去年在小木虫上成功找到导师，今年轮到我来发帖啦。还没有找到导师的学弟学妹快来试试！

我们是巴黎十一大（csc上官方名字是巴黎南大），学校和csc有合作项目。巴黎南大是法国排名第二的综合大学，物理系专业排名世界第十。

课题：structure, disorder and electron-phonon coupling in bi-dimensional kappa-(bedt-ttf)2x charge-transfer spin-liquids and multiferroics
注：这个课题是和巴黎六大（法国排名第一）的一个老师一起的，所以会有两个老师共同指导你。

要求：最好是凝聚态物理的，对固体物理知识比较熟练，当然英语好一点更有优势，2019年入学。我们这边对申请者要求不会太高，主要看你过不过得了csc了。
tips：不用担心不会法语，这边大多英语交流。

导师：pascale foury
邮箱：pascale.foury@u-psud.fr
个人主页：https://www.equipes.lps.u-psud.fr/foury/
实验室主页：https://www.lps.u-psud.fr/

以下是课题简介
strongly correlated systems show a number of exotic phenomena, like superconductivity and
magnetism, which cannot be understood without including many body interactions. subtle balance of different
factors: on-site correlation (u), exchange interaction (j), spin-charge or electron-phonon interaction, can result
in complex phase diagrams with different ground states dependent on pressure or doping. the competition of
different types of insulating and magnetic states with superconductivity is studied in organic conductors for
decades [1]. the discovery of the quantum spin liquid ground state or the multiferroïcity is more recent, but
even more intriguing [2,3]. the first appears in materials with a strong magnetic frustration, where, despite
strong antiferromagnetic coupling, quantum fluctuations prevent any spin ordering. the latter is characterized
by the co-existence of a magnetic and a ferroelectric order, which is rather rare.
bi-dimensional charge-transfer salts kappa-(bedt-ttf)2x are particularly interesting as they can
present all these properties, depending on the counter-anion (x), and external parameters, like pressure and
temperature. their structure is composed of layers of triangular constellation of dimers of bedt-ttf (bis-
(ethylene-thio)-tetra-thia-fulvalene) molecules, separated by anion layers parallel to the (b,c) plan. there is a
charge transfer of about one electron per dimer, from the bedt-ttf molecular layer (donor) to the anion layer
(acceptor). thus the sites of the triangular lattice of dimers are accommodated by approximately one hole
carrying a spin-1/2. we will be particularly interested in three of them, kappa-(bedt-ttf)2cu[n(cn)2]cl
(shortly k-cl), kappa-(bedt-ttf)2cu2(cn)3 (k-cu) and more recently synthesized kappa-(bedtttf)2ag2(cn)3 (k-ag). an extremely subtle competition between the spin frustration and coulomb interaction
results in their different ground states.
k-cl presents an antiferromagnetic order below 27 k, while k-cu and k-ag do not show any magnetic order
at low temperature [4-6]. moreover, signs of ferroelectricity are observed in k-cu and k-cl. they were
interpreted by a creation of electric dipoles on dimers of bedt-ttf molecules [7-10]. the competition between
the spin and the charge order should be here a result of an interaction between dipoles and spins [7-13]. in kcl, the ferroelectric order is proposed to be a motor of the magnetic order [10]. the situation seems to be inverse
compared to the transition metal multiferroïcs, where the magnetic order induces the ferroelectric order.
surprisingly, the presence of dipoles in k-cl, k-cu and k-ag has not been experimentally evidenced up to now
[14-16]. thus the discussion of the origin of the ferroelectric, spin-liquid, and multiferroïc behavior in kappa-
(bedt-ttf)2x systems is still under hot debate. it requires a complete revision of the results of their structure
and dynamics [17].
the object of the proposed work is to arrive at a consistent interpretation of the spin-liquid versus
multiferroïc behavior in kappa-(bedt-ttf)2x salts. combined effects of the disorder, the electronic
correlation and of the spin frustration will be examined as possible motors of the spin-liquid and multiferroïc
behaviors. particular attention will be given to the interplay between the anionic plane and the cationic molecular
(bedt-ttf) layer. the two subsystems are connected via hydrogen bonds, realized between the ethylene sidegroup of the bedt-ttf molecule, and the anionic-plane nitrogen of the cyanide cn group. side ethylene
groups, pointing to the n atoms in the anion layer, can take different conformations relative to the plane defined
by the rest of the molecule. moreover, in k-cu and k-ag, one third of anion plane n atoms is positionally
disordered due to the orientational disorder of bridging cn groups. the geometrical and charge frustration
resulting from the two disorders can result in a creation of globally non-polarized domains separated by domain
walls. excitations inside these domain walls can be responsible for the ferroelectric behavior.
questions about the ferroelectric, spin-liquid, and multiferroïc behavior in kappa-(bedt-ttf)2x will
3 | page
be addressed by studying the detailed structure and the elemental excitations of three kappa-(bedt-ttf)2x
salts: k-cu, k-ag and k-cl. three experimental techniques will mainly be used: x-ray diffraction, x-ray
absorption and resonant inelastic x-ray scattering (rixs).
new x-ray diffraction results show that k-cu presents lower symmetry than its conventional mean
structure [18]. this points to the necessity of a systematic and extremely detailed study of the structure of other
members of the family, particularly k-ag and k-cl. this through but extremely interesting work includes a
complete refinement of the structures, taking into account effects of the disorder in the anion plane and in the
molecular layer. diffraction experiments will be performed first using the 4 circle diffractometer in the
laboratoire de physique de solides, university paris-saclay, orsay, france. further, in order to record very
weak reflections associated with an eventual symmetry breaking, measurements will be performed using the 4
circle diffractometer of the high flux beam-line cristal at the synchrotron soleil, st. aubin, france.
recent n k edge x-ray absorption and rixs measurements reveal the importance of the electronphonon coupling related to the anion-plane phonon modes in kappa-(bedt-ttf)2cu2(cn)3 [19]. moreover,
it shows that the phonon excitation of two distinct nitrogen sites differs because they are more or less coupled
to the molecular layer. the study of the electron-phonon coupling will be extended to k-ag and k-cl
compounds. n k-edge (400 ev) and c k-edge (285 ev) x-ray absorption and rixs measurement will be
performed at sextants beamline at the synchrotron soleil. its rixs spectrometer, aerha, is unique as
it covers the widest energy range (50 ‐ 1000 ev), while having one of the best resolving powers in the world. it
permits to measure elemental excitations, like dd-excitations, charge transfer, collective magnetic and phonon
excitations.
the interpretation of the experimental data will be supported by theoretical ab-initio state-of-the-art
electronic structure calculations, as well as the calculations of the x-ray absorption (xas) and rixs spectra.
rixs calculations will be performed in laboratoire de chimie physique ╟ matière et rayonnement (lcp-mr),
upmc, sorbonne university, paris, while xas calculations will be performed in collaboration with institut
néel, grenoble, france.
we are confident that the study of the detailed structure and of the coupling of the lattice and charge
degrees of freedom will permit to gain better insight into the origins of the ferroelectric, spin-liquid, and
multiferroïc behavior in selected kappa-(bedt-ttf)2x salts.

