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[Balazs Dora <dora AT eik.bme.hu>]

                             MEGHIVO

      a BME Egzotikus kvantum fazisok csoport szeminariumara:


                          Hetenyi Balazs
                Bilkent University, Torokorszag


Baeriswyl based variational approach to the Bose-Hubbard model: is it 
supersolid or not?


One of the intriguing results of low temperature physics in the twentieth 
century was the discovery of superfluidity, a highly coherent quantum 
phase of matter exhibiting frictionless flow through cavities. Even more 
counterintuitive was the suggestion that this can occur in a phase of 
matter in which long-range crystalline order is maintained. In 2004 Kim 
and Chan [1], by measuring the rotational inertia of a solid sample in 
torsional oscillator experiment raised the possibility of a supersolid 
phase in helium II. The results of this experiment have been questioned 
since, the issue of supersolidity in helium II is still open. Anderson [2] 
argued that the ground state of the bosonic Hubbard model also exhibits a 
supersolid phase at integer fillings. Anderson`s argument is based on the 
leading terms of a perturbative expression of the Hamiltonian and 
analyzing the response to a boundary twist [3]. In this talk it is shown 
how a variational Monte Carlo method [4] can be constructed based on the 
Baeriswyl wavefunction [5]. The scheme is equivalent to the perturbation 
expansion used by Anderson, however, in this case the full expansion is 
performed. The phase diagram obtained is in excellent agreement with 
quantum Monte Carlo results. We also investigate the sensitivity of the 
system to a boundary twist, and find that it is sizeable even for integer 
fillings. To understand the nature of the phase we use a single-particle 
[6] and a many-particle localization quantity [7] and find that at
integer fillings the system exhibits many-particle localization, at the 
same time, single particles as a result of bosonic exchange, can 
delocalize over the entire lattice. Away from integer fillings, where the 
system is known to be superfluid, delocalization is found at both the 
single-particle and many-particle level. We interpret these results as a
signature of supersolidity in the Bose-Hubbard model at integer filling.

References:
[1] E. Kim and M. H. W. Chan, Science, 305 1941 (2004).
[2] P. W. Anderson, J. Low Temp. Phys., 169 124 (2012).
[3] W. Kohn, Phys. Rev., A133 171 (1964).
[4] B. Het?nyi and B. Tanatar, work in progress.
[5] D. Baeriswyl in Nonlinearity in Condensed Matter, Ed. A. R. Bishop, 
D. K. Campbell, D. Kumar, and S. E. Trullinger, Springer-Verlag (1986).
[6] A. Selloni, P. Carnevali, R. Car, and M. Parrinello, Phys. Rev. 
Lett. 59 823 (1987); E. S. Fois, A. Selloni, M. Parrinello, and R. Car, J. 
Phys. Chem. 92 3268 (1988).
[7] R. Resta, Phys. Rev. Lett., 80 1800 (1998); R. Resta and S. Sorella, 
Phys. Rev. Lett. 82 370 (1999).


  Helye:  BME Fizikai Intezet, Elmeleti Fizika Tanszek
        Budafoki ut 8. F-epulet, magasfoldszint


 Ideje: 2016. augusztus 29 (hetfo), 13.30.

    Minden erdeklodot szivesen latunk!




                                Dora Balazs