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 Wed Jun 29, 2022 (1401/4/8) Jun 291401/4/8 13:00 Fingerprinting local environments with application to machine learning interatomic potentials Show Abstract Abstract:A class of supervised machine learning approaches aims at predicting a quantity from an input data vector. For example, it is common practice to recognize a person's face from the set of data points (pixels) in a digital image frame. The same techniques are effectively useful in computational condensed matter physics problems for the prediction of atomic contributions to a given physical quantity from the arrangement of the neighboring atoms of the individual atoms. Then one needs a "descriptor" that quantifies the environment of an atom such that it can be fed as input to a supervised machine learning tool. We review the basic ideas, techniques, and challenges of fingerprinting the local environment as an essential ingredient of training interatomic potentials from e.g. ab initio reference samples. We exemplify the local fingerprinting by applying it to predict the excited and ground-state energy surfaces of large atomic systems. Lecturer(s): Ali Sadeghi From : Physics Department, Shahid Beheshti University, Tehran, Iran Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed Jun 22, 2022 (1401/4/1) Jun 221401/4/1 14:00 Optimization by a quantum reinforcement algorithm Show Abstract Abstract:Providing an optimal path to a quantum annealing algorithm is key to find good approximate solutions to computationally hard optimization problems. Reinforcement is one of the strategies that can be used to circumvent the exponentially small energy gaps of the system in the annealing process. We take a local entropy in the configuration space for the reinforcement and apply the algorithm to a number of easy and hard optimization problems. Lecturer(s): Abolfazl Ramezanpour From : Department of Physics, School of Sciences, Shiraz University Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed Jun 15, 2022 (1401/3/25) Jun 151401/3/25 14:00 Two-component density functional theory for muonic molecules Show Abstract Abstract: It is well-known experimentally that the positively charged muon and the muonium atom may bind to molecules and solids, and through muon�??s magnetic interaction with unpaired electrons, valuable information on the local environment surrounding the muon is deduced. Theoretical understanding of the structures and properties of the resulting muonic species requires accurate and efficient quantum mechanical computational methodologies. In this talk, the two-component density functional theory (TC-DFT) will be introduced as a possible candidate for the proper treatment of muonic systems. This approach is capable of treating the electrons and positive muon on an equal footing as quantum particles, which is beyond the domain of the purely electronic DFT framework. In addition, a novel electron-positive muon correlation functional will be offered for the first time, which serves as the main ingredient of the muonic TC-DFT methodology. The computational application of the developed method to a benchmark set of muonic organic molecules will also demonstrate its capability to elucidate the intricate interactions of the positive muon in complex molecular systems. Lecturer(s): Mohammad Goli From : IPM, School of Nano Science Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed Jun 08, 2022 (1401/3/18) Jun 081401/3/18 14:00 Magnetic blue-shift of Mott gaps enhanced by double-exchange Mechanism Show Abstract Abstract:Strong correlations in Mott insulators induce a substantial charge excitation energy known as the Mott gap. In this talk, we discuss how the Mott gap is affected by long-range antiferromagnetic ordering upon reducing the temperature below the Neel temperature. Our finding is that the Mott gap is increased by the magnetic ordering: a magnetic blue-shift (MBS) occurs. We unveil the origin of the MBS of the Mott gap by analyzing the Hubbard model and the Hubbard-Kondo model and clarify the subtle differences. We show that in the Hubbard model the MBS is determined by the magnetic exchange coupling. In the Hubbard-Kondo model, an additional contribution proportional to the hopping is induced by the double-exchange mechanism. We describe the magnetic contribution to the band gap blue-shift observed in the optical conductivity of α-MnTe and pinpoint a hopping contribution of 64 % and a magnetic exchange contribution of 36 %. A MBS with the energy scale of the hopping and the exchange interaction bears the potential to enable spin-to-charge conversion on extreme time scales, highly promising for spintronic and magnonic applications. Lecturer(s): Mohsen Hafez Torbati From : Department of physics, Technical University of Dortmund, Germany Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed Jun 01, 2022 (1401/3/11) Jun 011401/3/11 14:00 Collective cloaking of a cluster of electrostatically defined core-shell quantum dots in graphene Show Abstract Abstract:We study the cloaking of a cluster of electrostatically defined core-shell