Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C16: Focus Session: Spin Dynamics and EPR |
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Sponsoring Units: GMAG DMP Chair: Stephen Hill, Florida State University and NHMFL Room: 318 |
Monday, March 18, 2013 2:30PM - 2:42PM |
C16.00001: Synthesis and Physical Characterization of thin silicondioxide (SiO$_{2})$ layers with very high densities of E' centers K. Ambal, A. Payne, D.P. Waters, C. Williams, C. Boehme E' centers are paramagnetic (s$=$1/2) electronic states which are due to silicon dangling bonds in a-SiO$_{2}$ [1]. E' centers are able to trap electric charge, which can be detrimental to the performance of silicon based electronic devices. Therefore, most previous studies of E' centers have focused on a-SiO$_{2}$ layers with low E' center densities and material preparation techniques that allow to minimize it. Here, we present a study aiming at the opposite, the question of how E' center densities in a-SiO$_{2}$ can be maximized and whether E' centers in higher densities still exhibit similar spin dynamics (relaxation rates) in comparison to SiO$_{2}$ with low E' center densities. This study has been motivated by the need for a dielectric material containing very high spin densities as needed for single spin detection techniques. It is shown in this study that E' centers can be created at densities above $\sim$ 10$^{19}$ cm$^{-3}$ through exposure of a thin thermal oxide sample to an rf plasma containing Ar at low pressure. Most of the E' centers were found within 20 nm to 30 nm of the SiO$_{2}$ surface. While the high E' center densities can be annealed completely at 300 $^{\circ}$C, they are very stable at room temperature. Spin relaxation time measurements show that $T_{2}$ of high density E' centers does not strongly depend on temperature and $T_{1}$ is $\sim$ 600$\mu $s at 5K with an increase towards lower densities [1]. At room temperature$ T_{1}$ is $\sim$ 160$\mu $s, which agrees well with values found in literature for E' centers at low densities [2].\\[4pt] [1] J. G. Castle, \textit{J. Appl. Phys}. \textbf{36}, 124 (1965).\\[0pt] [2] S. S. Eaton, \textit{J. of Mag. Res. Series A, }\textbf{102}, 354-356 (1993). [Preview Abstract] |
Monday, March 18, 2013 2:42PM - 2:54PM |
C16.00002: Towards force detected single electron spin resonance at room temperature C.C. Williams, A. Payne, K. Ambal, C. Boehme Electrically detected magnetic resonance (EDMR) spectroscopy has shown that electron tunneling at or within silicon dioxide layers is strongly dependent on spin-selection rules [1]. Also demonstrated is the detection of single electron tunneling events by electrostatic force with sub-nanometer spatial resolution [2,3]. Here we propose to combine force detected single electron tunneling microscopy with EDMR to demonstrate a new kind of single spin force microscope. This approach has much better sensitivity than magnetic force based single spin microscopes [4], since electrostatic forces are much larger than corresponding magnetic forces. In this method, a paramagnetic state in an oxidized AFM probe tip is brought within tunneling range of a paramagnetic state in an oxide surface [5]. Under appropriate energy conditions, one of the unpaired electrons can randomly tunnel between the two states causing a random telegraph signal (RTS) to appear on the AFM cantilever frequency. Simulations predict that if magnetic resonance conditions are achieved, a measurable change in the RTS signal is detectable at room temperature. The theory and a quantitative simulation of this atomic scale spin resonance measurement will be presented, along with experimentally observed random telegraph signals.\\[4pt] [1] D. R. McCamey, et al., \textit{Phys. Rev. B}, \textbf{78}, 045302 (2008). [2] L. J. Klein and C.C. Williams, \textit{Appl. Phys. Lett.}\textbf{ 79}, 1828 (2001). [3] E. Bussmann and D.J. Kim, and C.C. Williams, \textit{Appl. Phys. Lett.} \textbf{85}, 2538 (2004). [4] D. Rugar et al., \textit{Nature} \textbf{430}, 329 (2004). [5] J.P. Johnson, Ph.D. Thesis, Dept. of Physics, University of Utah (2010). [Preview Abstract] |
Monday, March 18, 2013 2:54PM - 3:06PM |