[1] B. J. Powell et al, Rep.Prog.Phys. 74, 056501 (2011).
[2] Y.Zhou et al., Rev.Mod.Phys. 89, 025003 (2017).
[3] L. Savary et al., Rep.Prog.Phys. 80, 016502 (2017).
[4] A Kawamoto et al., Phys.Rev.B 52, 15522 (1995)
[5] Y. Shimizu et al., Phys. Rev. Lett. 91, 107001 (2003)
[6] Y. Shimizu et al., Phys. Rev. Lett. 117, 107203 (2016)
[7] M. Abdel-Jawad et al., Phys.Rev.B 82, 125119 (2010)
[8] C. Hotta, Phys.Rev.B 82, 241104 (2010)
[10] P. Lunkenheimer et al., Nat. Mater. 11, 755 (2012)
[11] S. Dalay et al., Phys. Rev. B 83, 245106 (2011);
[12] M. Naka et al., J. Phys. Soc. Jpn 79, 063707 (2010) and 82, 023701 (2013)
[13] H. Gomi et al., Phys.Rev.B 93, 035105 (2016)
[14] K. Sedlmeier et al., Phys. Rev. B 86, 245103 (2012)
[15] S. Tomic et al., J.Phys:Cond.Mat. 25, 436004 (2013).
[16] M. Pinteric et al., Phys.Rev.B90, 195139 (2014).
[17] S. Tomic et al., Rep.Prog.Phys. 78, 096501 (2015).
[18] P. Foury-Leylekian, Crystals 8, 158 (2018)


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