arXiv subjects

Georgios Papavassiliou

Publications and source records attributed to Georgios Papavassiliou.

4 recordsLinked to original sources

arXiv2024

The Role of Quantum Metastability and the Perspective of Quantum Glassiness in Kitaev Fractional Spin Dynamics: An NMR Study

The suppression of magnetic order and the detection of a half-quantized thermal Hall effect in α-RuCl3 under an external magnetic field have sparked significant debate, whether these phenomena point to spin fractionalization, as posited by the Kitaev quantum spin liquid (QSL) model, or if they arise from a more conventional mechanism in an antiferromagnetically ordered spin state. Here, through 23Na NMR relaxation measurements on the layered cobaltate Na2Co2TeO6 at two distinct magnetic fields (4.7 and 9.4 Tesla), we provide compelling evidence supporting a variant interpretation. While upon cooling, the NMR relaxation times align with the temperature dependence predicted by Kitaev fractional spin excitations, below 10 K, a dynamically heterogeneous state is detected with Quantum Spin Glass (QSG) characteristics. In this state, the spin fractionalization dynamics unfold over markedly different time scales, and are characterized by quantum metastability, inducing prolonged equilibration as T->0. This observation challenges the prevailing view of a primarily homogeneous QSL state, as the ground state of tangible Kitaev systems.

cond-mat.str-el↗

arXiv2021

Detection of Weyl Fermions and the Metal to Weyl-Semimetal phase transition in WTe$_2$ via broadband High Resolution NMR

Weyl Fermions (WFs) in the type-II Weyl Semimetal (WSM) WTe$_2$ are difficult to resolve experimentally because the Weyl bands disperse in an extremely narrow region of the (E-k) space. Here, by using DFT-assisted high-resolution $^{125}$Te solid-state NMR (ssNMR) in the temperature range $50$K - $700$K, we succeeded in detecting low energy WF excitations and monitor their evolution with temperature. Remarkably, WFs appear to emerge at T$\sim 120$K; at lower temperatures WTe$_2$ behaves as a metal. This intriguing metal-to-WSM phase transition is shown to be induced by the rapid raise of the Fermi level with temperature, crossing solely the electron and hole pockets in the low-T metallic phase, while crossing the Weyl bands near the nodal points - a prerequisite for the emergence of WFs - only for T$>120$K.

cond-mat.mtrl-sci↗

arXiv2019

Direct observation of Dirac states in Bi2Te3 nanoplatelets by 125Te NMR

Detection of the metallic Dirac electronic states on the surface of Topological Insulators (TIs) is a tribune for a small number of experimental techniques the most prominent of which is Angle Resolved Photoemission Spectroscopy. However, there is no experimental method showing at atomic scale resolution how the Dirac electrons extend inside TI systems. This is a critical issue in the study of important surface quantum properties, especially topological quasiparticle excitations. Herein, by applying advanced DFT-assisted solid-state 125Te Nuclear Magnetic Resonance on Bi2Te3 nanoplatelets, we succeeded in uncovering the hitherto invisible NMR signals with magnetic shielding influenced by the Dirac electrons, and subsequently showed how Dirac electrons spread and interact with the bulk interior of the nanoplatelets.

cond-mat.mtrl-sci↗

arXiv2009

Ultrafast control of nuclear spins using only microwave pulses: towards switchable solid-state quantum gates

We demonstrate the control of the alpha-proton nuclear spin, I=1/2, coupled to the stable radical CH(COOH)2, S=1/2, in a gamma-irradiated malonic acid single crystal using only microwave pulses. We show that, depending on the state of the electron spin mS=+/-1/2, the nuclear spin can be locked in a desired state or oscillate between mI=+1/2 and mI=-1/2 on the nanosecond time scale. This approach provides a fast and efficient way of controlling nuclear spin qubits and also enables the design of switchable spin-based quantum gates by addressing only the electron spin.

quant-ph↗