Special Issue "The Neutral–Ionic Phase Transition"
Deadline for manuscript submissions: closed (1 August 2017)
Prof. Dr. Anna Painelli
Prof. Dr. Alberto Girlando
The neutral–ionic phase transition was discovered in mixed-stack charge-transfer crystals almost 40 years ago. The transition, induced by temperature, pressure or light, involves two interrelated instabilities: (a) the crossover between a band-insulator (the neutral phase) and a Mott-insulator (the ionic phase), through a marginally metallic state; and (b) the lattice dimerization induced by a pure-Peierls mechanism close to the neutral–ionic interface and by a spin-Peierls mechanism far in the ionic regime. The subtle interplay between strongly correlated electrons and phonons in reduced dimensions drives the appearance of multiple competing phases, divergent responses, collective phenomena, ferroelectricity, multiferroicity, anomalous metallic states, etc., making the neutral–ionic phase transition and related materials an interesting playground for theoretical and experimental investigations.
The Special Issue will provide an international forum to cover a broad description of theoretical and experimental studies on the phenomenology of the transition, its mechanism and the properties of related materials. Scientists working in a wide range of disciplines are invited to contribute with original papers or short reviews on their activity in the field. The topics summarized under the keywords broadly cover the greater number of relevant sub-topics.
Prof. Dr. Anna Painelli
Prof. Dr. Alberto Girlando
Manuscript Submission Information
Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All papers will be peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.
Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Crystals is an international peer-reviewed open access monthly journal published by MDPI.
Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 1000 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.
- Valence and structural instabilities in charge-transfer crystals
- Correlated electrons and electron–phonon coupling
- Photoinduced phase transitions, multistability and domain-walls
- Organic ferroelectric and multiferroics
- Collective electron-transfer and excitations
The below list represents only planned manuscripts. Some of these manuscripts have not been received by the Editorial Office yet. Papers submitted to MDPI journals are subject to peer-review.
Back to the Structural and Dynamical Properties of Neutral-Ionic Phase Transitions
Marylise Buron-Le Cointe 1, Eric Collet 1, Marie-Hélène Lemée-Cailleau 2, Bertrand Toudic 1 and Hervé Cailleau1,*
1 Institut de Physique de Rennes, Université de Rennes 1 – CNRS, UMR 6251, 263 Avenue du Général Leclerc, 35042 Rennes Cedex, France
2 Institut Laue-Langevin, 71 avenue des Martyrs, 38000 Grenoble, France
Abstract: Although the Neutral-Ionic transition in mixed stack charge-transfer crystals was discovered almost forty years ago, many features of this intriguing phase transition, as well as open questions, remain at the heart of today’s science. First of all, there is a spectacular manifestation of electronic ferroelectricity, in connection with the fact that a significant change of the degree of charge-transfer between donor and acceptor molecules takes place concomitantly with the stack dimerization which breaks the inversion symmetry. On a second hand, these systems exhibit exceptional one-dimensional character which gives rise to original physical pictures for the dynamics of pre-transitional phenomena, from a pronounced Peierls-type instability to the generation of non-linear excitations along stacks. Last but not least, these mixed stack charge-transfer systems constitute a valuable test- bed to explore some of the key questions of ultrafast photo-induced phenomena, such as multiscale dynamics, selective coherent excitations and non-linearity. These different aspects will be discussed through the structural and dynamical properties of the neutral-ionic transition, considering old and recent results, open questions and future opportunities. In particular, we will revisit the structural changes and symmetry considerations, the pressure-temperature phase diagrams and the photo-induced dynamics.
Phenomenology of the Neutral-Ionic Valence Instability in Mixed Stack Charge-Transfer Crystals
Matteo Masino, Nicola Castagnetti and Alberto Girlando*
Dip. di Scienze Chimiche, della Vita e della Sostenibilità Ambientale and INSTM-UdR di Parma, Parco Area delle Scienze, Università di Parma, I-43124 Parma, Italy
Abstract: Organic charge-transfer (CT) crystals constitute an important class of functional materials, characterized by the directional charge-transfer interaction between p-electron Donor (D) and Acceptor (A) molecules, with the formation of one-dimensional stacks. The presence important electronic correlations and electron-phonon coupling, associated with low-lying CT excitation(s), gives rises to many different and often unique properties and phenomena. Among these, the Neutral-Ionic phase transition (NIT) occupies a special place, as it implies a collective electron transfer from D to A, involving charge, spin and phonon degrees of freedom. The analysis of such a complex yet fascinating phenomenon required many years of investigation, and still presents some open questions and challenges. Aim of the present paper is to give an updated summary of the phenomenology of NIT, with emphasis on the experimental aspects. The paper will privilege the work performed by our research group, also presenting some unpublished data from our laboratory.
