# Khandker Fazlul Quader

## Biography

** Postdoctoral positions:**

University of Illinois at Urbana-Champaign; Advisors: David Pines and Chris Pethick.

Rutgers University; Advisor: Elihu Abrahams

**Current Positions: **

Professor, Department of Physics, Kent State University.

Visiting Professor, Department of Physics, Carnegie-Mellon University.

**Research Collaborators:**

Michael Widom, Department of Physics, Carnegie-Mellon University.

Kevin Bedell, Department of Physics, Boston College.

Tom Ainsworth, Naval Research Laboratory.

**Graduate Students:**

*Present:* Pengtao ShenIan

Ian Lofgren

*Past:* George Levin

Natasha Kothekar

Dusan Volcko

Susan Killiches

Ranyuan Liao

Jason Ellis

Kelly Reidy

**Past Postdoctoral Fellows:**

Mark Flynn

Carl Greeff

George Levin

Florentin Popescu

**Department/University Activities:**

- Physics Majors Advisor: Applied Physics concentration
- KSU-ICAM Institute for Complex Adaptive Matter (ICAM) Coordinator

### Scholarly, Creative & Professional Activities

#### Research Interests/Activities

**Areas of Current Interest:** In recent years there have been fascinating discoveries of several types of quantum condensed forms of matter exhibiting novel phenomena arising from interactions among the constituents, such as, electrons in solids, or atoms & molecules in ultracold matter. Common to all of these is the emergence of spectacular phases with novel macroscopic behavior in the vicinity of quantum instabilities. Phenomena such as these give glimpses into the nature of the rich underlying quantum world, and provide motivation for theoretical study of correlated Fermi systems.

Examples of interacting electron systems are high temperature cuprate superconductors, iron pnictide materials, heavy fermions, copper-oxygen ladder systems, some of which are low-density, or low-dimensional or layered systems. Among issues of current interest are the underlying electronic structure and ionic vibrations; interplay of magnetism and superconductivity; nature of associated quantum fluctuations and collective modes; their role in pairing and nature of pairing symmetry; and physics close to quantum critical points (QCP).

In the field of ultracold matter, innovative experimentation, coupled with unprecedented level of experimental control has given birth to several novel classes of systems, such as, ultracold neutral atomic gases and optical lattices, and more lately, dipolar quantum gases. That these may also be prepared with unequal species population, and under non-adiabatic conditions, add to the breadth of the field. These systems present extraordinary opportunities to study novel physics of quantum many-particle systems, not only at extremely short (angstrom) length scales, but also at nanoscale in temperature, where quantum effects are expected to manifest dramatically. Tunability of particle density, kinetic energy and interparticle correlations render these systems highly controllable testing grounds for models in condensed matter physics. This is invaluable for fundamental understanding of complex materials and “designing” of newer materials. Thus, for example, atoms in optical lattices can be represented by the archetypal condensed matter Hamiltonians, e.g. Hubbard model and Bardeen-Cooper-Schrieffer (BCS) superconductivity model.

**Current Research Activities:** These are based on my interest in theoretical/computational understanding of novel phenomena in both correlated electron and ultracold atomic systems discussed above. My theory research combines phenomenological and microscopic approaches, with some emphasis on making connection with experiments. The theoretical methods include techniques of many-particle quantum statistical mechanics, for example, tractable crossing-symmetric equation (TCSE) approaches, Fermi liquid theory, quasi-classical transport equations, BCS theory of superconductivity, basic aspects of critical phenomena/phase transitions, and first principles density functional theory. I currently have research collaborations/ties with Boston College (K. Bedell’s group), Carnegie-Mellon University (M. Widom’s group), Rutgers University. In the past, I have had research ties with the condensed matter/materials division at Los Alamos National Lab and AT&T Bell Labs; I had been collaborator/consultant and regular visitor to these labs.

**Specific Research Efforts:**

- Study, in general terms, of quantum fluctuations, excitations, transport and superconducting properties of strongly correlated 3D electron systems in sectors delineated by Pomeranchuk instabilities (related to quantum critical points): paramagnetic (PM), ferromagnetic (FM), density/charge instability or phase separation (PS), mixed FM and PS sectors. For this, a reformulation of TCSE method for the non-PM sectors is being carried out. Specific system under study are ferromagnetic superconductors and systems exhibiting exotic magnetism and unconventional superconductivity. Study of excitations, thermodynamic, transport and superconducting properties involve use of TCSE method, concepts of local Fermi liquid theory, ferromagnetic Fermi liquid theory and quasi-classical kinetic theory. Exploration of physics around the Pomeranchuk instabilities connected with spin and charge instabilities is expected to shed perspective on behavior around the associated QCP.

- Development of TSCE formalism for correlated 2D systems, enabling applications to specific systems, such as, graphene, thin films, and anisotropic layered materials.The method is also being extended beyond spherically symmetric systems to for example 2D square lattices. Part of this work is being done in collaboration with K. Bedell’s group at Boston College.

- Linking of TCSE method with dynamical mean-field theory (DMFT) to complement the respective methods, thus properly treating dynamics and momentum dependence of interactions and self energies. This is at an initial phase and may involve collaboration with G. Kotliar at Rutgers University.

- Electronic Structure, vibrational modes and novel phases in iron pnictide materials. Currently studying the 122 pnictides in collaboration with M. Widom at CMU. There have been some promising results, a paper published, and another paper on Lifshitz transition submitted.