quantum dots in graphene. Guided by the generalized multiparticle Mie theory, the Dirac electron scattering from a cluster of quantum dots is addressed. Indeed distant quantum dots may experience a sort of individual cloaking. But despite the multiple scattering of an incident electron from a set of adjacent quantum dots, collective cloaking may happen. Via a proper choice of the radii and bias voltages of shells, the two most important scattering coefficients and hence the scattering efficiency of the cluster dramatically decrease. Energy-selective electron cloaks are realizable. More importantly, clusters simultaneously transparent to electrons of different energies, are achievable. Being quite sensitive to applied bias voltages, clusters of core-shell quantum dots may be used to develop switches with high on-off ratios. Lecturer(s): Mahdiyeh Sadrara From : IPM, School of Nano Science Research Group: Condensed Matter and Statistical Physics Group Wed May 25, 2022 (1401/3/4) May 251401/3/4 11:00 Experimentally simulating unexplored regimes of quantum walks via patterned photonic metasurfaces Show Abstract Abstract:The ability to control the couplings between either confined or propagating optical modes is the basis of photonic simulators. Reproducing temporally-long evolutions of particles across large lattices is a challenging task, as required setups are complex and lossy. We proposed a novel method to do a photonic quantum walk, even hundreds of steps, by propagating a light beam through a few birefringent optical elements. We achieved up to 320 timesteps of a one-dimensional quantum walk, far beyond state-of-the-art experiments [1]. In this talk, I will introduce an optical element which we named it g-plate. It is a patterned liquid crystal metasurface which has a lot of applications in classical and quantum optics. I will briefly explain how it is possible to measure picosecond displacement by this device [2]. I will also talk about in what way it is possible to do ultra-long timesteps quantum walk with a few g-plates. Lecturer(s): Amin Babazadeh From : Department of Physics, University of Naples Federico II, Naples, Italy Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed May 18, 2022 (1401/2/28) May 181401/2/28 14:00 Investigation of Hyperbolic Plasmon Polaritons in a Td-WTe2 Single Layer Show Abstract Abstract:Natural hyperbolic two-dimensional systems are a fascinating class of materials that could open alternative pathways to manipulating plasmon propagation and light-matter interactions. Here, we present a comprehensive study of the optical response in Td -WTe2 by means of density-functional and many-body perturbation theories. We show how monolayer WTe2 with in-plane anisotropy sustains hyperbolic plasmon polaritons, which can be tuned via chemical doping and strain. The latter is able to extend the hyperbolic regime toward the near-infrared with low losses. Moreover, WTe2 can even be switched between elliptic and hyperbolic regimes with a moderate strain. In addition, plasmons in WTe2 are characterized by low losses owing to electron-phonon scattering, which is responsible for the temperature dependence of the plasmon line width. Interestingly, the temperature can also be utilized to tune the in-plane anisotropy of the WTe2 optical response. Lecturer(s): Zahra Torbatian From : School of Nano Science,, IPM Research Group: Condensed Matter and Statistical Physics Group More Info. : Link Wed Apr 27, 2022 (1401/2/7) Apr 271401/2/7 14:00 Electron transport and interface effects in NbS2//WSe2 lateral and vertical heterostructures Show Abstract Abstract:Research on two-dimensional (2D) materials has drawn enormous attention since the pioneering studies of the novel properties of graphene. Among 2D materials, the interest in the transition metal dichalcogenide (TMD2) family has grown exponentially because of their outstanding electronic and optical properties. In this talk, we investigate transport in NbS2//WSe2 lateral heterostructure (LH) and vertical heterostructure (VH). Based on DFT simulations using Quantum Espresso suite of codes and an electrostatic potential analysis, we unveil electron transmission through the scattering region and study the effects of the electronic structure of each TMD2 and their interfaces on the electron transmission. Both LH and VH have nonzero transmission below the Fermi energy, while the LH has the higher transmission. For VH, we obtain the double peak transmission and bigger interval due to having a different electronic structure. The double peak in VH transmission will be further explained in terms of a simplified but accurate model. Lecturer(s): Zahra Golsanamlou From : CNR-ICCOM & IPCF, Consiglio Nazionale delle Ricerche, Pisa, Italy Research Group: Condensed Matter and Statistical Physics Group More Info. : link Wed Mar 02, 2022 (1400/12/11) Mar 021400/12/11 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 231400/12/4 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 *** Please Search (Top of the page) to see more seminars. ***

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