I discuss the nematic order in Fe-pnictides and argue that it has a magnetic origin. I introduce and analyze the effective low-energy itinerant model for collective magnetic excitations in Fe-pnictides, discuss the selection of long-range stripe magnetic order, and show that the stripe magnetic order is generally preempted by an Ising-nematic order which breaks C4 lattice symmetry but preserves O(3) spin-rotational symmetry. This leads to a rich phase diagram as a function of doping and pressure, with split magnetic and nematic tri-critical points. In some cases, the nematic transition instantly induces a magnetic transition, in other cases a magnetic transition occurs at a lower temperature. I show that the preemptive nematic transition is accompanied by either a jump or a rapid increase of the magnetic correlation length, what triggers magnetic precursors and psudogap-like electronic behavior. The spin-nematic transition also induces orbital order. I compare our results to various experiments on Fe-pnictides.
Monday, March 26, 12 noon, Bloomberg 475
Playing a lego game for a quasi-two-dimensional frustrated magnet
In this talk, we will consider complex magnetic interactions in a kagome-triangular-kagome trilayer antiferromagnet. I will present a simple and eloquent way of understanding the physics by mapping the magnetic interactions onto a problem of an ordered tricolor and a disordered binary sign degree of freedom. By doing so, we will show a systematic way of constructing different classical ground states, and will identify possible zero-energy excitations that involve 'partial but extended' numbers of spins in the system. Due to the unique properties of the ground state, we argue that a spin glass might be the ground state for this quasi-two-dimensional frustrated magnet.
Wednesday, February 22, 12 noon, Bloomberg 475(note special day)
Orbital order in iron-based superconductors
Weicheng Lv
University of Illinois, Urbana Champaign
Iron pnictides exhibit in-plane anisotropy in the absence of the long-range magnetic order. In this talk, I will argue that this nematic phase originates from a ferro-orbital order involving the Fe dxz and dyz orbitals. In particular, I will show that the localized states are coupled by spin-orbital interactions, and consequently the spin exchange constants acquire different values along the two in-plane axes with the onset of the orbital order. The itinerant electrons also undergo a nematic transition, resulting in the anisotropy observed in many transport and spectroscopic experiments. Furthermore, due to the double exchange mechanism, this ferro-orbital order in the itinerant bands causes the effective magnetic coupling to be negative along the ferromagnetic direction. I will closely compare our results with the experiments and demonstrate the importance of the orbital order.
Monday, January 30, 12 noon, Bloomberg 475
Iron pnictides and chalcogenides: A new class of strongly correlated electron systems
The recently discovered iron ?based superconductors pose a new challenge to ?many body theory methods. We ?will argue that these materials are Hund's metals, ?a new class of strongly correlated materials where the correlations are ?controlled by the strength of the ? Hunds rule ?coupling J, rather than by the Hubbard U. While correlations in these materials are strong, their physical properties are strikingly different from that of transition metal oxides. We will describe some of the LDA+DMFT based predictions for various spectroscopies: optical conductivity, photoemission and inelastic neutron scattering. We will compare the theoretical predictions with experiments. We will frame these questions in the more general context of our quest for describing, understanding and predicting the physical properties of strongly correlated materials.
Wednesday, January 25, 12 noon, Bloomberg 475(note special day)
Experimental clues to the ground state of the "quantum spin ice" pyrochlore
The nature of the low temperature magnetism in the magnetic pyrochlore Yb2Ti2O7 has been controversial?within the geometrical frustration community. Effective S=1/2 quantum spins are associated with Yb in Yb2Ti2O7. These are ferromagnetically-coupled to each other, and possess XY-like symmetry. ?This should imply that Yb2Ti2O7 possesses a non-frustrated ground state in spite of its underlying pyrochlore lattice. In contrast, many experimental studies have indicated a disordered ground state below a "transition" temperature (Tc ~ 240 mK), marked by a specific heat anomaly. Our diffuse neutron scattering above and below Tc show intriguing short range, low dimensional spin correlations, and application of a modest magnetic field at low temperatures induces a quantum phase transition to an ordered, polarized state. Recently, the spin waves we measured in the polarized state have been theoretically modeled. Strongly anisotropic exchange parameters from the microscopic Hamiltonian have been extracted, providing a possible explanation for the frustrated magnetism in Yb2Ti2O7, which is now revealed to have "quantum spin ice" type interactions. I will discuss our most recent paper in the context of these theoretical studies.
