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arXiv:1111.2375 · Physical Review A

Quantum criticality of spin-1 bosons in a one-dimensional harmonic trap

C. C. N. Kuhn, Xi-Wen Guan, Angela Foerster, Murray T. Batchelor

Published Jul 17, 2012• 10 citations
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Abstract

We investigate universal thermodynamics and quantum criticality of spin-1 bosons with strongly repulsive density-density and antiferromagnetic spin-exchange interactions in a one-dimensional harmonic trap. From the equation of state, we find that a partially polarized core is surrounded by two wings composed of either spin-singlet pairs or a fully spin-aligned Tonks-Girardeau gas depending on the polarization. We describe how the scaling behavior of density profiles can reveal the universal nature of quantum criticality and map out the quantum phase diagram. We further show that at quantum criticality the dynamical critical exponent $z=2$ and correlation length exponent $\ensuremath{\nu}=1/2$. This reveals a subtle resemblance to the physics of the spin-1/2 attractive Fermi gas.

DOI
10.1103/physreva.86.011605

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We investigate universal thermodynamics and quantum criticality of spin-1 bosons with strongly repulsive density-density and antiferromagnetic spin-exchange interactions in a one-dimensional harmonic trap. From the equation of state, we find that a partially polarized core is surrounded by two wings composed of either spin-singlet pairs or a fully spin-aligned Tonks-Girardeau gas depending on the polarization. We describe how the scaling behavior of density profiles can reveal the universal nature of quantum criticality and map out the quantum phase diagram. We further show that at quantum criticality the dynamical critical exponent z=2z=2z=2 and correlation length exponent \ensuremathν=1/2\ensuremath{\nu}=1/2\ensuremathν=1/2. This reveals a subtle resemblance to the physics of the spin-1/2 attractive Fermi gas.
Using the thermodynamic Bethe ansatz equations we study the quantum phase diagram, thermodynamics, and criticality of one-dimensional (1D) spin-1 bosons with strongly repulsive density-density and antiferromagnetic spin-exchange interactions. We analytically derive a high-precision equation of state from which the Tomonaga-Luttinger-liquid physics and quantum critical behavior of the system are computed. We obtain explicit forms for the scaling functions near the critical points yielding the dynamical exponent z=2z=2z=2 and correlation length exponent $\ensuremath{\nu}=1/2$ for the quantum phase transitions driven by either the chemical potential or the magnetic field. Consequently, we further demonstrate that quantum criticality of the system can be mapped out from the finite temperature density and magnetization profiles of the 1D trapped gas. Our results provide the physical origin of quantum criticality in a 1D many-body system beyond the Tomonaga-Luttinger-liquid description.
We propose the density-functional theory for one-dimensional harmonically trapped spin-1 bosons in the ground state with repulsive density-density interaction and antiferromagnetic spin-exchange interaction. The density distributions of spin-singlet-paired bosons and polarized bosons with different total polarization for various interaction parameters are obtained by solving the Kohn-Sham equations, which are derived based on the local density approximation and the Bethe ansatz exact results for a homogeneous system. Nonmonotonicity of the central densities is attributed to the competition between the density interaction and spin-exchange. The results reveal the phase separation of the paired and polarized bosons, the density profiles of which respectively approach the Tonks-Girardeau gases of Bose-Bose pairs and scalar bosons in the case of strong interaction. We give the radius-polarization phase diagram at strong interaction and find the critical polarization, which paves the way to directly observe the exotic singlet pairing in spinor gas experimentally.
We consider a strongly interacting one-dimensional (1D) Bose-Fermi mixture confined in a harmonic trap. It consists of a Tonks-Girardeau (TG) gas (1D Bose gas with repulsive hard-core interactions) and of a noninteracting Fermi gas (1D spin-aligned Fermi gas), both species interacting through hard-core repulsive interactions. Using a generalized Bose-Fermi mapping, we determine the one-body density matrices, exact particle density profiles, momentum distributions, and behavior of the mixture under 1D expansion when opening the trap. In real space, bosons and fermions do not display any phase separation: the respective density profiles extend over the same region and they both present a number of peaks equal to the total number of particles in the trap. In momentum space the bosonic component has the typical narrow TG profile, while the fermionic component shows a broad distribution with fermionic oscillations at small momenta. Due to the large boson-fermion repulsive interactions, both the bosonic and the fermionic momentum distributions decay as $C{p}^{\ensuremath{-}4}$ at large momenta, like in the case of a pure bosonic TG gas. The coefficient CCC is related to the two-body density matrix and to the bosonic concentration in the mixture. When opening the trap, both momentum distributions ``fermionize'' under expansion and turn into that of a Fermi gas with a particle number equal to the total number of particles in the mixture.
We investigate quantum criticality and universal scaling of strongly attractive Fermi gases confined in a one-dimensional harmonic trap. We demonstrate from the power-law scaling of the thermodynamic properties that current experiments on this system are capable of measuring universal features at quantum criticality, such as universal scaling and Tomonaga-Luttinger liquid physics. The results also provide insights on recent measurements of key features of the phase diagram of a spin-imbalanced atomic Fermi gas [Y. Liao et al., Nature (London) 467, 567 (2010)] and point to further study of quantum critical phenomena in ultracold atomic Fermi gases.
We investigate magnetism and quantum phase transitions in a one-dimensional system of integrable spin-1 bosons with strongly repulsive density-density interaction and antiferromagnetic spin-exchange interaction via the thermodynamic Bethe ansatz method. At zero temperature, the system exhibits three quantum phases: (i) a singlet phase of boson pairs when the external magnetic field HHH is less than the lower critical field Hc1{H}_{c1}Hc1​; (ii) a ferromagnetic phase of atoms in the hyperfine state ∣F=1, mF=1⟩|F=1,\text{ }{m}_{F}=1⟩∣F=1, mF​=1⟩ when the external magnetic field exceeds the upper critical field Hc2{H}_{c2}Hc2​; and (iii) a mixed phase of singlet pairs and unpaired atoms in the intermediate region Hc1<H<Hc2{H}_{c1}<H<{H}_{c2}Hc1​<H<Hc2​. At finite temperatures, the spin fluctuations affect the thermodynamics of the model through coupling the spin bound states to the dressed energy for the unpaired mF=1{m}_{F}=1mF​=1 bosons. However, such spin dynamics is suppressed by a sufficiently strong external field at low temperatures. Thus the singlet pairs and unpaired bosons may form a two-component Luttinger liquid in the strong coupling regime.
We obtain an analytical equation of state for one-dimensional strongly attractive Fermi gases for all parameter regimes in current experiments. From the equation of state, we derive universal scaling functions that control whole thermodynamical properties in quantum critical regimes and illustrate the physical origin of quantum criticality. It turns out that the critical properties of the system are described by those of free fermions and those of mixtures of fermions with masses mmm and 2m2m2m. We also show how these critical properties of bulk systems can be revealed from the density profile of trapped Fermi gases at finite temperatures and can be used to determine the T=0T=0T=0 phase boundaries without any arbitrariness.