NEWS: 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.
November 2016 – September 2017
We present a time-domain analysis of the response of a BCS
superconductor (in the low temperature limit) to a few cycle THz pulse
having spectral content limited to just below the absorption threshold
for breaking pairs. The analysis is based on the finite-difference
time-domain (FDTD) approach, in combination with a model susceptibility
for a superconductor that includes an explicit dependence on the energy
gap. The FDTD approach allows us to calculate the THz induced
current density, from which we determine the modified energy gap at each
instant of time during the THz wave’s passage. The resulting non-linear
susceptibility causes up-conversion of the incident THz wave into odd
harmonics. The FDTD results are compared with experiment for thin NbN
films in both linear and non-linear regimes. Additionally, some
experimental results for the response of a NbN film to a very strong,
single-cycle, sub-THz pulse will be shown, indicating that the
superconducting state can be disrupted on a ~100fs time scale.
1. Xioaxiang Xi and G.L. Carr, Supercon. Sci. & Technol. 26, 114001 (2013).
2. T. Hong et al, J. Appl. Phys. 114, 243905 (2013).
3. R. Matsunaga et al, Science 345, 1145 (2014).
Department of Materials Science and Chemical Engineering, Stony Brook University
As recently as 10-15 years ago, nanoscale metal catalysts were described in qualitative terms: oblate and hemispherical, discs and rafts. Today we admire their shapes that can be as beautiful as Platonic or Archimedian solids. We also know how to discriminate between them with unprecedented accuracy. Some of these advances are obtained using X-ray absorption fine-structure (XAFS) spectroscopy, which is a premier tool for studying structural, electronic and dynamic properties of nanoscale clusters. Negative thermal expansion, mono-metallic amorphization, metal-nonmetal transitions, increased (or decreased) Debye temperature are but a few examples of non-bulk behaviors. As an illustration, I will describe a prototypical catalytic system, a platinum particle in equilibrium with oxide support and adsorbate gas. By combining X-ray absorption and emission spectroscopies with DFT/MD simulations, I will show that many “anomalies” have their explanation in the heterogeneous structure, fluctuating over broad time-scale . By tracking the flow of charge to and from the nanoparticle in operando conditions, several competing interactions can be disentangled: metal-support, metal-adsorbate, and support-adsorbate . These results are of interest to energy sciences: by learning how to navigate these complex interactions and employ dynamics to tune up reactivity, one can learn how to rationally design a catalyst with the desired activity and selectivity.
 A. I. Frenkel, M. Cason, A. Elsen, U. Jung, M. W. Small, R. G. Nuzzo, F. D. Vila, J. J. Rehr, E. A. Stach, J. C. Yang. “Critical review: Effects of complex interactions on structure and dynamics of supported metal catalysts”, J. Vac. Sci. Technol. A 32, 020801 (2014)
 A. Elsen, U. Jung, F. D. Vila, Y. Li, O. V. Safonova, R. Thomas, M. Tromp, J. J. Rehr, R. G. Nuzzo, A. I. Frenkel. “Intracluster atomic and electronic structural heterogeneities in supported nanoscale metal catalysts “, J. Phys. Chem. C 119, 25615-25627 (2015)
In strongly correlated electron materials, the delicate interplay between spin, charge, and lattice degrees of freedom often leads to extremely rich phase diagrams exhibiting intrinsic phase inhomogeneities. The key to understand such complexities usually lies in the characterization and control of these materials at fundamental energy, time and length scales. I will use this opportunity to report the recent advances in the IR and THz spectroscopy and explain how they can be used to probe electronic/structural phase transitions with unprecedented spatial and temporal resolutions. Specifically, with scanning near-field infrared microscopy we resolved the insulator to metal phase transitions in 3d, 4d and 4f materials with ~10 nm resolution over a broad spectral range. Using ultrafast terahertz pump terahertz probe spectroscopy we can unambiguously demonstrated the insulator to metal transition at picosecond time scales via electric field-induced electron liberation. These results set the stage for future spectroscopic investigations to access the fundamental properties of complex materials.
We provide a unified description of aging in terms of record dynamics. “Aging” refers to the increasingly sluggish dynamics widely observed in the jammed state of disordered materials. Structural evolution in aging materials requires ever larger, record-sized rearrangements in an uncorrelated sequence of intermittent events (avalanches or quakes). According to record statistics, these (irreversible!) rearrangements occur at a rate ~1/t. Hence, in this log-Poisson statistics, the number of events between a waiting time t_w and any later time t integrates to ~ln(t/t_w), such that any observable inherits the t/t_w-dependence that is the hallmark of pure aging. Based on this description, we can explain the relaxation dynamics observed in a broad range of materials, such as in simulations of low-temperature spin glasses and in experiments on high-density colloids and granular piles. We have proposed a phenomenological model of record dynamics that reproduces salient aspects of the experiments, for example, persistence, intermittency, and dynamic heterogeneity. Here, we compare the predictions of the model with the data available from experiments by Yunker, et. al. [PRL103(2009)115701].
