the large N Gross-Neveu model analytically, mapping the time-dependent Hartree-Fock approach onto inverse scattering theory.

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arXiv2010

Kink dynamics, sinh-Gordon solitons and strings in AdS(3) from the Gross-Neveu model

Guided by a study of kink-antikink scattering in the Gross-Neveu model and other known solutions of the Hartree-Fock approach of a particularly simple type, we demonstrate a quantitative relationship between three different problems: Quantized 1+1-dimensional fermions in the large N limit, solitons of the classical sinh-Gordon equation and classical strings moving in 3-dimensional anti de Sitter space. Aside from throwing light on the relationship between quantum field theory and classical physics, this points to the full solvability of the dynamical N-kink-antikink problem in the Gross-Neveu model.

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arXiv2010

Competing mechanisms of chiral symmetry breaking in a generalized Gross-Neveu model

Chiral symmetry of the 2-dimensional chiral Gross-Neveu model is broken explicitly by a bare mass term as well as a splitting of scalar and pseudo-scalar coupling constants. The vacuum and light hadrons - mesons and baryons which become massless in the chiral limit - are explored analytically in leading order of the derivative expansion by means of a double sine-Gordon equation. Depending on the parameters, this model features new phenomena as compared to previously investigated 4-fermion models: spontaneous breaking of parity, a non-trivial chiral vacuum angle, twisted kink-like baryons whose baryon number reflects the vacuum angle, crystals with alternating baryons, and appearance of a false vacuum.

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arXiv2010

Covariant boost and structure functions of baryons in Gross-Neveu models

Baryons in the large N limit of two-dimensional Gross-Neveu models are reconsidered. The time-dependent Dirac-Hartree-Fock approach is used to boost a baryon to any inertial frame and shown to yield the covariant energy-momentum relation. Momentum distributions are computed exactly in arbitrary frames and used to interpolate between the rest frame and the infinite momentum frame, where they are related to structure functions. Effects from the Dirac sea depend sensitively on the occupation fraction of the valence level and the bare fermion mass and do not vanish at infinite momentum. In the case of the kink baryon, they even lead to divergent quark and antiquark structure functions at x=0.

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arXiv2010

Large N solution of generalized Gross-Neveu model with two coupling constants

The Gross-Neveu model in 1+1 dimensions is generalized to the case of different scalar and pseudoscalar coupling constants. This enables us to interpolate smoothly between the standard massless Gross-Neveu models with either discrete or continuous chiral symmetry. We present the solution of the generalized model in the large N limit including the vacuum, fermion-antifermion scattering and bound states, solitonic baryons with fractional baryon number and the full phase diagram at finite temperature and chemical potential.

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arXiv2009

Inhomogeneous Condensates in the Thermodynamics of the Chiral NJL_2 model

We analyze the thermodynamical properties, at finite density and nonzero temperature, of the (1+1)-dimensional chiral Gross-Neveu model (the NJL_2 model), using the exact inhomogeneous (crystalline) condensate solutions to the gap equation. The continuous chiral symmetry of the model plays a crucial role, and the thermodynamics leads to a broken phase with a periodic spiral condensate, the "chiral spiral", as a thermodynamically preferred limit of the more general "twisted kink crystal" solution of the gap equation. This situation should be contrasted with the Gross-Neveu model, which has a discrete chiral symmetry, and for which the phase diagram has a crystalline phase with a periodic kink crystal. We use a combination of analytic, numerical and Ginzburg-Landau techniques to study various parts of the phase diagram.

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arXiv2008

Phase structure of the massive chiral Gross-Neveu model from Hartree-Fock

The phase diagram of the massive chiral Gross-Neveu model (the massive Nambu-Jona-Lasinio model in 1+1 dimensions) is constructed. In the large N limit, the Hartree-Fock approach can be used. We find numerically a chiral crystal phase separated from a massive Fermi gas phase by a 1st order transition. Using perturbation theory, we also construct the critical sheet where the homogeneous phase becomes unstable in a 2nd order transition. A tricritical curve is located. The phase diagram is mapped out as a function of fermion mass, chemical potential and temperature and compared with the one of the discrete chiral Gross-Neveu model. As a by-product, we illustrate the crystal structure of matter at zero temperature for various densities and fermion masses.

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arXiv2008

Baryons in the large N limit of the massive two-dimensional Nambu-Jona-Lasinio model

Baryons in the massive Nambu-Jona-Lasinio model in 1+1 dimensions (the massive chiral Gross-Neveu model) are studied in the limit of an infinite number of flavors. The baryon mass is evaluated for a wide range of bare fermion masses and filling fractions, combining analytical asymptotic expansions with a full numerical Hartree-Fock calculation.

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arXiv2007

Integrating out the Dirac sea: Effective field theory approach to exactly solvable four-fermion models

We use 1+1 dimensional large N Gross-Neveu models as a laboratory to derive microscopically effective Lagrangians for positive energy fermions only. When applied to baryons, the Euler-Lagrange equation for these effective theories assumes the form of a non-linear Dirac equation. Its solution reproduces the full semi-classical results including the Dirac sea to any desired accuracy. Dynamical effects from the Dirac sea are encoded in higher order derivative terms and multi-fermion interactions with perturbatively calculable, finite coefficients. Characteristic differences between models with discrete and continuous chiral symmetry are observed and clarified.

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arXiv2007

Divergence of the axial current and fermion density in Gross-Neveu models

The divergence of the axial current is used to relate the spatial derivative of the fermion density to the bare fermion mass and scalar/pseudoscalar condensates in 1+1 dimensional Gross-Neveu models. This serves as a novel test of known results, to explain simple features of the continuous chiral model and to resolve a conflict concerning the assignment of baryon number to certain multi-fermion bound states.

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