CMSP - Live Broadcast on FCMP - Friday 15 September from 2.00 p.m. Stasi Seminar Room

[Sauleek, Doreen]

Condensed Matter and Statistical Physics - LIVE BROADCAST
Lectures on FCMP - Frontiers of Condensed Matter Physics
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FRIDAY 15 SEPTEMBER
Luigi Stasi Seminar Room,1st floor, ICTP Leonardo Bldg.


_Lecture 1_

14:10 to 15:25
Stephen J. BLUNDELL (Oxford University, U.K.)

Abstract:
Most superconductors are made using standard techniques involving 
inorganic components and modifications are made according to a standard 
set of simple chemical replacements. However, organic chemistry is 
potentially much more flexible and the addition of molecules into 
superconductors offers a new route to controlling properties. This has 
led to the field of organic superconductors and I will outline some of 
the results that have been achieved in this field [1-3]. I will also 
describe some new superconductors which have been made using a low- 
temperature intercalation method that is capable of inserting molecular 
species in between superconducting FeSe layers. It results in a large 
increase in superconducting transition temperature - more than a factor 
of 4 [4,5]. In this talk, I will review recent progress and describe new 
results on Lix[(NH2)y(NH3)1−y]zFe2Se2 (z = 1, 2) which have been carried 
out in order to study the effect on the superconducting properties of 
intercalating additional ammonia, via reversible adsorption and 
desorption. The reactions were carried out in situ on the muon beamline 
so that the superfluid stiffness could be measured using 
transverse-field muon-spin rotation experiment on a single sample, first 
having undergone exposure to 1 bar of ammonia at 250 K, and then again 
following desorption. These results illustrate some of the 
possibilities, but also the difficulties, of using molecules in 
developing new superconductors.

(Work performed in collaboration with S. J. Clarke and coworkers at 
Oxford, RAL and Durham, UK.)

References:
[1] T. Ishiguro, K. Yamaji, G. Saito, Organic Superconductors, second 
ed., Springer-Verlag, Berlin, (1998)
[2] F. L. Pratt and S. J. Blundell, Phys. Rev. Lett. 94, 097006 (2005)
[3] J. G. Analytis, A. Ardavan, S. J. Blundell, R. L. Owen, E. F. 
Garman, C. Jeynes and B. J. Powell, Phys. Rev. Lett. 96, 177002 (2006)
[4] M. Burrard-Lucas, D. G. Free, S. J. Sedlmaier, J. D. Wright, S. 
J.Cassidy, Y. Hara, A. J. Corkett, T. Lancaster, P. J. Baker, S. J. 
Blundell,
      and S. J. Clarke, Nat. Mater. 12, 15 (2013)
[5] H. Sun, D. N. Woodruff, S. J. Cassidy, G. M. Allcroft, S. J. 
Sedlmaier, A. L. Thompson, P. A. Bingham, S. D. Forder, S. Cartenet, N. 
Mary,
      S. Ramos, F. R. Foronda, B. H. Williams, X. Li, S. J. Blundell, 
and S. J. Clarke, Inorg. Chem. 54, 1958 (2015)


_Lecture 2_

15:30 to 16:45
Philippe MENDELS (Univ. Paris-Sud, France)

Abstract:
Finding new states of matter is one main goal of research in condensed 
matter which often gives rise to novel concepts and sometimes to 
remarkable technological innovations. In the field of magnetism, the 
frustration of the magnetic interactions in well-chosen lattice 
geometries is the key ingredient to promote truly original ground 
states. This field of research has grown tremendously during the last 15 
years with the emergence of new concepts such as the spin ices or the 
spin liquids and the corresponding original excitations, magnetic 
monopoles and spinons. Spin liquids for instance can be viewed as 
resonant singlets (antiferromagnetic spin pairs), a model quantum ground 
state first advocated for the interpretation of high Tc 
superconductivity [1].

After a long and basic introduction of this field of research and the 
challenges it has opened, I’ll focus on the emblematic case of the 
kagome geometry. Indeed the low connectivity of the kagome network 
together with the quantum fluctuations enhanced for low spin 1⁄2 
destabilize any ordered state in favor of a fluctuating ground state at 
T=0, coined a spin liquid. The recent discovery of model compounds for 
this physics, namely with a true kagome lattice decorated by quantum 
S=1/2 spins (Cu2+ or V4+), has boosted both the theoretical and the 
experimental research in this field. The spin liquid state is for 
instance realized in the archetypal ZnCu3(OH)6Cl2 “herbertsmithite” 
compound. The latter shows no sign of frozen on-site magnetism, ie no 
spontaneous breaking of symmetry, down to mK temperatures while the 
magnetic interaction is of the order of hundreds of Kelvin [2,3]! The 
precise nature of the ground state and of its elementary excitations, 
the phase transitions that can be induced by various parameters are at 
the heart of the current debates and a seed for new concepts in the 
field of frustrated magnetism. I will review the major and recent 
advances in the field and will underline the power of two local 
techniques, NMR and μSR to study such a field of research [4].

References
[1] P. W. Anderson, Science 235, 1196 (1987)
[2] P. Mendels et al., Phys. Rev. Lett. 98, 0772014 (2007); J. S. Helton 
et al. , Phys. Rev. Lett. 98 , 07204 (2007).
[3] P.A. Lee, Science, Perspectives 321, 1306 (2008).
[4] For a review, see P. Mendels and F. Bert, Special Topics Section on 
"Novel States of Matter Induced by Frustration", J. Phys. Soc. Jpn 1,
      011001 (2010); J. Phys. Conf. Series 320 , 012004 (2011). M.R. 
Norman, Rev. Mod. Phys. 88, 041002 (2016). P. Mendels and F. Bert,
      Comptes Rendus Physique, 17, 455 (2016).


All are most welcome to attend