- The scope of recent and ongoing work on cold atomic fermions is being expanded. Recent/ongoing research explores quantum instabilities and novel phases in population imbalanced 2D and 3D Fermi systems subject to repulsive, as well as attractive, interactions of arbitrary strength; this is relevant for cold fermion systems. Study includes consideration of different order parameter symmetry (e.g. s-, s*-, d-, p-wave), possibility of breached pair states (interior gap superfluidity), atoms trapped in optical lattices, and topological nature of superfluid ground states. Current work relates to instabilities in Fermi systems subject to dipolar interactions, which are long-range in nature. Calculation of many-body t-matrix will serve as input into calculations in random phase approximation and TCSE. This will be directly relevant to cold fermion systems.

#### Recent/Selected Publications:

1. Ammar Kirmani, Khandker Quader, and Maxim Dzero, Phase diagram for the trapped p-wave fermionic superfluid with population imbalance, Phys. Rev. B 95, 134503 (2017).

2. K. F. Quader and M. Widom, Pressure-Driven Enthalpic and Lifshitz Transition in 122-Pnictides, Contrib. Plasma Phys. 55, No. 2-3, 128 – 135 (2015).

3. Michael Widom and Khandker Quader, Elastic Instability of the Orthorhombic Antiferromagnetic Phase of 122-Pnictides Under Pressure, J Supercond Nov Magn, 28, (2015).

4. K. F. Quader and M. Widom, Lifshitz and Other Transitions in Alkaline 122 Pnictides, Phys. Rev. B (2014).

5. K. E. Reidy, K. F. Quader, and K. S. Bedell, Approaching Pomeranchuk Instabilities from Ordered Phase: A Crossing-symmetric Equation Method, Nucl. Phys. A, 928C, 168 (2014).

6. R. Liao, F. Popescu, and K. F. Quader, p-wave Pairing in Two-component Fermi Systems with Unequal Populations: Weak-coupling BCS to Strong-Coupling BEC Regimes, Phys. Rev. B 88, 134507 (2013).

7. M. Widom and K. F. Quader, First Principles Study of CaFe2As2 “Collapse” Under Pressure, Phys. Rev. B 88, 045117 (2013).

8. D. Volcko and K. F. Quader, Signatures of Fermion Pairing with Unconventional Symmetry around BCS-BEC Crossover in a Quasi-2D Lattice, Phys. Rev. Lett. 109, 235303 (2012).

9. R. Liao and K. F. Quader, Medium effects close to s- and p-wave Feshbach resonances in atomic Fermi gases, Phys. Rev. A 86, 012704 (2012).

10. R. Liao and K. F. Quader, Pairing in Asymmetrical Fermi Systems with Intra- and Inter-Species Correlations, Phys. Rev. B 76, 212502 (2007).

11. K. F. Quader, Fluctuations and Pairing in Fermi Systems: A Crossing-symmetric Approach, Int. J. Mod. Physics B, 20, Nos. 30-31, p. 5301 (2006).

12. K. F. Quader, Topology of Paired Fermion States in 2D, Condensed Matter Theories, 18, 181 (2004).

13. G. A. Levin and K. F. Quader, In-Plane Resistivity and an Explanation for the Characteristic T in High-Tc Cuprates, Phys. Rev. B 62, 11879 (2000).

14. G. A. Levin and K. F. Quader, Scaling in Spin Susceptibility and Specific Heat of High Tc Cuprates: A Model with Non-degenerate Fermions, Rapid Comm., Phys. Rev. B 52, 5899 (1996).

15. K. F. Quader and G. A. Levin, Origin and Consequences of the “Gap” in Cuprate Normal State, Phil. Mag. B 74, 6090 (1996).

16. N. Kothekar, K. F. Quader and D. W. Allender, Charge Instabilities and Phase Separation in 2D Extended Hubbard Model: The Role of O-O Hopping, Phys. Rev. B 51, 5899 (1995).

17. G. A. Levin and K. F. Quader, Tunneling Asymmetry in High-Tc Cuprates: Evidence for A Submerged Band with Non-degenerate Fermions?, Rapid Comm., Phys. Rev. B48, (1993).

18. Y. Bang, K. F. Quader, P. Littlewood and E. Abrahams, Pairing by Dynamic Charge Fluctuations in the Extended Hubbard Model, Rapid Comm., Phys. Rev. B42, 4865 (1990).

19. K. F. Quader and E. Abrahams, Superconducting Fluctuations in Specific Heat in Magnetic Field: Dimensional Crossover, Rapid Comm., Phys. Rev. B38, 11977 (1988).

20. K. F. Quader, K. S. Bedell and G. E. Brown, Strongly Interacting Fermions, Phys. Rev. B36, 156 (1987).

21. T. Fujita and K. F. Quader, Spin-Orbit Coupling In Fermi Liquid Theory, Phys. Rev. B36, 5152 (1987).

22. R. Anderson, C. J. Pethick and K. F. Quader, Transport Properties of a Multi-component Fermi Liquid, R. Anderson, C. J. Pethick and K. F. Quader, Phys. Rev. B35, 1620 (1987).

23. C.J. Pethick, D. Pines, K.F. Quader, K.S. Bedell and G.E. Brown, One Component Fermi Liquid Theory and the Properties of UPt3, Phys. Rev. Lett. 57, 1955 (1986).

### Courses Teaching

**Spring 2017**

- PHY 76162 - Quantum Mechanics II
- PHY 80299 - 014 Dissertation II