Condensed-Matter Physics seminars: Fall 2007

Wednesdays in the Physics Reading Room

Date & time Speaker & affiliation Talk title & abstract

Oct 3 4:00pm Kumar S. Raman (UCR)
On the construction of microscopic models with exotic phases
Abstract: The past two decades have seen the discovery of a number of strongly correlated materials with unconventional physical properties and complex phase diagrams. This has led to a number of proposals of non-trivial quantum ground states, including the resonating valence bond (RVB) spin liquid phases, the related valence bond crystal phases, and striped phases with periods that are large (even incommensurate) compared with the scale on the underlying lattice. In this talk, I will discuss mechanisms by which such exotic phases can arise in strongly correlated quantum systems with only local interactions and no explicitly broken lattice or spin symmetries.I will do this by constructing microscopic Hamiltonians that contain these phases.
References: PRB 72, 064413 (2005); PRB 75, 094406 (2007).

Oct 10 4:00pm TBA

Oct 17 4:00pm TBA

Oct 24 4:00pm Jeanie Lau (UCR)
Phase Coherent Charge Transport in Carbon Nanotubes and Graphene
Abstract: Graphene and carbon nanotubes, the one- and two-dimensional forms of carbon, have emerged as promising candidates for electronic materials, as well as new model systems for condensed matter physics. In this talk I will discuss phase coherent transport of electrons and holes in these systems, where Fabry-Perot interference of multiply-reflected charge waves gives rise to conductance oscillations. When graphene and carbon nanotubes are coupled to superconducting electrodes, they form novel Josephson junctions and give rise to enhanced conductance at zero-bias, multiple Andreev reflections, and gate-tunable switching current. Our results underscore the fascinating properties of graphene and nanotubes as prototypical 1D and 2D systems.

Oct 31 4:00pm TBA

Nov 2 Friday
4:00pm Ilya Gruzberg (U. Chicago)
Anderson localization transitions, multifractal wave functions, conformal invariance, and entanglement entropy.
Abstract: Anderson metal-insulator transitions (MIT) caused by disorder in electronic systems exhibit many fascinating features making them quite special. In particular, electronic wave functions that are extended in the metal and localized in the insulator, appear to have intricate self-similar structure right at the transition. This structure leads to an infinite number of scaling multifractal exponents that describe the moments of the wave functions. We have recently developed the concept of multifractality of wave functions for finite samples with boundaries and corners. It appears that the critical behavior of various observables related to wave functions near a boundary at a MIT is different from that in the bulk. In two dimensions (2D), the multifractal exponents at a corner are found to be directly related to those at a straight boundary through a relation arising from conformal invariance. This provides direct evidence for conformal invariance at a 2D MIT. Moreover, the presence of boundaries modifies the multifractal spectrum of the whole sample even in the thermodynamic limit. Finally, multifractal scaling of critical wave functions directly leads to a singular behavior of the entanglement entropy across an Anderson transition.

Nov 7 4:00pm TBA

Nov 14 4:00pm Oleg Tchernyshyov (JHU)
Topological defects in nanomagnets
Abstract: Formation of magnetic domains in a macroscopic ferromagnet is a familiar phenomenon caused by a competition between local and long-range forces. The physics of domains becomes drastically different in nanosized magnets. In particular, domain walls in magnetic nanowires are composite objects containing a few elementary topological defects: vortices with integer and fractional winding numbers. Dynamics of such domain walls can be reduced to the motion of these "elementary particles". A nonzero skyrmion charge of a vortex strongly influences the dynamics of composite domain walls.

Nov 21 4:00pm Jing Shi (UCR)
Dissipationless Hall and Nernst Currents in Mn-Doped GaAs
Abstract: The physical origin of the anomalous Hall effect (AHE) discovered in ferromagnets has been a topic of long-standing debate. The Hall current may or may not depend on the scattering rate, which is called extrinsic or dissipationless, respectively, and it has not been completely settled experimentally. To help resolve this issue, we carried out a systematic study of both electrical and thermoelectric transport properties in Mn-doped GaAs ferromagnetic semiconductors. We found the thermoelectric counterpart of AHE - the anomalous Nernst effect (ANE), which is directly related to the magnetization instead of the applied magnetic field. Unlike AHE that remains positive below the Curie temperature, ANE can switch its sign at low temperatures. We show that the sign change in ANE is the first indication that the Hall and Nernst currents are not extrinsic. We further fit the temperature dependence of ANE using all measured transport coefficients and obtained excellent agreement which quantitatively yields the exponent of the power-law between R_xy and R_xx. Our results show the dissipationless nature of the Hall and Nernst currents in ferromagnets. Meanwhile, we experimentally established the Mott relation for the first time for dissipationless anomalous transport in ferromagnets.

