Curriculum Vitae

Education

Professional Employment

Awards and honors

Research Experience

Ab Initio- Based Studies of Structural Properties of Binary and Pseudo-Binary Alloys

While triumphant in prediction of materials properties, ab initio calculations require a large amount of computational power. I furthered the development of an approach called Mixed-basis cluster expansion (MBCE), that goes beyond the traditional ab initio framework and allows to accurately predict the structure-dependent properties of bulk alloys. Together with Zunger, I developed the machinery that allowed application of MBCE to various non-trivial cases, such as stability of mixed lattice and mixed magnetism systems (MBCE applied to Fe-Ni and Fe-Pt in collaboration with Blum and to Fe-Pd in collaboration with Chepulskii), ferromagnetism in GaMnAs (MBCE applied in collaboration with Franceschetti), and phase-separation in GaMnAs (MBCE applied in collaboration with Osorio-Guillen). I also conducted the extensive MBCE-guided ab initio studies of the structural properties of Fe-(Ni,Pt) and Au-Pd binary alloys. I demonstrated that Fe-rich alloys have a strong tendency for layering in (100) direction. For example, while the stable Fe₃Ni configuration had been previously commonly expected to be L1₂ structure, I demonstrated that it is unstable with respect to forming Fe₃Ni (100) superlattice. Similar conclusion holds at T=0 even for Fe₃Pt, despite the fact that Fe₃Pt L1₂ is stable at finite temperatures. For Au-Pd, I predicted the existence of both long-sought and unsuspected ground states, and gave theoretical explanation to the difficulties in experimental observation of Au-Pd ground states. All these conclusions, as well as other predictions for the structural stability in Fe-(Ni,Pt) and Au-Pd, are confirmed by direct ab initio calculations.

Earlier, in collaboration with Stroud, I used ab initio total energy calculations for ordered structures in the Al-doped superconductor MgB₂ to show that this material tends to form superstructures, with alternating layers of Al and Mg atoms, separated by layers of B atoms. This calculated result is in good agreement with experiment. Starting from those numerical results, I developed a model that successfully explained the phase separation observed in Mg_1-xAl_xB₂ for x~0.2 and x~0.7, previously considered a mysterious phenomenon at odds with the usual crystalline stability arguments.

My current cluster expansion research includes studies of structural properties of Pb₂Te₂ - AgSbTe₂ alloys and related systems, and of some superconducting and f-element oxide alloys.

Superconductivity and Magnetism

In collaboration with Zunger and co-workers, I have developed and applied the machinery for quantitative prediction of structure-dependent ferromagnetic transition temperature T_c in dilute magnets. Our methodology begins with ab initio calculation of long-range Heisenberg magnetic interactions, followed by the Monte-Carlo simulation of thermal fluctuations (resulting in accurate T_c estimate) and combined, if necessary, with MBCE and percolation studies of the interplay between the structure and T_c. My particular contribution (in addition to the assistance with the cluster expansion procedure) was the development of a Heisenberg Monte-Carlo code capable to simulate the realistic distance- and environment-dependent interactions ranging far beyond the nearest neighbors. The quantitative predictions obtained by this machinery can confidently guide the experimental search of materials with desired properties. For example, in a DARPA-sponsored project on the Prediction of Real Optimized Materials, we have demonstrated that the highest ferromagnetic transition temperature in GaMnAs would be achieved if the material is grown in such a way that Mn atoms form a (201) superlattice. These results have prompted current development of experimental methods for such a growth. We also have reconciled the disagreement between the theoretical and experimental studies of vacancy-induced ferromagnetism in CaO. This material had been argued theoretically to exhibit bulk ferromagnetism, yet experimentally such a ferromagnetism is hard to observe. We showed that the concentration of vacancies required for the onset of bulk ferromagnetism in CaO cannot be achieved under the near-equilibrium bulk growth conditions.

In collaboration with Stroud, I have explored the effects of inhomogeneities and phase fluctuations on the AC conductivity of high-T_c superconductors and Josephson-junction arrays. In particular, I have shown that spatial variations in the superfluid density produce additional electromagnetic absorption similar to that seen experimentally in Bi₂Sr₂CaCu₂O_8+x; I have also derived several sum rules for this absorption. Such spatial variations in superfluid density have been observed in several recent imaging experiments. For high-T_c materials containing nonmagnetic impurities such as Zn, I showed that the concentration-dependence of both the superfluid density and the microwave conductivity can be accounted for, in part, by a simple percolation model. Finally, I have shown that the observed infrared absorption spectrum of superconducting MgB₂ is consistent with the multiple gap model proposed for this compound, and studied the effects of Al doping on the superconducting properties of this compound.

Physics of Heterogeneous Materials

In collaboration with Bergman and Stroud, I have developed theories and numerical models for the behavior of composite systems in a magnetic field. In this case, the effective resistivity of the composite is known to depend on magnetic field, even if none of the constituents have any magnetoresistance. Using both numerical and analytical techniques, I have shown that the phase diagram of certain ternary composites contains a critical line separating regions of saturating and non-saturating magnetoresistance. The percolation problem which describes this line is a generalization of anisotropic percolation. I have used scaling theory, duality symmetry, and numerical simulations based on a resistor-network model to study the critical properties of this system. I have also proposed a simple model for the magnetoresistance in some ferromagnetic domain structures, based on similar concepts drawn from the theory of heterogeneous materials. This theory leads to the negative magnetoresistance which is observed in some ferromagnetic thin films.
I have also studied of the effective properties of heterogeneous materials, using Bergman's spectral theory. This theory was originally developed to treat composite materials with isotropic constituents. We have generalized this spectral theory so that it describes the effective properties of a polycrystalline material with anisotropic crystallites; we have also used this theory to develop approximations for the third-order nonlinear properties of a polycrystal. I have further extended this theory to treat a weakly disordered periodic composite, such as a colloidal crystal. In this case, I have shown how the introduction of disorder converts the discrete spectrum of a periodic composite into a continuous spectrum, similar to that which characterizes other disordered composites. Recently, together with Zunger and co-workers, I have studied the effect of magnetic interaction radius onto the percolation-governed critical concentration necessary for establishing the ferromagnetic order in disordered magnetic alloys.

