SCHOOL OF PHYSICS :: Seminars

Seminars

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Wed Mar 02, 2022 (1400/12/11)

	Mar 02 1400/12/11	14:00	RGreen-X library: Exascale Green-Function-Based Methods Show Abstract Abstract:A new open-source exascale library of Green-function-based methodologies is under developing. Its layered design will separate higher-level functionalities from architecture-dependent numerical routines, common to all code families. Considerations of scaling with system size favour the choice of algorithms based on real space sparseness and time-frequency transforms, like the real-space / imaginary-time approach (cubic scaling instead of the quartic scaling of other algorithms), whose larger amount of numerical evaluations as compared to data communication is also well suited for the massive parallelism in exascale machines. The new library, Green-X, will include time-frequency transformations, space transformations, sparse or full basis-set dependent transformations, and solutions of the Poisson equation. Lecturer(s): Maryam Azizi From : Universite Catholique de Louvain Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Feb 23, 2022 (1400/12/4)

	Feb 23 1400/12/4	14:00	Random Long-range interacting spin chains: Entanglement properties Show Abstract Abstract:Quantum information theoretical measures are useful tools for characterizing quantum dynamical phases. However, employing them to study excited states of random spin systems is a challenging problem. Here, we report results for the entanglement entropy (EE) scaling of excited eigenstates of random XX antiferromagnetic spin chains with long-range (LR) interactions decaying as a power law with distance with exponent $\alpha$. To this end, we extend the real-space renormalization group technique for excited states (RSRG-X) to solve this problem with LR interaction. For comparison, we perform numerical exact diagonalization (ED) calculations. From the distribution of energy level spacings, as obtained by ED for up to $N\sim 18$ spins, we find indications of a delocalization transition at $\alpha_c \approx 1$ in the middle of the energy spectrum. With RSRG-X and ED, we show that for $\alpha>\alpha^$ the entanglement entropy (EE) of excited eigenstates retains a logarithmic divergence similar to the one observed for the ground state of the same model, while for $\alpha<\alpha^$ EE displays an algebraic growth with the subsystem size $l$, $S_l\sim l^{\beta}$, with $0<\beta<1$. We find that $\alpha^* \approx 1$ coincides with the delocalization transition $\alpha_c$ in the middle of the many-body spectrum. An interpretation of these results based on the structure of the RG rules is proposed, which is due to {\it rainbow} proliferation for very long-range interactions $\alpha\ll 1$. We also investigate the effective temperature dependence of the EE allowing us to study the half-chain entanglement entropy of eigenstates at different energy densities, where we find that the crossover in EE occurs at $\alpha^* < 1$. Lecturer(s): Javad Vahedi From : Jacobs University Bremen Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Feb 02, 2022 (1400/11/13)

	Feb 02 1400/11/13	14:00	Glycosylation promotes the cancer regulator EGFR-ErbB2 heterodimer formation â?? molecular dynamics study Show Abstract Abstract:ErbB family of receptor tyrosine kinases play significant roles in cellular differentiation and proliferation. Mutation or overexpression of these receptors leads to several cancers in humans. The family has four homologous members including EGFR, ErbB2, ErbB3, and ErbB4. From which all except the ErbB2 bind to growth factors via the extracellular domain to send signals to the cell. However, dimerization of the ErbB receptor occurs in extracellular, transmembrane, and intracellular domains. The ErbB receptors are known to form homodimers and heterodimers in the active form. Heterodimerization increases the variety of identified ligands and signaling pathways that can be activated by these receptors. Furthermore, glycosylation of the ErbB receptors has shown to be critical for their stability, ligand binding, and dimerization. Here, atomistic molecular dynamics simulations on the glycosylated and unglycosylated heterodimer showed that the EGFRErbB2 heterodimer is more stable in its dynamical pattern compared to the EGFR-EGFR homodimer. This increased stability is regulated by maintaining the dimeric interface by the attached glycans. It was also shown that the presence of various glycosylation sites within the ErbB2 growth factor binding site leads to occlusion of this site by the glycans that inhibit ligand binding to ErbB2 and participate in further stabilization of the heterodimer construct. Putting together, glycosylation seems to promote the heterodimer formation within the ErbB family members as the dominant molecular mechanism of activation for these receptors. DOI: https://doi.org/10.1007/s00894-021-04986-9 Lecturer(s): Maryam Azimzadeh Irani From : Shahid Beheshti University Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Jan 19, 2022 (1400/10/29)