Dr. Kin Chung Fong, Raytheon BBN Technologies
Putting hydrodynamic into solid state
Despite of the strong Coulomb interaction, electrons in simple metal behave as a non-interacting Fermi gas with long-lived quasi-particle excitation. However, weak screening near the charge neutrality point of the massless Dirac fermions in graphene can lead to a new collective behavior described by hydrodynamics. By listening to the Johnson noise of the electrons, we are able to probe simultaneously the thermal and electrical transport of the Dirac fluid and observe how it departs from Fermi liquid physics. At high temperature near the neutrality point, we find a strong enhancement of the thermal conductivity and breakdown of Wiedemann-Franz law in graphene. This is attributed to the non-degenerate electrons and holes forming a strongly coupled Dirac fluid.
Ref: Science 351, 1058 (2016)
Iron based superconductors have been attracting considerable attention since their discovery in 2008 . In particular, simple binary iron chalcogenides have recently emerged to the frontier of research due to traces of superconducting critical temperatures (Tc’s) similar to copper oxide high-Tc superconductors . In this talk I will discuss characteristics of FeX and KxFe2-yX2 (X=Se,S). I will mention in a nutshell pair breaking mechanism [4,5,6,7,8], magnetic states [9,10] and critical currents whereas I with focus on the normal states in high magnetic fields as T → 0 connected with crystal structure characteristics [18,19]. The presentation will also include brief discussion on magnetic states in semiconducting crystal structures with FeX building blocks.
Electronic polarization plays a crucial role in determining the structural and dynamical properties of water with different boundary conditions. Although it is well known that the molecular polarization in condensed phases behaves substantially differently from that in the vacuum due to the intermolecular interaction, the environmental effects have not been fully understood from first principles methods. As a result, how to rigorously define and calculate the molecular polarizability of a water molecule in different chemical environments remains an open question. A main challenge to this puzzle arises from the intrinsic non-local nature of the electronic susceptibility. We propose a fully ab initio theory to compute the electron density response under the perturbation in the local field. This method is based on our recently developed local dielectric response theory [Phys. Rev. B 92, 241107(R) (2015)], which provides a rigorous theoretical framework to treat local electronic excitations in both finite and extended systems beyond the commonly employed dipole approximation. We have applied this method to study the electronic part of the molecular polarizability of water in ice Ih and liquid water. Our results reveal that the crystal field of the hydrogen-bond network has strong anisotropic effects, which significantly enhance the out-of-plane component and suppress the in-plane component perpendicular to the bisector direction. The contribution from the charge transfer is equally important, which increases the isotropic molecular polarizability by 5–6%. Our study provides insights into the dielectric properties of water, which form the basis to understand electronic excitations in water and to develop accurate polarizable force fields of water.
This research used resources of the Center for Functional Nanomaterials, which is a U.S.
DOE Office of Science Facility, at Brookhaven National Laboratory under Contract No. DESC0012704.
This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science
of the US Department of Energy under Contract No. DE-AC02- 05CH11231.
In recent years, 2D materials, such as black phosphorus and transition metal dichalcogenides, have attracted much attention due to their excellent transport and opticalproperties. Using a tight-binding model of the electron-phonon interaction we explore phonon limited mobility in black phosphorous monolayer as a function of temperature and doping level. Using a Bethe-Salpeter equation, we investigate optical and excitonic properties of MoS2 monolayers in an applied in-plane electric field. We predict a quadratic Stark shift and its scaling with the exciton binding energy, determined by the dielectric environment. Finally, I will discuss electrical contacts in 1D carbon nanotubes and the role of electronic structure modifications caused by the nanotube deformations due to the metal wetting.
Vasili Perebeinos is a Fellow of the American Physical Society. He received Diploma in 1997 in Physics from the Moscow State University, Russia, and PhD degree in 2001 in Physics from the State University of New York at Stony Brook, USA. He worked as a Research Associate in the condensed matter theory group at Brookhaven National Lab (for 2 years) and as a Visiting Scientist (for 2 years) and as Research Staff Member (for 9 years) at IBM T. J. Watson Research Center. His research interests are in the area of advanced materials and nanostructures for electronics and optoelectronics, specifically 1D carbon nanotubes and novel 2D materials. In 2014 he become an Associate Professor at Skolkovo Institute of Science and Technology (Skoltech). He published over 75 papers cited ~11000 (h index 46).
In relativistic quantum field theory, Dirac fermions in 3D space and time exhibit so-called chiral anomaly – the non-conservation of chiral charge induced by the external gauge fields with non-trivial topology. A consequence of the chiral anomaly is the chiral magnetic effect – the generation of electric current in a magnetic field induced by the chirality imbalance between the left-handed and the right-handed fermions – which was recently discovered in Dirac semimetal ZrTe5 [Q. Li, et al. arXive:1412.6542 (2014), Nature Physics 12, 550 (2016)].The powerful notion of chirality, originally discovered in high-energy and nuclear physics, underpins a wide palette of new and useful phenomena. In this seminar, I will focus on several condensed matter systems explored experimentally. Transport coefficients arising from the chiral anomaly do not break time reversal symmetry, enabling charges, provided chirality is conserved, to travel without resistance, like Cooper pairs in superconductors. In addition, the non-dissipative charge transport supported by the chiral magnetic effect does not require any condensates in the ground state, thus, can be potentially more robust and survive to much higher temperatures. I will try to accentuate the similarities and differences between the chiral magnetic effect and conventional superconductivity. Finally, I will discuss the prospect of harnessing the power of chirality for transmission of information and energy at virtually zero energy loss.