RESEARCH INTERESTS:
Theoretical Condensed Matter Physics, first-principles electronic structure calculations, many-body physics, high-temperature superconductivity.
ACADENIC DEGREE:
Ph.D. Physics, the Hebrew University of Jerusalem, 1975.
ACADEMIC APPOINTMENT:
Associate Professor, Physics Department, University of Miami.
RECENT PUBLICATIONS:
J. Ashkenazi: "Stripe Fluctuations, Carriers, Spectroscopies, Transport, and BCS-BEC Crossover in the High-Tc Cuprates", J. Phys. Chem. Solids 63, 2333-2240 (2002). http://arxiv.org/abs/cond-mat/0108383
J. Ashkenazi: "Stripe-Like Inhomogeneities, Carriers, and BCS-BEC Crossover in the High-Tc Cuprates", in "New Trends in Superconductivity", edited by J. F. Annett and S. Kruchinin (Kluwer Academic Publishers, 2002), pp. 51-60. http://arxiv.org/abs/cond-mat/0203170
J. Ashkenazi: "Stripe-Like Inhomogeneities, Spectroscopies, Pairing, and Coherence in the High-Tc Cuprates", J. Phys. Chem. Solids 65, 1461-1472 (2004). http://arxiv.org/abs/cond-mat/0308153
J. Ashkenazi: "Stripe-like Inhomogeneities, Coherence, and the Physics of the High Tc Cuprates", in "New Challenges in Superconductivity: Experimental Advances and Emerging Theories", edited by J. Ashkenazi, M.V. Eremin, J. L. Cohn, I. Eremin, D. Manske, D. Pavuna, and F. Zuo (Springer, 2005), pp. 187-212. http://arxiv.org/abs/cond-mat/0407175
J. Ashkenazi: "Towards a global theory for the high Tc cuprates: explanation of the puzzling optical properties", http://arxiv.org/abs/cond-mat/0506515
J. Ashkenazi: "A unified theory for the cuprates, iron-based and similar superconducting systems: application for spin and charge excitations in the hole-doped cuprates", J. Supercond. Nov. Magn. 22, 3-11 (2009). DOI: 10.1007/s10948-008-0370-8, http://arxiv.org/abs/0809.4237,
J. Ashkenazi: "A unified theory for the cuprates, iron-based and similar superconducting systems: non-Fermi-liquid to Fermi-liquid crossover, low-energy and waterfall anomalies", http://arxiv.org/abs/0811.4561
J. Ashkenazi: "A Theory for the High-Tc Cuprates: Anomalous Normal-State and Spectroscopic Properties, Phase Diagram, and Pairing", J. Supercond. Nov. Magn. 24 1281-1308 (2011), DOI: 10.1007/s10948-010-0823-8, http://arxiv.org/abs/0912.4735
Neil F. Johnson, Josef Ashkenazi, Zhenyuan Zhao, Luis Quiroga: "Equivalent dynamical complexity in a many-body quantum and collective human system", AIP Advances 1, 012114 (2011), DOI: 10.1063/1.3563072, http://arxiv.org/abs/1011.6398
Josef Ashkenazi and Neil F. Johnson: "Pairing Glue Activation in Cuprates within the Quantum Critical Regime", http://arxiv.org/abs/1111.5033
BOOKS:
J. Ashkenazi, S.E. Barnes, F. Zuo, G.C. Vezzoli and B.M. Klein (editors): "High Temperature Superconductivity: Physical properties, Microscopic Theory, and Mechanisms" (Plenum Press, NY, 1992).
J. Ashkenazi, S. E. Barnes, J. L. Cohn and F. Zuo (editors): "High Temperature Superconductivity: Physical properties and Mechanisms", special volume of J. Supercond. 8 (Plenum Press, NY, 1995).
S. E. Barnes, J. Ashkenazi, J. L. Cohn, and F. Zuo (editors): "High-Temperature Superconductivity" (AIP Conference Proceedings 483, 1999).
J. Ashkenazi, M.V. Eremin, J. L. Cohn, I. Eremin, D. Manske, D. Pavuna, and F. Zuo (editors): "New Challenges in Superconductivity: Experimental Advances and Emerging Theories" (Springer, 2005).
ORGANIZING CONFERENCES:
In January 26-27, 1990 I participated in organizing a nation-wide Workshop on Electronic Mechanisms for High Temperature Superconductivity at the University of Miami. The number of participants was approximately 20.
