NEWS: During the spring 2020 semester, the Long Island Condensed Matter Social Distancing Journal Club will replace most seminars.
Journal Club events will be posted on this calendar and take place via Zoom. To join the mailing list and get the link, email Mengkun Liu (first firstname.lastname@example.org)
The CQM Distinguished Lecture series has been established in the Fall of 2015 to bring to Stony Brook University the renown experts in the physics of quantum matter.
December 2019 – March 2020
Title: Incommensurate transitions and twist disorder in microscopic models of twisted bilayer graphene
Abstract: Recent experiments in twisted bilayer graphene have set off a flurry of work due to the observation of purportedly correlated phases at the so-called “magic-angle.” However, the current models in the literature for magic-angle graphene suffer from two flaws: they assume commensurate twist angles and have great difficulty modeling the exact experimental setup where patches of different twist angles appear (“twist disorder”). We introduce and study a family of microscopic models that begins to address these concerns. In these models, the twist angle enters as a free parameter in real space. We can use this to simulate both incommensurate effects and disorder effects. We find that incommensuration leads to an Anderson-like delocalization transition in momentum space. The result is a small metallic phase at the “magic-angle” (that we speculate is unstable to correlated phases). We further study twist-disorder effects and find that while the minibandwidth is renormalized substantially, the Fermi velocity is not significantly altered.
Host: Jen Cano
Title: Microscopic view of heat conduction in solids
Microscopic quantum mechanical interactions among heat carriers called phonons govern the macroscopic thermal properties of semiconducting and electrically insulating crystalline solids, which find applications in thermal management of electronics, thermal barrier coatings and thermoelectric modules. In this talk, I will describe my recent work on how our newly developed first-principles computational framework to predict these microscopic interactions among phonons unveils a new paradigm for heat conduction in several of these materials. As an example, I will describe a curious case of heat conduction in boron arsenide (BAs), where the lowest order interactions involving three phonons are unusually weak and higher-order scattering among four phonons affects the thermal conductivity significantly, in stark contrast with several other semiconductors such as silicon and diamond . I will show that this competition between three and four phonon scattering can be exquisitely tuned with the application of hydrostatic pressure, resulting in an unusual non-monotonic pressure dependence of the thermal conductivity in BAs unlike in most other materials . I will also briefly describe my prior experimental effort to probe the scattering of phonons at atomically rough surfaces of a nanoscale silicon film, where they showed extreme sensitivity to the changes in surface roughness of just a few atomic planes .
 Fei Tian, Bai Song, Xi Chen, Navaneetha K. Ravichandran et al., Science 361 (6402), 582-585, 2018
 Navaneetha K. Ravichandran & David Broido, Nature Communications 10 (827), 2019
 Navaneetha K. Ravichandran, Hang Zhang & Austin Minnich, Physical Review X 8 (4), 041004, 2018
Intertwined Orders in Cuprate Superconductors
Condensed Matter Physics & Materials Science Division
Brookhaven National Laboratory, Upton, NY 11973-5000
While the nature of cuprate superconductors remains controversial, the concept of intertwined orders provides a consistent way to understand multiple types of superconductivity in these hole-doped antiferromagnets . Neutron and x-ray scattering experiments have demonstrated the tendency for the doped holes to segregate into stripes that are separated by locally-antiferromagnetic regions. As originally proposed by Emery, Kivelson, and Zachar , the hole stripes can develop pairing correlations, but superconducting order is limited by the ability to establish phase order by Josephson coupling through the intervening magnetic regions. When those intervening spin correlations can be gapped, spatially-uniform superconductivity can develop, where the coherent gap is limited by the spin gap . When the spin-stripe correlations are strong, an alternative superconducting state involves a spatially-modulated pair wave function (pair density wave) intertwined with spin stripe order. A sufficiently strong magnetic field destroys the superconducting order without disrupting the pair correlations within the stripes .
- E. Fradkin et al., Rev. Mod Phys. 87, 457 (2015).
- V. J. Emery et al., Phys. Rev. B 56, 6120 (1997).
- Yangmu Li et al., Phys. Rev. B 98, 224508 (2018).
- Yangmu Li et al., Sci. Adv. 5, eaav7686 (2019).
Host: Mengkun Liu
Mapping Spatiotemporal Dynamics of Photo-generated Carriers
Semiconducting transition-metal dichalcogenides exhibit remarkable electrical and optical properties. Using laser-illuminated microwave impedance microscopy, we were able to perform simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers with different defect densities. For chemical-vapor deposited (CVD) samples, the diffusion length and carrier lifetime, extracted from the spatial profile and temporal relaxation of microwave signals, respectively, are in good agreement with the diffusion equation and Einstein relation. Time-resolved experiments indicate that the critical process for photo-excited carriers is the escape of holes from trap states. As a result, the photoconductivity is weaker in exfoliated monolayers with lower disorders than the more defective CVD samples. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in van der Waals materials.