C16.00003: Local control of single-electron spin using spin-orbit coupling Miguel Angel Rodriguez-Moreno, Lilia Meza-Montes, David Hernandez de la Luz It has been demonstrated that CNOT quantum gates combined with single qubit operations form a universal set for quantum computing. In spin-based quantum qubits both conditions can be achieved by using a double quantum dot with two electrons. This configuration also allows for the realization of a completely electrical control of the spins, provided that hyperfine and spin-orbit interactions exist in the system. In this work, we simulate numerically the dynamics of the spin of two electrons in a double quantum dot. We use a combination of finite differences, direct diagonalization and a time propagator approach in order to solve the time-dependent two-electron Schr\"odinger equation. The single qubit operation is simulated by bringing the system into a separated charge state and then applying a time-varying electric field locally to one of the dots. It is shown that the spin-orbit coupling induces Rabi oscillations and that the frequency and amplitude of these oscillations can be varied by changing the magnitudes of the electric and static magnetic fields. We also analyze the role of the direction of the static magnetic field; in particular, we determine the variation of the spin dynamics with respect to direction of an in-plane static magnetic field. [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C16.00004: Analytical description of spin-Rabi oscillation controlled electronic transitions rates between weakly coupled pairs of paramagnetic states with S=(1/2) Rachel Glenn, William Baker, Christoph Boehme, Mikhail Raikh We study theoretically and experimentally the Fourier content, ${\mathcal {\bf F}} (s)$, of the Rabi oscillations in photoconductivity coming from pairs of spin-$\frac{1}{2}$ localized carriers. Upon increasing the ac drive, the Fourier spectrum evolves from a single peak at $s= \Omega_R$, where $\Omega_R$ is the Rabi frequency, to {\em three} peaks at $s= \Omega_R$, $s=2\Omega_R$, and at low $s\ll \Omega_R$. The crossover between the two regimes takes place when $\Omega_R$ exceeds the broadening, $\delta_0$, of Zeeman levels due to disorder, e.g., hyperfine field. We capture this crossover within the analytical treatment by calculating the shapes of all three peaks at arbitrary relation between $\Omega_R$ and $\delta_0$. When the peaks are well-developed their widths are ${\Delta} s \sim \delta_0^2/\Omega_R$. Good agreement of theory and experiment allowed us to infer the experimental value of $\delta_0$. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C16.00005: The quantum to classical transition in atomic scale magnets Fernando Delgado, Joaquin Fernandez-Rossier Understanding the emergence of classical behavior in a world governed by quantum mechanics at the microscopic scale is one of the main fundamental open problems in physics. The radical differences between the two behaviors is dramatically represented by quantum systems that are, at the same time, in two classically different states. The quantum to classical transition is empirically linked to the size of the systems and conceptually related to the concept of environmental decoherence [1], but no general and clear rules have been determined. Here we consider it in the context of atomically engineered magnetic nanostructures [2,3] and we address fundamental questions such as the conditions under which a single adatom can behave classically or quantum mechanically. We show that the phase transition depends on the relative strength of its exchange coupling to surface and the renormalized zero-field splitting induced by quantum spin tunneling.\\[4pt] [1] W. H. Zurek, Physics Today 44, 36 (1991).\\[0pt] [2] C. F. Hirjibehedin et al., Science 312, 1021 (2006).\\[0pt] [3] C. Hirjibehedin et al., Science 317, 1199 (2007). [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C16.00006: Spin Fluctuation and Coherence in Concentrated systems Johan van Tol, Jingfang Wang, Zhenxing Wang, Susumu Takahashi In materials with a relatively high density of electron spins without direct exchange pathways, the spin decoherence tends to be dominated by dipolar-interaction mediated spin-exchange/diffusion processes. These spin exchange processes will significantly be reduced at high magnetic fields and low temperatures when the spin polarization approaches the saturation limit. We will show some examples of single crystals of molecular magnetic complexes in which the decoherence is measured experimentally at high frequencies, and which form a reference for direct theoretical models that predict the spin decoherence in these systems, and their dependence on orientation, temperature and field. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C16.00007: High Field Electron Paramagnetic Resonance (HFEPR) study on a Mn(IV) monomer Asma Amjad, Enrique del Barco, Stephen Hill, Johan Van Tol, Andrzej Ozarowski, Mahammad Ali In this work we investigated the magnetic anisotropy of a Mn (IV) monomer via axial and rhombic zero field splitting terms $D, E$. The d$^{3}$ ion sits in an octahedral environment in a P 21/c space group. The complex is studied via single crystal and powder HFEPR over a wide range of frequencies 49GHz to 416GHz and temperatures 2 to 60K. The angle dependence at low temperature and frequency ($\sim$88GHz) reveals a minimum of the resonance field, when the long axis of the crystal is along the magnetic field. The same behavior is observed at higher frequency ($\sim$240GHz). Furthermore, pulse EPR experiments in high frequency quasi-optical spectrometer at low temperature ($\sim$1.487K) a spin echo could be observed and we were able to observe the variation of the T$_{2}$ times as a function of the magnetic field orientation, and as a function of the temperature. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C16.00008: Cavity Perturbation Technique: The Effects of Crystal Size on the EPR Spectra of Fe$_{8}$ Single-molecule Magnets Muhandis Shiddiq, Christopher C. Beedle, Stephen Hill The Cavity Perturbation Technique (CPT) is a contact-free technique that measures the change of the characteristics of a cavity resonator upon the introduction of the sample. In this experiment, we study the effect of crystal size with regards to the CPT transmission spectra for a single crystal of the Fe$_{8}$ single-molecule magnets. It is interesting to study the interaction between these two resonance systems, i. e. a cavity and a crystal of Fe8. We want to know whether it is a quantum mechanical or a classical interaction. The frequency shift and suppression of the cavity Q value increase linearly with increasing sample size. These observations are in agreement with the theoretical expectation for a classical coupling between the Fe$_{8}$ crystal and the cavity. From cavity perturbation theory, these phenomena may be explained by the following classical formula: $\Delta \omega $/$\omega \quad =-\beta \chi $, where $\omega $ is the complex frequency, $\beta $ is the filling factor that depends on the sample volume and the resonant mode of the cavity, and $\chi $ is the complex susceptibility. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C16.00009: Single molecule magnets from magnetic building blocks W. Kroener, A. Paretzki, C. Cervetti, S. Hohloch, S. Rauschenbach, K. Kern, M. Dressel, L. Bogani, P. M\"{u}ller We provide a basic set of magnetic building blocks that can be rationally assembled, similar to magnetic LEGO bricks, in order to create a huge variety of magnetic behavior. Using rare-earth centers and multipyridine ligands, fine-tuning of intra and intermolecular exchange interaction is demonstrated. We have investigated a series of molecules with monomeric, dimeric and trimeric lanthanide centers using SQUID susceptometry and Hall bar magnetometry. A home-made micro-Hall-probe magnetometer was used to measure magnetic hysteresis loops at mK temperatures and fields up to 17 T. All compounds show hysteresis below blocking temperatures of 3 to 4 K. The correlation of the assembly of the building blocks with the magnetic properties will be discussed. [Preview Abstract] |
Monday, March 18, 2013 4:18PM - 4:54PM |
C16.00010: Theoretical calculations of spin dynamics and quantum effects in rare earth SMMs Invited Speaker: Alejandro Gaita-Ari\~no Rare-earth single-molecular magnets constitute a hot emerging topic in molecular magnetism. It also constitutes a promising field to study and eventually remedy the processes that lead to decoherence. In fact, experiments show some success in the design of rare-earth spin qubits with long coherence times. Furthermore, these long-lived quantum states of rare-earth SMMs can in principle be manipulated for quantum information processing. In particular, a simple quantum error correction protocol might be realizable using ElectroNuclear DOuble Resonance. Going further on this path will require a detailed knowledge of the wave function of the low-energy multiplet, and an understanding of how it can be tailored by chemical means. An inexpensive point-charge model has been presented recently that is able to reproduce the main features of the Crystal Field Hamiltonian of both lanthanoids (such as Dysprosium, Holmium, Terbium) and actinoids such as Uranium. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C16.00011: Probing magnetic interactions in molecule-based materials using high-pressure electron paramagnetic resonance K. Thirunavukkuarasu, C.C. Beedle, S. Winter, A. Kovalev, S. Tozer, R.A. Oakley, S. Hill Multi-frequency electron paramagnetic resonance (EPR) spectroscopy is a powerful technique for investigating magnetic exchange interactions in quantum matter. EPR spectroscopy when combined with techniques such as high pressure will enable us to probe various quantum phase transitions that give rise to novel electronic and magnetic phases in correlated electron systems. However, this particular combination of experimental tools has remained uncommon for several decades. Recently, our group has successfully implemented high pressure technique together with EPR spectroscopy. Cavity-based high-frequency EPR measurements can now be performed in the frequency range from 40~GHz to 200~GHz at temperatures down to 1.6~K under quasi-hydrostatic pressures up to 30~kbar. With the application of pressure, the inter-atomic/molecular correlations can be tuned continuously to reveal the nature of magnetic anisotropy and exchange interaction. In this talk, the realization of high pressure EPR spectroscopy will be briefly described using one of the molecule-based materials such as single-molecule magnet, organic radical-based ferromagnet etc., as an example. [Preview Abstract] |
Monday, March 18, 2013 5:06PM - 5:18PM |
C16.00012: Magnetoelectric coupling in 4, 4'-stilbenedinitrene J.L. Musfeldt, O. Gunaydin-Sen, P. Chen, J. Fosso-Tande, T. Allen, J. Cherian, T. Tokumoto, S. McGill, P.M. Lahti, R.J. Harrison We investigated the optical properties of 4,4${'}$-stilbenedinitrene at low temperature and in high magnetic fields and compared the results with complementary first principles calculations. Both physical tuning parameters allow us to manipulate the singlet-triplet equilibrium, and by so doing, control the optical contrast (which is on the order of -2.5$\times$10$^2$ cm$^{-1}$ at 555 nm and 35 T). Moreover, analysis of the magneto-optical response using a combined population and Beer's law framework reveals the singlet-triplet spin gap and identifies particular features in the absorption difference spectrum as deriving from singlet or triplet state excitations. These findings deepen our understanding of coupling in open shell molecules and show how highlight opportunities where chemical structure modification can amplify charge-spin interactions in organic biradicals. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C16.00013: Effective model and spin/charge ordering in molecular conductors X[Pd(dmit)$_2$]$_2$ Hitoshi Seo, Takao Tsumuraya, Masahisa Tsuchiizu, Tsuyoshi Miyazaki, Reizo Kato The family of molecular conductors, $\beta$'-type X[Pd(dmit)$_2$]$_2$ (X: monovalent cation) salts, show a variety of electronic states: dimer-type Mott insulator, magnetic order, spin-liquid behavior, metallic/superconducting states, and a peculiar charge ordering involving multi-orbitals[1]. In this work, we construct an effective low-energy model which takes into account the multi-orbital degree of freedom. We consider fragments of molecular orbital as a basis set, nearly localized on either one of the dmit ligands. The transfer integrals are obtained for a series of salts by fitting to the first-principles band calculations[2]. We find that all the intra-dimer transfer integrals including the diagonal ones are of the same order; this results in a modification of the orbital scheme in strongly dimerized [Pd(dmit)$_2$]$_2$ discussed in the literatures, then to the effective one-band model. We calculate possible spin and charge ordering based on mean-field approximation to the extended Hubbard model incorporating the fitted parameters. [1] R. Kato, Chem. Rev. 104 (2004) 5319; K. Kanoda and R. Kato, Annu. Rev. Condens. Matter Phys. 2 (2011) 167. [2] T. Miyazaki and T. Ohno, Phys. Rev. B 59 (1999) 5269; T. Tsumuraya, H. Seo, M. Tsuchiizu, R. Kato, and T. Miyazaki, in preparation. [Preview Abstract] |
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