In January 3-9, 1991, I participated in organizing the University of Miami Workshop on: Electronic Structure and Mechanisms for High Temperature Superconductivity. In attendance were 106 participants, including most of the nation's leading scientists working on the problem of high temperature superconductivity, and a distinguished representation for eleven other countries. The workshop established the current standing of both theory and experiment in the search the process which leads to high temperature superconductivity. The workshop was supported by a grant from the Department of the Navy.
In January 5-11, 1995, I participated in organizing the University of Miami Workshop on High Temperature Superconductivity: Physical Properties and Mechanisms. In attendance were 149 participants, including leading scientists working on the problem of high temperature superconductivity from the US and many other countries. The workshop established the current standing of both theory and experiment in the search the process which leads to high temperature superconductivity. The workshop was supported by a grant from the Department of the Navy.
I was a member of the Organizing Committee of the International Conference on Stripes, Lattice Instabilities and High-Tc Superconductivity, which took place in Rome, Italy, on December 8-12, 1996.
I was a member of the Organizing Committee of the Second InternationalConference on Stripes, and High-Tc Superconductivity , which took place in Rome, Italy, on June 2-6, 1998.
In January 7-13, 1999, I participated in organizing the Third University of Miami Conference on High-Temperature Superconductivity and Related Topics. In attendance were 105 participants, including leading scientists working on the problem of high temperature superconductivity and related topics from the US and many other countries. The conference established the current standing of both theory and experiment in the search the process which leads to high temperature superconductivity. The conference was supported by a grant from DARPA through ONR. From comments of participants it is understood that this conference is considered as the best in the field in the recent years.
In January 11-16, 2004, I was the co-director of the 2004 University of
Miami Workshop on Unconventional Superconductivity.
It consisted of two consecutive events:
(i) NATO Advanced Research Workshop (ARW) on New Challenges in
Superconductivity: Experimental Advances and Emerging Theories
(supported by NATO),
(ii) Symposium on Emerging Mechanisms for High Temperature
Superconductivity (SEMHTS).
In attendance there were about a hundred participants (including many
leading scientists in the field), representing about twenty countries.
The workshop has been commented for his first-rate excellence. The ARW
was supported by a 27,000 EURO grant from NATO.
In March 28 - April 2, 2009 I have organized, together with Davor Pavuna, a workshop on High-Tc Facts and Open Questions, as a part of the SMEC2009 conference which took place on a ship during a Miami - Western Caribbean cruise.
SCIENTIFIC ACHIEVEMENTS:
My first scientific publication was in experimental nuclear physics, but since then I have been working mainly in the field of theoretical condensed matter physics, and involved in pioneering works in this field. My scientific approach takes account of both single-particle and many-body effects, and I have been carrying out both analytical and numerical works, including also calculations based on first-principles band-structure codes. Such calculations included also an application for astrophysics. My main scientific accomplishments include (years of publications are specified):
1973-1976: Performing a pioneering work (as a graduate student) on incorporating the effects of band-structure and the Hubbard-U in a study of the metal-insulator transition in V2O3 and Ti2O3 as a combined electronic band-structure and correlation effect.
1977-1978: Performing a pioneering work (as a post-doc) on an unrestricted solution of the Eliashberg equations for niobium, incorporating the effects of the band-structure, the phonons, and the electron-phonon coupling.
1978-1981: Calculation of the elastic-constant anomalies in the Zr-Nb-Mo system as a function of composition and temperature.
1979: Proof of the equivalence of the Bloch and the Frohlich approaches to the electron-phonon coupling.
1981-1982: Generalization of the force theorem, within the density-functional approach, for first-principles calculations of elasticity.
1983-1984: Explaining the interrelation between superconductivity and antiferromagnetic, or long-wavelength modulated magnetic order.
1985-1989: Calculating the three-dimensional band structure of trans-polyacetylene, and finding on the basis of total energy calculations that the Peierls mechanism is insufficient for its dimerization energy.
1987-2005:
My major research effort in this period has been on the resolution of the mystery of the cuprates, and specifically the occurrence of high-Tc superconductivity (SC) in them. The problem of the cuprates is one of the forefront problems in physics at present, and its resolution would open a new era in condensed-matter physics research in systems with complexity.
Beside the anomalously high SC transition temperatures, the cuprates are characterized by an anomalous behavior in almost any of their physical properties. No consensus has been established so far concerning the physics of the cuprates, and its understanding may open a new era in condensed-matter physics research in complex systems. Since the cuprates are close to a metal-insulator transition induced by electron correlations, many physicists have reached the conclusion that their behavior is due to an unconventional metallic state close to such a transition, where ordinary theoretical approaches do not work.
