Advanced Seminar on Condensed Matter Physics
INF 227, SR 1.403
Electron paramagnetic resonance (EPR) is a very powerful method due to its enhanced sensitivity to unpaired electrons. In order to understand the defect structure in functional nano-materials we use multi-frequency EPR spectroscopy. In this presentation i) basics of EPR spectroscopy, ii) quantum confinement effects in ferroelectric nano-materials and iii) EPR and Photoluminescence (PL) investigations of intrinsic defect centers in semiconductor zinc oxide (ZnO) quantum dots will be given. Starting with the introductory information about EPR spectroscopy; poling, aging, doping and nano-size effects will be discussed for the ferroelectric materials such as, PbTiO3, BaTiO3, PbZrTiO3 (PZT) etc. In the last part of the talk, surface and core defects and their reactivity under temperature and light will be presented for ZnO semiconductor quantum dots. Defect models will be discussed. Metal ion doping of ZnO nanomaterials will be presented through the application of diluted semiconductor materials (DMS) that have high potential in spintronic devices.
Interdisciplinary research into the utility of magnetic molecules for quantum computing applications represents one of the frontiers of materials science. This lecture will describe recent results of continuous-wave (cw) and pulsed EPR studies on related families of lanthanide containing molecules that have attracted tremendous interest as potential hybrid electron-nuclear spin qubits. A molecular approach is attractive because it enables systematic control of the quantum states of the lanthanide (the qubit) via molecular geometry, and allows functionalization of the molecule in order to engineer interactions between qubits.
The first example involves a HoIII (4f 10) ion encapsulated within a (W5O18)2 cage. The Ho ion experiences a significant magnetic anisotropy due to crystal-field splitting of the spin-orbit coupled total angular momentum (J = L + S = 8) ground state, resulting in a pair of low-lying mJ = ±4 singlets that are further split by a strong hyperfine interaction with the I = 7/2 nuclear spin . A small departure from a square antiprismatic (D4d symmetry) coordination geometry results in a Zeeman diagram (with B parallel to the molecular symmetry axis) with multiple avoided crossings between the 16 [(2I + 1) ´ 2] lowest-lying electron-nuclear sub-levels. Right at these avoided crossings, the EPR transition frequencies are insensitive to dipolar field fluctuations associated with the surrounding electron/nuclear spin bath, which represent the main source of decoherence. These so-called ‘atomic clock transitions’ (named after the principle which gives atomic clocks their exceptional phase stability) give rise to long coherence (T2) times . Formally forbidden ΔmI= ±1 hybrid electron/nuclear clock transitions are also observed upon application of a transverse field.
The second example involves a bis-phthalocyanine radical coupled to a TbIII ion, revealing a highly anisotropic signal that is attributed to the radical, suggesting a significant coupling to the lanthanide spin ; the radical EPR spectrum would be expected to be essentially isotropic otherwise. This work is important given the recent demonstration that radical bearing ligands provide a means of addressing lanthanide qubits integrated into single-molecule devices.
 S. Ghosh, S. Datta, L. Friend, S. Cardona-Serra, A. Gaita-Ariño, E. Coronado, S. Hill, Dalton Trans. 41, 13697 (2012).
 M. Shiddiq, D. Komijani, Y. Duan, A. Gaita-Ariño, E. Coronado, S. Hill, Nature 531, 348-351 (2016).
 D. Komijani, A. Ghirri, M. Affronte, M. Ruben, S. Hill, in preparation.
Graphene is a one‐atom thick two‐dimensional monolayer material with amazing physical properties. Carrier mobilities higher than those of silicon raised great expectations for disruptive carbon‐based electronics. Graphene represents the ideal two‐dimensional electron gas with negligible spin‐orbit coupling (SOC) as well as hyperfine interaction, which are prerequisites for long electron spin lifetimes. Thus graphene is very appealing for applications in spintronics. The essential benchmark for spintronics devices, i.e. long electron spin lifetimes, has been theoretically predicted on the order of 1 ms. Experimental work using spin‐FET (Field Effect Transistor) or nonlocal spin‐valve measurement devices yielded early on spin lifetimes in the ps range. In 2011 we reached at least 2 ns at room temperature. It was concluded that extrinsic effects are responsible for this shortcoming due to imperfect device technology (exfoliation and handling in air; imperfect tunnel barriers for spin injection from ferromagnets into graphene; charged impurities inducing extrinsic SOC fields). Recent improvements of the device concept, e.g., by “flattening” graphene on top of an h‐BN flake, yielded at room temperature an increase of the carrier mobilities from 1.000 to 20.000 cm2/V×s, spin lifetimes of 12 ns and spin diffusion lengths of 31 μm. These results are encouraging, but yet leave room for improvement and alternative concepts. Our findings rule out previous scalings of spin lifetime vs. momentum scattering time or mobility, which favored a D’yakonov‐Perel’ spin scattering mechanism.
Low-dimensional quantum magnets are presently an interesting field of study because of their unique and non-trivial properties. Sr14Cu24O41, a well-studied quasi-one-dimensional quantum magnet consisting of a hybrid chain/ladder structure. The ladders in this compound are known to exhibit a spin-singlet ground state, whereas the chains harbor spin-1/2 dimers that form an intervening Zhang-Rice singlet. The presence of some residual ‘free’ spins in the chain exhibit long-distance quantum entanglement at low temperatures . We have grown high quality single crystals of Sr14Cu24O41doped with dilute magnetic and nonmagnetic impurities, using the TSFZ method . We investigate the effect of dilute doping (< 1 %) on the spin-dimerization in the chain sublattice using susceptibilities and specific heat. The dimerization gap remains isotropic and constant in magnitude upon doping with Zn, Ni and Al. However, with Co-doping, we witness an anisotropic closing of the dimerization gap; for H || chain, the gap supresses rapidly, whereas the gap remains almost unchanged for the perpendicular to chain.
Last part, I will be talking on a new system, named orthoferrites (RFeO3). Recently, we have performed highly sensitive torque magnetometry measurement on single crystals of HoFeO3 along crystallographic directions. Since the paramagnetic contribution exerts no torque, a detailed investigation of the ordered Fe moments as a function of temperature and magnetic field is carried out. In addition to the spin reorientation transition, we found a weak and continuous transition sets in below a temperature of ~220 K and a spin-switching transition above the room temperature.
 S. Sahling et. al. Nature Physics 11, 255-260 (2015)
 R. Bag et. al. Journal of Crystal Growth 458, 16-26 (2017)