Quantum annealing | EPFL Graph Search

Quantum annealing (QA) is an optimization process for finding the global minimum of a given objective function over a given set of candidate solutions (candidate states), by a process using quantum fluctuations. Quantum annealing is used mainly for problems where the search space is discrete (combinatorial optimization problems) with many local minima; such as finding the ground state of a spin glass or the traveling salesman problem. The term "quantum annealing" was first proposed in 1988 by B. Apolloni, N. Cesa Bianchi and D. De Falco as a quantum-inspired classical algorithm. It was formulated in its present form by T. Kadowaki and H. Nishimori (ja) in "Quantum annealing in the transverse Ising model" though an imaginary-time variant without quantum coherence had been discussed by A. B. Finnila, M. A. Gomez, C. Sebenik and J. D. Doll, in "Quantum annealing is a new method for minimizing multidimensional functions". Quantum annealing starts from a quantum-mechanical superposition of

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

This thesis presents investigations of magnetic and structural properties of two dimensional metal-organic frameworks and of single atoms, both adsorbed on decoupling layers grown on metal surfaces, with focus on rare earth elements.We first report on a series of attempts at realizing self-assembled metal-organic networks on different decoupling layers, probed by scanning tunneling microscopy. The aim is to produce regular structures with high surface filling factors such that the magnetic properties of the rare earth atoms in the metal-organic networks can be investigated by X-ray ensemble measurements. The combinations of three molecular ligands (QDC, BDA, ZnTPyP) on three insulating layers (MgO, NaCl, graphene) with Tb as rare earth are reported. Of the three decoupling layers, only graphene is found to allow the synthesis of metal-organic structures. We successfully synthesized large islands of a rare earth-QDC coordination complex, with a majority of fivefold coordination of the rare earth atoms. The same structure is reproduced with Dy and Er.X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism measurements are then carried out on the Dy- and Er-QDC/gr/Ir(111) networks. Compared to single atoms on gr/Ir(111), both Dy and Er change their

4f

occupancy from 4

f^n

4f^{n-1}

and change their easy axis of magnetization. Both species are paramagnetic when incorporated into the metal-organic structure. The last part of the thesis is devoted to the study of Dy, Ho, and Gd on NaCl thin films. All three atoms display

4f

occupancy of the gas phase. The top-Cl adsorption site is determined with scanning tunneling microscopy. The magnetic properties are probed with X-ray absorption spectroscopy, X-ray magnetic circular and linear dichroism. Ho and Gd are paramagnetic. Dy has out-of-plane anisotropy with open hysteresis at non zero magnetic field. However, thermally assisted quantum tunneling of the magnetization prevents remanence at zero field.

EPFL2022

Universal Aspects of Metamagnetic Transitions in Frustrated Ising Chains

Nagabhushan Ganesh Hegde

The study of magnetic frustration has been interesting due to the variety of magnetic ordering observed at low temperatures. At these temperatures thermal fluctuations become redundant and quantum fluctuations play a significant role in the determining the ground state of the system. Frustration can happen due to randomness or geometry. In our research, we have focused on triangle-based geometrically frustrated systems where stronger FM coupling is present along the c- axis and weaker AFM coupling in the ab- plane. Such quasi-1D behavior gives rise to several interesting features and metamagnetic phase transitions are one among them. In triangle based Ising geometry 1st order metamagnetic phase transitions are observed between 1/3Ms and Ms (saturation magnetization). Since most of the research has been dedicated to explain the formation of these plateaus, there is a lack of clear understanding of the dynamics of this transition. Our primary aim is to construct analytical models that can explain the dynamics of metamagnetic transitions using bulk and microscopic methods. Further, we would like to expand our quest to focus on finding out universal behavior of these transitions in different materials.

EPFL2021

In-situ and post annealing effect on the microstructure and the optical properties of black Cu-Co-Mn oxide spinel coating for Parabolic Trough Collector (PTC) applications

Samaneh Daviran, Anna Krammer, Josef Andreas Schuler

The present paper deals with the deposition of Cu-Co-Mn oxides thin film by magnetron sputtering to produce a black selective coating for solar Parabolic Trough Collector (PTC) systems. The effect of temperature was studied between the nominal temperature range of 300-600°C, using both in-situ heating and post annealing. The optical behavior was investigated by advanced spectrophotometry devices in visible, near and infrared spectral ranges. Scanning Electron Microscopy (SEM) was used to show the microstructure, and X-ray Diffraction (XRD) to identify the phases and to compare the samples'pattern prepared at different temperatures.

IOP Science2019

Magnetism and metal-organic self-assembly of rare-earth atoms on decoupling layers

Sébastien Reynaud

4f

occupancy from 4

f^n

4f^{n-1}

4f

EPFL2022