While the state of the electrons in ordinary metals is qualified as a Fermi liquid (FL), the cuprates may very well present a non-FL behavior. A feature existing in the cuprates, as well as in other compounds which are close to a metal-insulator transition is that the atomic-scale structure is not homogeneous, as is generally the case in simple metals. The recent discovery of high-Tc SC, and a similar behavior to that of the cuprates, in iron pnictides and selenides (referred to below as FeSCs), introduced an additional test case for high-Tc theories.
I have been searching for the most appropriate theoretical framework, and checking approximate solutions, since 1987. My physical description of the system has been evolving with the years, combining my experience in theoretical superconductivity, electronic-structure, and many-body physics, with careful monitoring and analysis of various experimental results. It turned out, repeatedly, that ideas that I have raised concerning the physics of the cuprates, which may had sounded doubtful in earlier stages of my research on the system, have become obvious later, in view of newer experimental and theoretical data.
2005-2010:
My research on high-Tc SC has been going through basic revisions. During this period I have strengthened the theoretical basis of my work; this resulted in an improved description of the anomalous physical properties (including the occurrence of high-Tc SC) in the cuprates, proving that I have been in the right track. My theory has been successfully generalized for the recently discovered FeSCs, and it applies to similar quasi-two-dimensional highly-correlated electron systems.
The strong intra-atomic Coulomb interactions in such systems restricts the hopping of electrons between atoms; consequently instead of applying an independent-electron picture, under the mean field of the ions and other electrons (as is done in conventional metals), an auxiliary-particles approach, based on combinations of atomic-like electron configurations, has been adopted. These auxiliary particles are treated as fermions or bosons under the constraint that each site can be occupied by one (and only one) configuration.
This constraint is imposed on the auxiliary particles through a Lagrange Bose field representing an effective fluctuating potential, and its coupling to them is analogous to the coupling of lattice vibrations to electrons in conventional metals. Therefore, it provides an intrinsic pairing mechanism for high-Tc SC, similarly to the pairing of electrons through phonons in regular SCs, but with coupling constants that are so strong that they would result in lattice instabilities in electron-phonon systems (but not within this theory). One of the consequences of this approach in the cuprates is the existence of dynamical atomic-scale inhomogeneities, as has been observed. In 2010, this theoretical approach was named the "auxiliary Bose condensates" (ABC) theory.
2010:
A related research which took place towards the end of 2010, in collaboration with Neil Johnson and others, is explaining why the fractal behavior of ordered interstitial oxygen atoms, observed in a family of high-Tc cuprates, has a positive effect on Tc. This was found to be connected with the existence of a temperature range above Tc, where SC fluctuations exist but the global SC phase coherence is incomplete. The mathematical model of Complexity applied reflects the existence of dynamical equivalence between the behavior of a many-body SC system (i.e. scale of 10-9-10-4 meters and 10-12-10-6 seconds) and various systems of human/social groups (i.e. scale of 103-106 meters and 104-108 seconds).
Recent Work:
A continuation of my collaboration with Neil Johnson in 2011 revealed that my ABC approach to the high-Tc SC problem, discussed above, actually answers the long-standing question why quantum criticality (QC) coincides with the occurrence of high temperature SC (HTSC) in systems like the cuprates. QC is is believed by many in the high-Tc community to be behind major anomalous normal-state properties in these systems.
The reason why HTSC coincides with QC lies in the fact that (as has been known for many years) when the coupling constants driving SC become strong, SC is generally suppressed by competing symmetry-breaking instabilities, and thus HTSC is prevented. However, the occurrence of a quantum-critical regime results in a combination state of the different broken-symmetry states. Consequently, SC is allowed, under certain conditions, for very strong coupling constants, resulting in HTSC.
The ABC approach exactly assumes such a combination state. It is understood now that the existence of such a state is the consequence of QC. It results from the fact that the system lies within the critical regime of a hidden quantum phase transition at zero temperature between a non-FL striped state (for low doping levels) and a FL state (for high doping levels).
UNIVERSITY ACTIVITIES:
1. I am a member of the Graduate Recruitment Committee of the UM Physics Department.
2. I am the Chair of the Library Committee of the UM Physics Department.
COMMUNITY SERVICE:
I am involved in the politics of the local Israeli and Jewish communities.
I am a member of the "Riviera Parliament" of Israelis living in South Florida.
I am as board member of the South-Florida Jewish-Arab Dialogue Association (JADA).