Thursday, January 12, 12 noon, Bloomberg 462 (note special day and room)
Theory of spin liquids in integer-spin pyrochlores
Rare-earth pyrochlores, with a chemical formula A2B2O7, exhibit many interesting features in A-site spin systems. Depending on the A-site rare earth elements, spin ice and magnetically ordered phases are found in several experiments. Moreover, they have been also proposed as possible candidates for a U(1) spin liquid. In order to explore such versatile phases, we study a pseudospin-1/2 model applicable to rare-earth pyrochlores with integer spins, in the presence of spin-orbit coupling and crystalline electric field. Using a new "gauge mean-field theory", we find possible ground states corresponding to several of the phases listed above.
Thursday, December 15, 12 noon,Bloomberg 274(note special day and room)
Challenges in the discovery of magnetic materials
Adam Phelan
Louisiana State University
Discovery-driven synthesis of new materials is motivated by the interesting physical properties displayed by compounds comprised of lanthanides, transition metals, and main group elements.? Emphasis is placed on the discovery of highly correlated materials which have a tendency to exhibit unusual magnetic, transport, and thermodynamic properties.? Moreover, we employ the flux-growth synthetic method in order to grow large single crystals of these materials so that we may accurately determine their structure-physical property relationships. New magnetic materials will play an important role in the fields of information technology, alternative energies, and basic research. However, the discovery and optimization of new magnetic materials is not a trivial task.? The synthetic and analytical challenges encountered in my quest to discover new magnetic materials will be presented.
Friday, December 9, 12 noon, Bloomberg 475(note special day)
Ground state degeneracy of chiral superconductors allows the existence of? complicated topological defects -- domain walls formed between regions of opposite chirality. A tentative candidate material for a chiral superconductor is Sr2RuO4 -- the observation which is corroborated, in particular, by muSR? and Kerr measurements. The story however is complicated by the fact that scanning measurements aimed to detect? magnetic field produced the chiral domains yielded a null result.? A possible resolution is that domains form an alternating structure which leads to a substantial field cancellation. In this talk I examine thermodynamic stability of chiral domain walls and vortices? and demonstrate that at sufficiently small applied and chiral fields their existence is not favored? and the sample's configuration is a single domain. The particular chirality of the single-domain configuration is neither favored nor disfavored by the applied field. Increasing the field leads to an entry of a domain wall loop or a vortex into the sample. Formation of a straight domain wall is never preferred in equilibrium.
Monday, December 5, 12 noon, Bloomberg 475
Orbital Order and Orthorhombic Anisotropy in Iron Pnictides
Orthorhombic anisotropy has been reported in iron-pnictide superconductors by a broad range of experiments, including neutron scattering, transport, and a variety of spectroscopic measurements. In this talk, I will discuss the idea that these observed anisotropies of broken tetragonal symmetry stem from an ordering of the partially-filled iron 3d-orbitals. In particular, I will consider a model Hamiltonian that couples the spin and orbital variables, and show that several properties of this spin-orbital model are reminiscent of the observed behaviors in the iron-pnictide materials. I will conclude the talk by discussing light-polarization studies of x-ray absorption and other recent experiments supportive of theories highlighting the importance of the orbital degrees of freedom.
Monday, November 21, 12 noon, Bloomberg 475
Dirac fermions and quantum transport in Bi/Sb layered compounds
Dirac fermions have raised great interest in condensed matter?physics, as seen on the example of materials such as graphene and?topological insulators. Here we reported the two-dimensional quantum?transport behavior of four Bi/Sb based compounds with layered structure.?The linear energy dispersion leads to the unusual non-saturated linear?magnetoresistance (MR) since all Dirac fermions occupy the lowest Landau?level in the quantum limit. The transverse magnetoresistance exhibits a?crossover at a critical field B* from semiclassical quadratic-field?dependent MR to the high-field linear MR. The temperature dependence of?the B* satisfies the quadratic behavior. This plus the large effective?magnetoresistant mobility indicates the existence of Dirac fermions in?this kind of materials.
