CMP Kid's journal club at Stanford

Beth Nowadnick, Deveraux group
Quantum Monte Carlo simulations of strongly correlated materials

Strongly correlated materials display a rich phase diagram due to the presence of many competing interactions. However, analytical solutions to models describing strongly correlated materials are often lacking, and numerical approaches are needed. One class of numerical approaches for studying strongly correlated materials is Quantum Monte Carlo, which uses Monte Carlo techniques to simulate interacting quantum many body problems.

I will first review the Monte Carlo method in general, and then I will discuss a particular algorithm called Determinant Quantum Monte Carlo (DQMC), which is useful for studying systems of many interacting fermions. I will then present results from a DQMC simulation of the Hubbard-Holstein model, which includes both electron-electron and electron-phonon interactions. This allows us to study the role of electron-phonon coupling in systems with strong electron correlations, which is relevant for strongly correlated systems where signatures of electron-phonon coupling have been observed, such as the cuprates.

Ronny Thomale, KQEZG group, SITP fellow
Realization of topological spin liquids in condensed matter - the Kitaev model in the iridates?

I report on recent progress to identify promising material candidates of correlated electrons to host topological quantum phases. Specifically, I present the Heisenberg-Kitaev model to characterize the Na₂IrO₃ and Li₂IrO₃ compounds, as well as renormalization group calculations to obtain the phase diagram as a function of different Hamiltonian couplings. Our predictions following from there have been recently confirmed in detail by specific heat measurements which I will discuss from a theoretical perspective. I outline further directions of the field.

Wonhee Ko, Manoharan group
Realizing topological phases on molecular graphene

Graphene exhibits special electronic properties stemming from its two-dimensional (2D) structure and embedded relativistic Dirac cones. However, many proposed topologically ordered ground states remain elusive in conventional measurements due to the difficulty in arranging the necessary quantum textures into natural graphene. By exploiting atomic manipulation with a custom-built ultrastable scanning tunneling microscope, we have constructed graphene-like structures by arranging molecules to create a honeycomb lattice of electrons drawn from normal 2D surface states. Spectroscopy reveals a spectacular transformation of nonrelativistic massive 2D electrons into massless Dirac fermions carrying a chiral pseudospin symmetry. We also demonstrate the tailoring of this new class of graphene to reveal signature topological properties, ranging from simple energy gap to exotic states related to quantum Hall effect.

Inna Vishik, Shen group
Phase competition in cuprate phase diagram

The momentum-resolved nature of angle-resolved photoemission spectroscopy (ARPES) has made it a key probe of emergent phases in the cuprates, such as superconductivity and the pseudogap, which both have anisotropic momentum-space structure. I will begin with an introduction to the cuprates and the ARPES technique, followed by a discussion of our recent laser-ARPES experiments in Bi₂Sr₂CaCu₂O_8+d (Bi-2212) exploring near-nodal superconductivity and dynamic competition between the pseudogap and superconducting gap. The phenomenology uncovered therein leads to a revised cuprate phase diagram.

Hello all,

I hope you had a good Christmas break and wish everyone the very best for 2012. The Journal Club for this Winter quarter is filling up nicely, but we still have some spots left open. If you're interested in giving a talk for the Journal Club, check out the Winter 2012 Schedule for open dates.

One idea that I had, was for a 'review' of the APS March Meeting in the Journal Club on March 6. If I can find enough volunteers who would like to take 10 minutes to talk about the most interesting thing they saw at the Meeting, I think it would be an interesting Journal Club. So, if you are going to the APS March Meeting and are willing to talk 10 minutes about one of the presentations there (not your own:-), send me an email. Or talk to me in the hallways. Or something.

See you all tomorrow!

Gerwin Hassink
Journal Club organizer

Special Announcement: This Journal Club will be held in McCullough 130 instead of its usual location of 335. Spread the word!

Ming Yi, Shen group,
ARPES studies of electronic nematicity in iron pnictide superconductors

In most iron pnictide families that are capable of being doped to superconduct, there exists in the underdoped region a tetragonal to orthorhombic structural transition (i.e. in-plane C4 symmetry breaking) that is closely correlated with a collinear spin density wave transition. Evidence for large in-plane anisotropy that persists above both transitions in detwinned compounds has been reported by transport measurements, suggesting the existence of an electronic nematic phase. In this talk, I will present our ARPES studies of the electronic structures of two families of iron pnictide compounds, Co-doped Ba122 and NaFeAs. We observe distinct signatures of electronic reconstructions that occur at each of the transitions, which also reveal an interesting way by which these two transitions might be related. More importantly, we observe a C4 symmetry breaking in the orbital degree of freedom that is associated with the higher structural transition above the magnetic transition, hence suggesting that it is not a trivial result of the low temperature magnetic ground state, but likely an important player in pnictide physics.

Minu Kim, Hwang group
Exploring the interplay between high-mobility electrons and superconductivity in delta-doped SrTiO₃

d-electron systems exhibit versatile and exciting physical phenomena, which can be employed to study novel electronic properties in low dimensions. Perhaps the most well known material is SrTiO₃ (STO), not only as a building block of complex oxide heterostructures [1], but also as a high-mobility d⁰-electron semiconductor, and a superconductor with the lowest known carrier density. Using thin-film growth techniques, we have fabricated “delta-doped” STO structures that confine the superconducting Cooper pairs in 2D without surface or interface scattering. Crucially these samples simultaneously show quantum oscillations that demonstrate the presence of 2D electron states [2]. Furthermore, by increasing the mobility, we discovered a crossover of the electronic structure from 3D to 2D [3], as well as unconventional spin properties revealed by the robustness of the superconductivity to parallel magnetic fields [4]. These results provide an ideal system to investigate quantum phase transitions into a variety of ground states, and novel superconductivity in the 2D clean limit.

[1] A. Ohtomo & H. Y. Hwang, Nature 427, 423 (2004)
[2] Y. Kozuka et al., Nature 462, 487 (2009)
[3] M. Kim et al., Phys. Rev. Lett. 107, 106801 (2011)
[4] M. Kim et al., arXiv:condmat/1106.5193

Nicholas Breznay, KGB group
Weak localization and metal-insulator-transition phenomena in GeSbTe phase-change materials

In this talk, I will discuss electrical transport studies of weak localization phenomena in crystalline phase change materials. These materials, made from the Ge-Sb-Te system, are used for optical and electronic data storage and are a promising candidate for universal memory and multi-level storage applications. However, there remain significant open quesitons about the apparent metal-insulator transition in 3D samples and the relevant physical mechanisms affecting electrical transport. Thin 2D films show quantum interference effects at low temperatures, also known as weak localization. I will give a short introduction to these nifty materials, and then review the theory of weak localization and explain how we can use magnetoresistance techniques to learn about the quantum transport behavior of these materials.

Hanoh Lee, Fisher group
Superconducting gap symmetry study in CeIrIn₅ under hydrostatic pressure

CeIrIn₅ is one of heavy fermion superconductors with a T_C of 0.3 K at ambient pressure. I will present my past work data with collaborators about the field rotation ac-calorimetry study of CeIrIn₅ under pressures up to 2.05 GPa deep inside its second superconducitng dome to identify its nodal gap structure. A fourfold oscillation in the heat capacity at 0.3 K is clearly observed and our study shows d_x²-y² order parameter symmetry in this compound like CeCoIn₅.