	Jan 19 1400/10/29	14:00	Survival probability in Generalized Rosenzweig-Porter random matrix ensemble Show Abstract Abstract:This is talk about the dynamics of the generalized Rosenzweig-Porter model, which is known to possess three distinct phases: ergodic, multifractal and localized phases. Our focus is on the survival probability R(t), the probability of finding the initial state after time t . In particular, if the system is initially prepared in a highly-excited non-stationary state (wave packet) confined in space and containing a fixed fraction of all eigenstates, we show that R(t) can be used as a dynamical indicator to distinguish these three phases. Three main aspects are identified in different phases. The ergodic phase is characterized by the standard power-law decay of R(t) with periodic oscillations in time, surviving in the thermodynamic limit, with frequency equals to the energy bandwidth of the wave packet. In multifractal extended phase the survival probability shows an exponential decay but the decay rate vanishes in the thermodynamic limit in a non-trivial manner determined by the fractal dimension of wave functions. Localized phase is characterized by the saturation value of R(t â??â??)=k, finite in the thermodynamic limit Nâ??â?? , which approaches k=R(t â??0) in this limit. Lecturer(s): Mohsen Amini From : University of Isfahan Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Jan 05, 2022 (1400/10/15)

	Jan 05 1400/10/15	14:00	Nonlinear & Quantum Integrated Photonics Show Abstract Abstract:A photon with a well-defined quantum state can be generated via spontaneous parametric down conversion (SPDC). Therefore, the beginning of the seminar is a brief review of the nonlinear optics with emphasis on the SPDC. Then some of recent works done in my group, related to the topic, will be presented. The optical mode will be explained and then results of very recent projects related to this area of research will be presented. Lecturer(s): Rahman Nouroozi From : Institute for advanced studies in Basic Sciences (IASBS) Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Dec 08, 2021 (1400/9/17)

	Dec 08 1400/9/17	14:00	A critical look at theoretical predictions and experimental discoveries of topological states of matter Show Abstract Abstract:Topology is a branch of mathematics addressing the global properties of objects being independent of local smooth deformation, which has led to the concepts of topological invariants. The use of quantum topological concepts, however, have become common in condensed matter physics over the last 40 years. The first topological state discovered experimentally is the integer quantum Hall pointing to the topological character of the wave functions of a 2D electron gas. The topological materials have been, however, discovered in 2000-2021, mostly following theoretical predictions. While this might be tantalizing at first look, the way of looking for what is theoretically known might be misleading. In this talk I briefly review the recent discoveries with a critical look at some them. Lecturer(s): Mehdi Kargarian From : Sharif University of Technology Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Nov 24, 2021 (1400/9/3)

	Nov 24 1400/9/3	14:00	Environmental fate of 2D materials in aquatic ecosystems: the case of graphene oxide 2D membranes Show Abstract Abstract:An increasing number of 2D materials are being deployed for different applications across energy generation, conversion and storage sectors. Among these materials, however, graphene oxide and not graphite oxide, as the most technologically relevant 2D material of choice for energy-related applications, has gained a stronghold and is near the final stage of commercialization ready to enter the mass market. This calls for a thorough understanding of the behaviour of this intriguing atomically thin material and possibly other members of the mushrooming family of 2D materials in aquatic environment as their toxicity can easily pose a threat to aquatic life at global levels not seen before due to the huge surface area of 2D materials (~2000 m2/g). Although a great deal of research has been dedicated to this subject, a key misconception is the treatment of these materials as a sphere with an effective radius or that their flat surface would fold upon itself resulting in a much lower aspect ratio and consequently lower available surface area. This essentially implies a much higher critical concentration at which it is alleged that these 2D materials are toxic to the aqueous environment. Our findings presented here point out that this is not a valid assumption by showing that 2D materials in saline waters hold their flat morphology and therefore pose a greater challenge for the environment rather than their professed 3D conformed counterparts. Lecturer(s): Seyed Hamed Aboutalebi From : Condensed Matter National Laboratory, IPM Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Nov 10, 2021 (1400/8/19)