Quantum materials such as topological insulators, Weyl-semimetals, and atomically thin two-dimensional crystals have intriguing electronic properties, which makes them promising candidates for their potential applications in next-generation technology. Therefore, it is desirable to investigate the electronic properties of these materials, perhaps through various methods such that their full potential, as well as the limitations, can be identified. In this talk, I will introduce a novel spectroscopic approach based on the use of strong ultrafast laser pulses and the generation of high-order harmonics. Analysis of temporal and spectral properties of high-order harmonics from these materials reveals their valence charge density distributions, electronic band-structure, topology, as well as driven electron dynamics in the natural time scales of electrons. The advantages of this approach over conventional methods include the use of the all-optical setting, no physical contacts to samples, and much of the measurements that can be performed in ambient conditions. More importantly, the non-perturbative interaction to strong laser pulses forms novel quantum phases, which could be probed by analyzing high-order harmonics.
Short Bio: Dr. Shambhu Ghimire is a Staff Scientist at Stanford PULSE Institute, where he leads Attosecond X-ray Photonics Group. He is a recipient of the prestigious Young Investigator award from U.S. Department of Energy. His research interests are on Strong-field Physics and Nonlinear X-ray Science. He received his Ph.D. in physics from Kansas State University in 2007 and went to University of Michigan for a post-doc job before he joined Stanford in 2009.
- J. Lu, E. Cunningham, Y. S. You, D. A. Reis and S. Ghimire, Interferometry of dipole phase in high harmonics from solids, Nature Photonics 13, 96100 (2019)
- S. Ghimire and D. Reis, Review: High-order harmonic generation from solids, Nature Physics Online November (2018).
- Y. You, Y. Yin, Y. Wu, A. Chew, X. Ren, F. Zhuang, S. Gholam-Mirzaei, M. Chini, Z. Chang and S. Ghimire, High-harmonic generation in amorphous solids, Nature Comm 8, 724 (2017)
- Y. You, D. Reis and S. Ghimire, Anisotropic high-harmonic generation in bulk crystals, Nature Physics, 13, 345-349 (2017).
- N. Ddabashimiye, S. Ghimire, M. Wu, D Browne, K. Schafer, M. Gaarde and D. Reis, Solid-state harmonics beyond atomic limit, Nature 534, 520-523 (2016).
“Electronic materials and states for quantum computing and networks”
Valley Phenomena and Phase Diagrams of Transition Metal Dichalcogenide Alloys
Dr. Patrick Vora
Assistant Professor | Department of Physics and Astronomy
Director | Quantum Materials Center
George Mason University
Atomically thin materials derived from layered crystals have occupied much of the condensed matter community since the discovery of graphene in 2004. Transition metal dichalcogenides (TMDs) are among the most versatile members in the family of layered materials due to the opportunities for tuning electronic behaviors with chemical composition, layer number, and structural phase. Achieving on-demand transitions between different structural phases could enable a new class of atomically-thin non-volatile memories known has phase change memories (PCMs).1 MoTe2 is an ideal candidate for this technology as it exhibits the smallest energy difference between the 2H semiconducting and 1T′ semi-metallic structural phases.2 However, the energy cost for driving a phase transition could be further reduced by alloying MoTe2 with WTe2, which naturally crystallizes in the 1T′ phase,3 and has led to substantial interest in the properties of TMD alloys.
In this colloquium I will discuss our team’s exploration of TMD alloys that are candidates for PCM applications. I will first present our work on the phase diagram of Mo1-xWxTe2 (x=0..1).4 In this study, we used polarization-resolved Raman spectroscopy to explore the composition-dependent optical properties of MoxW1-xTe2 alloys. These data provided clear signatures of the 2H, 1T′, and Td structural phases and their evolution with W composition x. Combining these results with aberration-corrected transmission electron microscopy and x-ray diffraction measurements allows for the construction of the alloy phase diagram. This interdisciplinary study clarified significant disagreements in the literature regarding the structural phase diagram and has proved foundational in future attempts to create MoxW1-xTe2 – based PCMs. The second half of this talk will focus on a previously unstudied aspect of PCM-candidate TMD alloys: valleytronic behaviors. 5 Here we investigate WSe2(1-x)Te2x alloys which are also potentially useful for PCMs but are more resistant to oxidation. Polarization-resolved photoluminescence measurements show new low energy emission features unique to this alloy system that may originate from buckling due to the large W-Te bond lengths. Despite the significant disorder, we find that valley polarization and coherence in alloys survive at high Te compositions and are larger than in pure WSe2 at elevated temperatures. These findings illustrate the persistence of valley properties in alloys with highly dissimilar parent compounds and suggest a novel class of devices combining PCM characteristics with valleytronics.