Monday, October 31, 12 noon, Bloomberg 475
Terahertz electrodynamics of surface states of topological insulator Bi2Se3
3D Topological insulators are a class of band insulators in which strong spin-orbit coupling leads to interesting 2D properties derived from their bulk band structures. This leads to topologically protected surface states that show massless Dirac dispersion as well as protection to backscattering. This talk will center on our recent experiments measuring the THz response of thin films of the topological insulator Bi2Se3. At low frequencies, transport is essentially thickness independent showing the dominant contribution of the surfaces. Despite their extended exposure to ambient conditions, these surfaces exhibit robust properties including narrow, almost thickness-independent Drude peaks, and an unprecedentedly large polarization rotation of linearly polarized light reflected in magnetic field. This Kerr rotation can be as large as 65 degrees and can be explained by a cyclotron resonance effect of the surface states. Our results are evidence for the intrinsic response of the topologically protected surface states and may serve as a benchmark in the search for the universal "axionic" Kerr and Faraday magneto-electric effects predicted in these materials. I will also discuss our recent work on the THz response of superconductor-Bi2Se3 layered structures and evidence for a inverse-proximity effect at the interface.
Monday, October 24, 12 noon, Bloomberg 475
A field theory for a fermionic ladder with generic intrachain interactions
An effective low energy field theory is developed for a system of two chains. The novelty of the approach is that it allows to treat generic intrachain repulsive interactions of arbitrary strength.? The chains are coupled by a direct tunneling and four-fermion interactions. At low energies the individual chains are described as Luttinger liquids with an arbitrary ratio of spin vs and charge vc velocities. A judicious choice of the basis for the decoupled chains greatly? simplifies the description and allows one to separate high and low energy degrees of freedom. In a direct analogy to the bulk cuprates the resulting effective field theory distinguishes between three qualitatively different regimes: (i) small doping (vc << vs), (ii) optimal doping (vc \approx vs) and (iii) large doping (vc >> vs). I discuss the excitation spectrum and derive expressions for the electron spectral function which turns out to be highly incoherent. The degree of incoherence increases when one considers an array of ladders (stripe phase).
Spin ice is a highly frustrated ferromagnet displaying rich emergent phenomena. In its ground states, magnetization of spin ice satisfies a zero-divergence constraint leading to an effective cancellation of the internal magnetic field. Spin excitations violate this constraint and thus behave as magnetic monopoles. New pyrochlore materials such as Yb2Ti2O7 stand out as possible candidates for quantum spin ice, in which quantum fluctuations could play a major role. The low-energy effective theory of such a system is quantum electrodynamics with emergent photons and quantum monopoles.
In this talk, I will discuss new emergent phenomena in quantum spin ice in an external magnetic field applied along a (100) lattice direction. When? quantum monopoles are confined by the external field, the magnetic flux tube (an open string binding a monopole pair) becomes a dynamical object with a field-dependent tension. The motion of an open string includes longitudinal expansion and contraction and transverse fluctuations. Much different from the difficult quantum string theory in 26 dimensions, the emergent quantum string theory in this context allows for simple analytical solution and straightforward numerical simulation. Moreover, vibrational modes of the string can be detected by experimental techniques such as neutron scattering and THz spectroscopy.
Topological insulators are a new type of band insulator. They are distinguished by the fact that their edges (in the 2D case) or surfaces (in the 3D case) support gapless transport which is extremely robust. In the two dimensional case, topological insulators can be thought of as time reversal invariant analogues of integer quantum Hall states. This analogy is intriguing since integer quantum Hall states are a special case of the far richer class of fractional quantum Hall states. It is natural to wonder: can topological insulators also be generalized? In this talk, I will investigate this question. I will show that, just as in quantum Hall systems, electron interactions allow for a whole new class of states - which we call "fractional topological insulators."
Untitled Document
Institute for Quantum Matter Department of Physics & Astronomy Bloomberg Center Johns Hopkins University 3400 North Charles Street Baltimore, MD 21218