	Nov 10 1400/8/19	14:00	Floquet Engineering of Topological Insulators Show Abstract Abstract:Recent technological progresses in mid-infrared lasers have established a new route for engineering of the electronic band structure. Indeed, an interplay between the periodicity of lattice and time originated from the oscillating field extends the Hilbert space by inducing the so-called Floquet-Bloch states and enables one to modify the band structure (opening the gaps) and to engineer phase transitions at will by using photon-assisted processes. Fortunately, such light-induced states have been experimentally observed on the surface of irradiated topological insulators, optical lattices, graphene etc. Many striking phenomena are predicted or observed in this field: light-induced Hall conductivity, light-induced QAHI on TIï¿½??s, etc. However, experimental challenge would be huge heating rate when the system is irradiated by the laser. In this talk, I will present a short review on the Floquet theory including on- and off- resonant regimes, the gap opening and emergence of topological phases accompanied by anomalous edge states. Then I will focus on the special case of magnetically topological insulator thin film irradiated by circularly-polarized electromagnetic field in the off-resonant regime. Fascinating feature of distinct phases emerges in the phase diagram depending on the frequency and intensity of the light as well as the system parameters and also magnetic field. We also try to go through the on-resonant regime and calculate optical conductivity of this driven system. Lecturer(s): Hosein Cheraghchi From : Damghan University Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Oct 27, 2021 (1400/8/5)

	Oct 27 1400/8/5	11:00	An Amelioration for the Sign Problem: Adiabatic Quantum Monte Carlo Show Abstract Abstract:In this talk, I will introduce the adiabatic quantum Monte Carlo (AQMC) method, where we gradually crank up the interaction strength, as an amelioration of the sign problem. It is motivated by the adiabatic theorem and will approach the true ground-state if the evolution time is long enough. I will demonstrate that the AQMC enhances the average sign exponentially such that low enough temperatures can be accessed and ground-state properties probed. It is a controlled approximation that satisfies the variational theorem and provides an upper bound for the ground-state energy. I will first benchmark the AQMC vis-Ã -vis the undoped Hubbard model on the square lattice which is known to be sign-problem-free within the conventional quantum Monte Carlo formalism. Next, I will test the AQMC against the density-matrix-renormalization-group approach for the doped four-leg ladder Hubbard model and demonstrate its remarkable accuracy. As a nontrivial example, I will apply our method to the Hubbard model at p=1/8 doping for a 16Ã?8 system and discuss its ground-state properties. I will finally utilize our method and demonstrate the emergence of U(1)2â?¼SU(2)1 topological order in a strongly correlated Chern insulator. Lecturer(s): Abolhassan Vaezi From : Sharif University of Technology Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
Wed Oct 13, 2021 (1400/7/21)

	Oct 13 1400/7/21	14:00	Localization and delocalization in one-dimensional Anderson model with a general hopping matrix Show Abstract Abstract:Much of our present understanding of wave-function localization in one spatial dimension is based on the original Anderson model on a one-dimensional lattice. The experimental simulation of this model using cold atoms, owing to the high degree of control over system parameters, has made possible the direct observation of localization of matter waves. In this talk, after giving an introduction to Anderson localization, I will present some new results on localization properties of this model with a general hopping matrix. Lecturer(s): Reza Sepehrinia From : University of Tehran Research Group: Condensed Matter and Statistical Physics Group Weekly Seminar More Info. : Link
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