Spin quantum number | EPFL Graph Search

In physics, the spin quantum number is a quantum number (designated s) that describes the intrinsic angular momentum (or spin angular momentum, or simply spin) of an electron or other particle. It has the same value for all particles of the same type, such as s = 1/2 for all electrons. It is an integer for all bosons, such as photons, and a half-odd-integer for all fermions, such as electrons and protons. The component of the spin along a specified axis is given by the spin magnetic quantum number, conventionally written ms. The value of ms is the component of spin angular momentum, in units of the reduced Planck constant ħ, parallel to a given direction (conventionally labelled the z–axis). It can take values ranging from +s to −s in integer increments. For an electron, ms can be either +1/+ or −1/

Magnetic Properties of Surface Adsorbed Metal Adatoms and Dimers

Dante Philippe Sblendorio

This thesis investigates the magnetic properties of single atoms and dimers adsorbed on graphene and oxide decoupling layers supported by single crystal metal substrates, using scanning tunneling microscopy (STM) and spin-polarized scanning tunneling microscopy (SP-STM). The goal is twofold: to use SP-STM to further advance the understanding of the interactions that determine the magnetic stability of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), and to use insights from these systems to motivate the study of new systems-- mainly, IrCo heterodimers on graphene/Ir(111).For Dy adatoms on graphene/Ir(111), the characteristic lifetime of the spin system is measured as a function of temperature and tunnel bias, probing the available magnetization reversal pathways in the energy level diagram. The necessity of including the intra-atomic exchange to correctly describe spin lifetimes is demonstrated. In addition, naturally abundant Dy isotopes possess two possible nuclear spin values. Models of both nuclear spin cases of Dy are compared, and shown to produce similar behavior in the temperature and bias ranges probed by SP-STM. Furthermore, accounting for both nuclear spin cases allows for modeling of X-ray magnetic circular dichroism (XMCD) magnetization sweeps. For Ho adatoms on MgO/Ag(100), novel measurement protocols are used to determine the zero-field stability, and the correct ground state model. These protocols are also used to induce magnetic state reversal via Landau-Zener tunneling at avoided level crossings due to the hyperfine interaction. Finally, the study of the IrCo heterodimer adsorbed on graphene is motivated with stability considerations derived from the studies of Dy adatoms on graphene/Ir(111) and Ho adatoms on MgO/Ag(100), in addition to calculations based on density function theory (DFT). Strategies of engineering these heterodimers using statistical growth and atomic manipulation are successfully implemented. The heterodimer is highly mobile on graphene/Ir(111) and displays geometric instability, consistent with its predicted upstanding geometry. Despite this, measurement via STM and SP-STM is demonstrated. Observations are consistent with the robust magnetic stability expected from DFT studies, but additional investigation is needed to disentangle the measurements made thus far.

EPFL2022

Spinon-like excitations in spin-1/2 quantum antiferromagnets

Irina Safiulina

Quantum magnetism remains a hot topic in condensed matter physics due to its complexity and possible powerful and significant applications in data storage and memory. To understand how the materials can achieve these goals, one should have a clear idea about the fundamentals behind it. In this thesis, we focus on three examples that can help us deepen the knowledge in many-body effects, which stand to be crucial for quantum magnetism.(1) The well-known \textbf{CuSO

_4\cdot

_2

O} material has already demonstrated the model behaviour as one-dimensional Heisenberg antiferromagnet in zero and high (

H>H_{sat}

) fields. The fully-polarized magnetic ground state is described by linear spin-wave theory with magnons, whereas at zero field, the excitations are pairs of topological excitations called spinons. In an intermediate field, the dynamic properties are even more complicated. The inelastic spectrum cannot be reproduced without considering exotic elementary excitations and bound states such as psinons and Bethe strings. Although Bethe in 1931 provided an exact solution for 1D Heisenberg systems, there is still no quantitative comparison between theory and experiment.(2) The magnetic ground state and hence the dynamic properties of the gemstone mineral green dioptase, \textbf{Cu

_6[

_6

_{18}]\cdot6

_2

O} are under debate: starting from controversial theories and continuing with non-explained experimental observations. Dioptase is a quasi-one-dimensional spin chain with dominant antiferromagnetic interactions. Recent studies claim the classical spin chain behaviour and absence of any quantum fluctuations in the system. In contrast, our experimental findings indicate the presence of continuous excitations above and below T

_N

.(3) Newly synthesized material \textbf{CuSb

_2

_6

} of rosiaite-type structure tends to become a quantum spin-liquid (QSL) candidate since the magnetic cations Cu

^{2+}

are arranged in trigonal layers, and no long-range order is observed down to 2~K. The idea of QSL on the triangular lattice was proposed by P. Anderson in 1973. Since his work, a lot of efforts have been made to explore deeper, both theoretically and experimentally, this state. As for now, there are several requirements needed to be met -- small spin number, absence of long-range order or spin freezing, long-range entanglement, and the associated fractional spin excitations. We aim to establish whether or not CuSb

_2

_6

can be considered as a potential quantum spin-liquid candidate employing different techniques suitable for a powder sample.

EPFL2022

First-order Neel to columnar valence bond solid transition in a model square-lattice S=1 antiferromagnet

We study the Neel to fourfold columnar valence bond solid (cVBS) quantum phase transition in a sign-free S = 1 square-lattice model. This is the same kind of transition that for S = 1/2 has been argued to realize the prototypical deconfined critical point. Extensive numerical simulations of the square-lattice S = 1/2 Neel-VBS transition have found consistency with the deconfined critical point scenario with no direct evidence for first-order behavior. In contrast to the S = 1/2 case, in our quantum Monte Carlo simulations for the S = 1 model, we present unambiguous evidence for a direct conventional first-order quantum phase transition. Classic signs of a first-order transition demonstrating coexistence including double-peaked histograms and switching behavior are observed. The sharp contrast from the S = 1/2 case is remarkable; we hypothesize that this is a striking demonstration of the role of the size of the quantum spin in the phase diagram of two-dimensional lattice models.

AMER PHYSICAL SOC2020