Nov 30
Friday
2:00pm
Note the unusual day and time Yaroslav Tserkovnyak (UCLA)
Spin-charge coupling and separation in magnetic wires
Abstract: In this talk, I will discuss an interplay of electron flows and magnetic texture dynamics. Electric currents turn out to offer very efficient means to exert spin-transfer torques on the collective magnetization. In particular, various instabilities of the magnetic order and domain-wall motion driven by electric currents have recently attracted much attention both theoretically and experimentally. At the same time, nonequilibrium magnetic textures give rise to an SU(2) gauge field, which in some simple limits can be understood in terms of spin-dependent fictitious electromagnetic forces acting on electrons. After constructing a self-consistent mean-field theory describing the interaction of textured ferromagnetism and quasiparticle transport in quasi-three-dimensional diffuse wires, I will also briefly discuss an amusing departure from the mean-field picture in strongly-interacting one-dimensional ferromagnetic nanowires, where spin waves intricately interplay with soundlike plasmons.

Dec 5 4:00pm Gil Refael (Caltech)
Superfluid-insulator transition in strongly disordered Boson chain: new results from real-space RG.
Abstract: The superfluid-insulator transition of bosons has constantly attracted much attention - it describes the physics of Cooper pairs in films and nanowires, as well as Helium and other bosonic atoms. Little is known, however, about this phase transition in the presence of strong disorder. In my talk I will present recent progress in the study of strongly disordered one-dimensional Bosonic chains. Using real-space renormalization group, we analyzed the U(1) rotor model with randomness in hopping, charging, and chemical potential, and found the universal low energy properties of the insulating phases in the system. The real-space RG also allows a discussion of the nature and universality of the critical point at large disorder, which we compare to what is known about the superfluid-insulator transition in weakly-disordered bosonic chains.

Dec 5 4:00pm Finals week

Seminars for Winter 2008
Seminars for Winter 2007
Seminars for Spring 2007
Seminars for Fall 2006
Seminars for Spring 2006
Seminars for Winter 2006
Seminars for Fall 2005