Modeling the Neutral-Ionic Transition with Correlated Electrons Coupled to Soft Lattices and Molecules
Gabriele D’Avino1, Anna Painelli2 and Zoltán G. Soos3
1 CNRS, Institut Néel, 25 avenue des Martyrs, F-38042 Grenoble, France
2 Dip. di Scienze Chimiche, della Vita e della Sostenibilità Ambientale and INSTM-UdR di Parma, Parco Area delle Scienze, Università di Parma, I-43124 Parma, Italy
3 Department of Chemistry, Princeton University, Princeton, New Jersey 08544, USA
Abstract: Neutral-Ionic transitions (NITs) occur in organic charge-transfer (CT) crystals of planar p-electron donors (D) and acceptors (A) that form mixed stacks …DrA-rDrA-r…with variable ionicity 0 < r < 1 and electron transfer t along the stack. The microscopic NIT model presented combines a modified Hubbard model for strongly correlated electrons delocalized along the stack with mean-field Coulomb interactions and linear coupling of electrons to a harmonic molecular vibration (e-mv) and the Peierls phonon (e-ph). This simple framework captures the observed complexity of NITs with continuous and discontinuous r on cooling or under pressure, together with the stack’s instability to dimerization. The interplay of charge, molecular and lattice degrees of freedom at NIT amplifies the nonlinearity of responses, accounts for the dielectric anomaly, and generates strongly anharmonic potential energy surfaces (PES). Dynamics on the ground state PES address vibrational spectra using time correlation functions. When extended to the excited state PES, the NIT model describes the early ( < 1 ps) dynamics of transient NIT induced by optical CT excitation with a fs pulse. Although phenomenological, the model parameters are broadly consistent with density functional calculations.
Quantum Phenomena Emerging Near a Ferroelectric Critical Point in a Donor-Acceptor Organic Charge-Transfer Complex
Fumitaka Kagawa1, Sachio Horiuchi,2, Yoshinori. Tokura1,3
1 RIKEN Center for Emergent Matter Science (CEMS), Wako 351-0198, Japan
2 Flexible Electronics Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8565, Japan
3 Department of Applied Physics, The University of Tokyo, Tokyo 113-8656, Japan
Abstract: When a second-order transition point is decreased to zero temperature, a continuous quantum phase transition between different ground states is realized at a quantum critical point (QCP). A recently synthesized organic charge-transfer complex, TTF-2,5-QBr2I2, provides a platform to explore quantum phenomena that accompany a ferroelectric QCP. Here, we summarize recent results showing quantum phenomena associated with the ferroelectric QCP in TTF-2,5-QBr2I2. Whereas the enhanced quantum fluctuations lead to quantitative changes in the critical exponents of the critical phenomena, they qualitatively change the nature of the domain-wall kinetics, from thermally activated motions to temperature-independent tunnelling motions. The present findings highlight great influences of quantum fluctuations on physical properties at low temperatures and suggest that TTF-2,5-QBr2I2 is one of the model systems of the uniaxial ferroelectric QCP.
Ultrafast Electron and Molecular Dynamics in Photoinduced and Electric-Field-Induced Neutral-Ionic Transitions
Takeshi Morimoto, Tatsuya Miyamoto and Hiroshi Okamoto
Department of Advanced Materials Science, Univ. of Tokyo
Abstract: Mixed-stacked organic molecular compounds near the neutral-ionic phase boundary, represented by tetrathiafulvalene-p-chloranil (TTF-CA), show neutral (N) to ionic (I) transitions not only by decreasing temperature or applying pressure but also by the irradiation of a femtosecond laser pulse. In this paper, we focus on the ultrafast electron and molecular dynamics in photoinduced and electric-field-induced transitions between the N and I states in TTF-CA. In the first half, we review the photo-induced NI transition studied by femtosecond pump-probe reflection spectroscopy. We show that in the early stage of the photoinduced NI transition, the photoinduced collective charge transfer occurs within 20 fs after the photoirradiation, and this ultrafast charge dynamics is followed by molecular deformations and displacements, which play important roles on the stabilization of photoinduced ionic domains. We also show that the time characteristics in the picosecond time domain in the photoinduced NI and IN transitions are considerably different from each other. In the second half of this paper, we review two phenomena induced by a strong terahertz electric field in TTF-CA; the generation of a large macroscopic polarization in the neutral phase and the modulation of a ferroelectric polarization in the ionic phase. The dynamics of NI domain walls is also discussed.