Publications in Peer-Reviewed Journals

1. S. Barabash and D. Stroud,
"Spectral Representation for the Effective Macroscopic Response of a Polycrystal: Application to Third-Order Nonlinear Susceptibility",
J. Phys.: Condens. Matter 11, 10323 (1999).

2. S. Barabash, D. Stroud and I.-J. Hwang,
"Conductivity Due to Classical Phase Fluctuations in a Model For High-T_c Superconductors",
Phys. Rev. B 61, R14924 (2000) .

3. Sergey V. Barabash, David J. Bergman, and D. Stroud,
"Magnetoresistance of Three-Constituent Composites: Percolation Near a Critical Line",
Phys. Rev. B 64, 174419 (2001).

4. Sergey V. Barabash, D. Stroud,
"Negative Magnetoresistance Produced by Hall Fluctuations in a Ferromagnetic Domain Structure",
Appl. Phys. Lett. 79, 979 (2001).

5. Jeng-Da Chai, Sergey V. Barabash and D. Stroud,
"Simple Model for the Variation of Superfluid Density with Zn Concentration in YBCO",
Physica C 366, 13 (2001).

6. Sergey V. Barabash and David Stroud,
"Structural and Superconducting Transitions in Mg_1-xAl_xB₂",
Phys. Rev. B 66, 012509 (2002).

7. Sergey V. Barabash and David Stroud,
"Transition Spectra for a BCS Superconductor with Multiple Gaps: Model Calculations for MgB₂",
Phys. Rev. B 66, 172501 (2002).

8. Sergey V. Barabash and David Stroud,
"Superfluid Inhomogeneity and Microwave Absorption in Model High-T_c Superconducting Films",
Physica B 338, 224 (2003) .

9. Sergey V. Barabash and David Stroud,
"Effective Macroscopic Response of a Composite with Small Deviations from Periodicity: Application to Colloidal Crystals",
Physica B 338, 4 (2003) .

10. Sergey V. Barabash and David Stroud,
"Models for Enhanced Absorption in Inhomogeneous Superconductors,"
Phys. Rev. B 67,144506 (2003).

11. Jorge Osorio-Guillen, Yu-Jun Zhao, Sergey V. Barabash, Alex Zunger,
"Structural stability of (Ga,Mn)As from first-principles: Random alloys, ordered compounds, and superlattices",
Phys.Rev. B 74, 035305 (2006).

12. J. Osorio-Guillen, S. Lany, S.V. Barabash, A. Zunger,
"Magnetism without magnetic ions: Percolation, exchange, and formation energies of magnetism-promoting intrinsic defects in CaO",
Phys.Rev.Lett. 96, 107203 (2006).

13. Sergey V. Barabash, Volker Blum, Stefan Müller, Alex Zunger,
"Prediction of unusual stable ordered structures of Au-Pd alloys via a first-principles cluster expansion",
Phys.Rev. B 74, 035108 (2006).

14. A. Franceschetti, S.V. Dudiy, S.V. Barabash, A. Zunger, J. Xu, and M. van Schilfgaarde,
"Design of high T_c ferromagnetic semiconductors from first principles",
Phys.Rev.Lett. 97, 047202 (2006).

15. A. Franceschetti, S.V. Barabash, J. Osorio-Guillen A. Zunger, and M. van Schilfgaarde,
"Enhancement of interactions between magnetic ions in semiconductors due to declustering",
Phys.Rev. B 74, 241303(R) (2006).

16. J. Osorio-Guillen, S. Lany, S. V. Barabash, and A. Zunger
"Nonstoichiometry as a source of magnetism in otherwise nonmagnetic oxides: Magnetically interacting cation vacancies and their percolation",
Phys. Rev. B 75, 184421 (2007).

17. Hakan Gunaydin, Sergey V. Barabash, K. N. Houk, and V. Ozolins,
"First-Principles Theory of Hydrogen Diffusion in Aluminum",
Phys. Rev. Lett. 101, 075901 (2008).

18. S. V. Barabash, V. Ozolins, and C. Wolverton,
"First-Principles Theory of Competing Order Types, Phase Separation, and Phonon Spectra in Thermoelectric AgPb_mSbTe_m+2 Alloys",
Phys. Rev. Lett. 101, 155704 (2008) .

19. S. V. Barabash, V. Ozolins, and C. Wolverton,
"First principles theory of the coherency strain, defect energetics, and solvus boundaries in the PbTe-AgSbTe₂ system",
Phys. Rev.B 78, 214109 (2008) .

Preprints:

20. Sergey V. Barabash, Volker Blum, and Alex Zunger, "First-principles determination of low-T order and new ground states of Fe-Ni, Fe-Pt, and Fe-Pd",
submitted to Phys. Rev. Lett..

21. Roman Chepulskii, Sergey V. Barabash, and Alex Zunger,
"Theory of phase stability and structural selectivity of Fe-Pd alloys",
in preparation.

22.Sergey V. Barabash, Roman Chepulskii, Volker Blum, and Alex Zunger,
"Mixed fcc/bcc, high-low spin alloys from first-principles: unsuspected ground states in Fe-(Ni,Pd,Pt)",
in preparation.

Talks and Presentations