Leonid Pryadko <my first name at landau dot ucr dot edu>

Last modified: Tue Dec 4 17:39:02 PST 2007

Date & time	Speaker & affiliation	Talk title & abstract
Oct 3 4:00pm	Kumar S. Raman (UCR)	On the construction of microscopic models with exotic phases Abstract: The past two decades have seen the discovery of a number of strongly correlated materials with unconventional physical properties and complex phase diagrams. This has led to a number of proposals of non-trivial quantum ground states, including the resonating valence bond (RVB) spin liquid phases, the related valence bond crystal phases, and striped phases with periods that are large (even incommensurate) compared with the scale on the underlying lattice. In this talk, I will discuss mechanisms by which such exotic phases can arise in strongly correlated quantum systems with only local interactions and no explicitly broken lattice or spin symmetries.I will do this by constructing microscopic Hamiltonians that contain these phases. References: PRB 72, 064413 (2005); PRB 75, 094406 (2007).
Oct 10 4:00pm	TBA
Oct 17 4:00pm	TBA
Oct 24 4:00pm	Jeanie Lau (UCR)	Phase Coherent Charge Transport in Carbon Nanotubes and Graphene Abstract: Graphene and carbon nanotubes, the one- and two-dimensional forms of carbon, have emerged as promising candidates for electronic materials, as well as new model systems for condensed matter physics. In this talk I will discuss phase coherent transport of electrons and holes in these systems, where Fabry-Perot interference of multiply-reflected charge waves gives rise to conductance oscillations. When graphene and carbon nanotubes are coupled to superconducting electrodes, they form novel Josephson junctions and give rise to enhanced conductance at zero-bias, multiple Andreev reflections, and gate-tunable switching current. Our results underscore the fascinating properties of graphene and nanotubes as prototypical 1D and 2D systems.
Oct 31 4:00pm	TBA
Nov 2 Friday 4:00pm	Ilya Gruzberg (U. Chicago)	Anderson localization transitions, multifractal wave functions, conformal invariance, and entanglement entropy. Abstract: Anderson metal-insulator transitions (MIT) caused by disorder in electronic systems exhibit many fascinating features making them quite special. In particular, electronic wave functions that are extended in the metal and localized in the insulator, appear to have intricate self-similar structure right at the transition. This structure leads to an infinite number of scaling multifractal exponents that describe the moments of the wave functions. We have recently developed the concept of multifractality of wave functions for finite samples with boundaries and corners. It appears that the critical behavior of various observables related to wave functions near a boundary at a MIT is different from that in the bulk. In two dimensions (2D), the multifractal exponents at a corner are found to be directly related to those at a straight boundary through a relation arising from conformal invariance. This provides direct evidence for conformal invariance at a 2D MIT. Moreover, the presence of boundaries modifies the multifractal spectrum of the whole sample even in the thermodynamic limit. Finally, multifractal scaling of critical wave functions directly leads to a singular behavior of the entanglement entropy across an Anderson transition.
Nov 7 4:00pm	TBA
Nov 14 4:00pm	Oleg Tchernyshyov (JHU)	Topological defects in nanomagnets Abstract: Formation of magnetic domains in a macroscopic ferromagnet is a familiar phenomenon caused by a competition between local and long-range forces. The physics of domains becomes drastically different in nanosized magnets. In particular, domain walls in magnetic nanowires are composite objects containing a few elementary topological defects: vortices with integer and fractional winding numbers. Dynamics of such domain walls can be reduced to the motion of these "elementary particles". A nonzero skyrmion charge of a vortex strongly influences the dynamics of composite domain walls.
Nov 21 4:00pm	Jing Shi (UCR)	Dissipationless Hall and Nernst Currents in Mn-Doped GaAs Abstract: The physical origin of the anomalous Hall effect (AHE) discovered in ferromagnets has been a topic of long-standing debate. The Hall current may or may not depend on the scattering rate, which is called extrinsic or dissipationless, respectively, and it has not been completely settled experimentally. To help resolve this issue, we carried out a systematic study of both electrical and thermoelectric transport properties in Mn-doped GaAs ferromagnetic semiconductors. We found the thermoelectric counterpart of AHE - the anomalous Nernst effect (ANE), which is directly related to the magnetization instead of the applied magnetic field. Unlike AHE that remains positive below the Curie temperature, ANE can switch its sign at low temperatures. We show that the sign change in ANE is the first indication that the Hall and Nernst currents are not extrinsic. We further fit the temperature dependence of ANE using all measured transport coefficients and obtained excellent agreement which quantitatively yields the exponent of the power-law between `R_xy` and `R_xx`. Our results show the dissipationless nature of the Hall and Nernst currents in ferromagnets. Meanwhile, we experimentally established the Mott relation for the first time for dissipationless anomalous transport in ferromagnets.
Nov 30 Friday 2:00pm Note the unusual day and time	Yaroslav Tserkovnyak (UCLA)	Spin-charge coupling and separation in magnetic wires Abstract: In this talk, I will discuss an interplay of electron flows and magnetic texture dynamics. Electric currents turn out to offer very efficient means to exert spin-transfer torques on the collective magnetization. In particular, various instabilities of the magnetic order and domain-wall motion driven by electric currents have recently attracted much attention both theoretically and experimentally. At the same time, nonequilibrium magnetic textures give rise to an `SU(2)` gauge field, which in some simple limits can be understood in terms of spin-dependent fictitious electromagnetic forces acting on electrons. After constructing a self-consistent mean-field theory describing the interaction of textured ferromagnetism and quasiparticle transport in quasi-three-dimensional diffuse wires, I will also briefly discuss an amusing departure from the mean-field picture in strongly-interacting one-dimensional ferromagnetic nanowires, where spin waves intricately interplay with soundlike plasmons.
Dec 5 4:00pm	Gil Refael (Caltech)	Superfluid-insulator transition in strongly disordered Boson chain: new results from real-space RG. Abstract: The superfluid-insulator transition of bosons has constantly attracted much attention - it describes the physics of Cooper pairs in films and nanowires, as well as Helium and other bosonic atoms. Little is known, however, about this phase transition in the presence of strong disorder. In my talk I will present recent progress in the study of strongly disordered one-dimensional Bosonic chains. Using real-space renormalization group, we analyzed the `U`(1) rotor model with randomness in hopping, charging, and chemical potential, and found the universal low energy properties of the insulating phases in the system. The real-space RG also allows a discussion of the nature and universality of the critical point at large disorder, which we compare to what is known about the superfluid-insulator transition in weakly-disordered bosonic chains.
Dec 5 4:00pm	Finals week