This week's CM seminars

[Johannes Grassberger]

JOINT ICTP/SISSA STATISTICAL PHYSICS SEMINAR



Tuesday, 26 February   -   11:00 a.m.




SISSA, Santorio Building,  Room 128 (1st Floor)




Tamas PALMAI    ( SISSA)



"Finite volume matrix elements in the   sine-Gordon model"



Abstract



I will discuss finite volume matrix elements of local operators in 
integrable quantum field theories and present a conjecture relating 
finite and infinite volume form factors in the most general setting, 
i.e. including the case of form factors containing disconnected pieces 
in the presence of non-diagonal scattering.  Specializing to the 
sine-Gordon model, finite volume multi-soliton form factors are

compared to numerical data coming from a numerical renormalization group 
calculation in the truncated conformal space.  I find excellent 
agreement between the two approaches, thus verifying both bootstrap 
for multi-soliton form factors and the theory linking finite and 
infinite volume matrix elements.  Evaluation of the nontrivial 
multi-soliton form factors is achieved by a newly developed

regularization scheme.

==


JOINT ICTP/SISSA   COLLOQUIUM   ON CONDENSED MATTER PHYSICS



Wednesday, 27 February -  4:30 p.m.




Luigi Stasi Seminar Room,   ICTP Leonardo Building (1st floor)



Andrey ZHELUDEV     ( ETH Zürich )



" The end of the quantum spin ladder problem (almost)"


Abstract


For the past 25 years, the Heisenberg S = 1/2 ladder remains the most 
important model in quantum magnetism.  It combines the essence of 
quantum spin fluctuations with topological peculiarities of one 
dimension.  The vast amount of theoretical work performed over the years 
yielded a comprehensive picture of spin correlations in this model, and 
their evolution in applied magnetic fields.  Unfortunately, good 
experimental realizations and sufficiently accurate numerical results 
remained elusive.  The situation changed drastically in the past couple 
of years, due to progress in material synthesis, neutron 
scattering instrumentation and algorithm development.  We are now close 
to claiming that the spectral and thermodynamic properties of the 
quintessential J_rung-J_leg Heisenberg ladder model are completely 
understood at the quantitative level. Particularly striking are recent 
results on spin excitations in the strong-leg ladder material DIMPY, 
obtained in the gapped spin liquid [1-3] and gapless 
Tomonaga-Luttinger spin liquid phases [3].  At the same time, just 
slightly more complex ladder systems, those showing disorder [4], 
geometric frustration [5,6] or a coupling to the crystal lattice [7], 
present new challenges that we are beginning to tackle.

References
1.        T. Hong et al., Phys. Rev. Lett. 105, 137207 (2010).
2.        D. Schmidiger et al., Phys. Rev. B 84,144421 (2011)
3.        D. Schmidiger et al., Phys. Rev. Lett. 108, 167201 (2012).
4.        T. Hong et al., Phys. Rev. B 81, [Rapid] 060410 (2010).
5.        V. O. Garlea, et al., Phys. Rev. Lett. 100, 037206 (2008).
6.        V. O. Garlea, et al., Phys. Rev. B 79, [Rapid] 060404 (2009).
7.        F. Schrettle, et al., Phys. Rev. B [Rapid]-in press, 
arXiv:1203.3127v1.