(1) Rehn, D. A.; Li, Y.; Pop, E.; Reed, E. J. Theoretical Potential for Low Energy Consumption Phase Change Memory Utilizing Electrostatically-Induced Structural Phase Transitions in 2D Materials. npj Comput. Mater. 2018, 4 (1), 2.
(2) Duerloo, K.-A. N.; Li, Y.; Reed, E. J. Structural Phase Transitions in Two-Dimensional Mo- and W-Dichalcogenide Monolayers. Nat. Commun. 2014, 5 (1), 4214.
(3) Duerloo, K.-A. N.; Reed, E. J. Structural Phase Transitions by Design in Monolayer Alloys. ACS Nano 2016, 10 (1), 289–297.
(4) Oliver, S. M.; Beams, R.; Krylyuk, S.; Kalish, I.; Singh, A. K.; Bruma, A.; Tavazza, F.; Joshi, J.; Stone, I. R.; Stranick, S. J.; et al. The Structural Phases and Vibrational Properties of Mo1−xWxTe2 Alloys. 2D Mater. 2017, 4 (4), 045008.
(5) Oliver, S. M.; Young, J.; Krylyuk, S.; Reinecke, T. L.; Davydov, A. V.; Vora, P. M. Valley Phenomena in the Candidate Phase Change Material WSe2(1-x)Te2x. ArXiv e-prints 2019, 1908.00506.
“Exploring Quantum Materials with Atomic Qubit Sensor”
We are witnessing a revolution in which quantum phenomena are being harnessed for next-generation technology. In this context, atomic qubits associated with defects in solids, such as nitrogen-vacancy (NV) centers in diamond, provide versatile building blocks for quantum technologies due to their optical addressability, atomic size, and excellent coherence. Among the applications being explored with such platform, quantum sensing technology realized with NV centers has emerged as a powerful probe of quantum materials. Due to its ability to sense magnetic field with high spatial resolution over wide temperature and dynamic range, NV sensors enable the exploration of condensed matter phenomena in parameter space inaccessible to existing probes. In this talk, I will discuss our application of NV quantum sensing technology to study correlated electronic and spin phenomena. We have directly imaged, for the first time, the viscous Poiseuille flow of the Dirac fluid in neutral graphene, a finding that holds implications for other strongly correlated electrons such as those in high-Tc superconductors. Enabled by the NV platform, we have developed new capabilities for probing coherent spin-waves, which can be applied to study novel magnetic materials and spintronic devices, and a technique for characterizing low-dimensional high-Tc cuprates without electrical contacts. Looking forward, I will highlight opportunities for advancing the frontiers of quantum materials and quantum technology enabled by NVs and other solid-state atomic qubits.
Non-volatile quantized states are ideal for the realization of classical Boolean logics. Abrikosov vortex represents the most compact magnetic object in superconductors with the size determined by the London penetration depth ~100 nm. Therefore, it can be utilized for creation of high-density digital cryoelectronics. In this talk we will describe operation of memory cells, in which a single vortex is used as an information bit . The vortex is pinned at a nano-scale trap and is read-out by a nearby Josephson junction [2,3]. Unlike SQUID-based memory cells, such cells have non-degenerate 0 and 1 states, which greatly simplify the device architecture. Furthermore, SQUID-based devices have a problem with increasing write current upon decreasing the SQUID loop size, preventing a straightforward miniaturization. To the contrary, write current for a vortex is determined by the depinning current density and, therefore, scales with the size. All together this allows simple miniaturization down to sub-micron sizes. We demonstrate that vortex memory cells have a high-endurance operation, are characterized by an infinite magnetoresistance, do not require external magnetic field, have a short access time, and a low write energy. Non-volatility and perfect reproducibility are inherent for such devices due to the quantized nature of the vortex. We argue that vortex-based memory can be used in superconducting digital supercomputers.
 T. Golod, A. Iovan, and V. M. Krasnov, Nat. Commun. 6, 8628 (2015).
 T. Golod, A. Rydh, and V. M. Krasnov, Phys. Rev. Lett. 104, 227003 (2010).
 T. Golod, A. Pagliero, and V. M. Krasnov, Phys. Rev. B 100, 174511 (2019).
Acknowledgements: The work was done in collaboration with Taras Golod, Adrian Iovan, Alessandro Pagliero, Olena Kapran and Lise Morlet-Decarnin. The work was supported by the European Union H2020-WIDESPREAD-05-2017-Twinning project SPINTECH under Grant Agreement No. 810144.
Vladimir Krasnov has graduated from Moscow Institute of Physics and Technology in 1990. He completed his PhD in 1995 from the Institute of Solid State Physics, Chernogolovka, Russia and postdoctoral studies from Danish Technical University and Chalmers University of Technology, Sweden. Since 2005 he is professor and head of the Experimental Condensed Matter Physics group at the Department of Physics, Stockholm University.