Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session H15: Low Dimensional Magnetism (including molecules and surfaces) |
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Sponsoring Units: GMAG Chair: William Ratcliff,, National Institute of Standards and Technology Room: 316 |
Tuesday, March 17, 2009 8:00AM - 8:12AM |
H15.00001: Reversible switching of charge states of single TCNE molecules on Cu(111) Taeyoung Choi, Jay Gupta The interplay of electronic structure and magnetic properties is of interest in various organic materials. TCNE (TCNE = tetracyanoethylene) is one component of well-known organic magnets with ferromagnetism up to room temperature. TCNE has a strong electron affinity that facilitates chemical bond formation and charge transfer with metals. We use scanning tunneling microscopy and spectroscopy to study single TCNE molecules on Cu(111) and Cu(100) surfaces. On Cu(111), we find that TCNE can be reversibly switched among three configurations via a controlled voltage pulse. We determine the adsorption sites for these configurations by co-adsorbing CO molecules, which are well known to adsorb atop Cu atoms. We believe these states represent different adsorption configurations and charge states. One of the configurations shows a strong Kondo resonance at low temperature; spectroscopic imaging indicates that this state is strongly localized at the corners of the TCNE molecule. Several features symmetric about $V$=0 suggest a convolution of the Kondo density of states with inelastic electron tunneling spectroscopy of vibrational modes. http://www.physics.ohio-state.edu/$\sim $jgupta [Preview Abstract] |
Tuesday, March 17, 2009 8:12AM - 8:24AM |
H15.00002: Spin-Electric Coupling in Molecular Magnets Mircea Trif, Filippo Troiani, Dimitrije Stepanenko, Daniel Loss We study the triangular antiferromagnet Cu$_3$ in external electric fields, using symmetry group arguments and a Hubbard model approach. We identify a spin-electric coupling caused by an interplay between spin exchange, spin-orbit interaction, and the chirality of the underlying spin texture of the molecular magnet. This coupling allows for the electric control of the spin (qubit) states, e.g. by using an STM tip or a microwave cavity. We propose an experimental test for identifying molecular magnets exhibiting spin-electric effects. [Preview Abstract] |
Tuesday, March 17, 2009 8:24AM - 8:36AM |
H15.00003: Magnetic excitation in artificially designed oxygen molecule magnet Takatsugu Masuda, Satoshi Takamizawa, Kazuma Hirota, Masaaki Ohba, Susumu Kitagawa We performed inelastic neutron scattering experiment to study magnetic excitation of O$_2$ molecules adsorbed in microporous compound. The dispersionless excitation with characteristic intensity modulation is observed at $\hbar \omega = 7.8$ meV at low temperature. The neutron cross section is explained by spin dimer model with intradimer distance of 3.1 \AA . Anomalous behaviour in the temperature dependence is discussed in the context of enhanced magnetoelasticity in the soft framework of O$_2$ molecule. [Preview Abstract] |
Tuesday, March 17, 2009 8:36AM - 8:48AM |
H15.00004: Influence of the Dzyaloshinskii-Moriya exchange interaction on quantum phase interference of spins Wolfgang Wernsdorfer, T.C. Stamatatos, G. Christou Magnetization measurements of a Mn$_{12}$mda wheel single-molecule magnet (SMM) with a spin ground state of $S = 7$ show resonant tunneling and quantum phase interference, which are established by studying the tunnel rates as a function of a transverse field applied along the hard magnetization axis. We show how the Dzyaloshinskii-Moriya (DM) exchange interaction can affect the tunneling transitions and quantum phase interference of a SMM. Of particular novelty and importance is the phase-shift observed in the tunnel probabilities of some transitions as a function of the DM vector orientation. Such observations are of importance to potential applications of SMMs that hope to take advantage of the tunneling processes that such molecules can undergo. Ref.: W. Wernsdorfer, T.C. Stamatatos, G. Christou, Phys. Rev. Lett., 101, (28 Nov. 2008). [Preview Abstract] |
Tuesday, March 17, 2009 8:48AM - 9:00AM |
H15.00005: Coupling between molecular spin cluster qubits Marco Affronte Supramolecular chemistry enables nanoscale engineering of scalable structures, by introducing controlled interactions between well defined molecular building blocks. The ability to assemble weakly-interacting subsystems is a prerequisite for implementing quantum-information processing (QIP) and generating controlled entanglement. In recent years, molecular nanomagnets (MNMs) have been proposed as suitable candidates for the qubit encoding and manipulation. In particular, antiferromagnetic Cr7Ni rings at low temperature behave as effective spin-1/2 systems and exhibit long decoherence times. Here we show that these rings can be linked to each other through supramolecular functional groups, which allow an extensive tuning of the coupling between their spins. We demonstrate that maximally entangled states can be deterministically generated in tripartite supramolecular assemblies, formed by two Cr7Ni rings and a Cu ion, by simulating the system's time evolution under the effect of realistic microwave pulse sequences (under consideration Nature Nanotechnology). [Preview Abstract] |
Tuesday, March 17, 2009 9:00AM - 9:12AM |
H15.00006: High Resolution Neutron Scattering Studies of Spin Excitations in the 2D Singlet Ground State Systems of SrCu2(BO3)2, SrCu(2-x)Mg(x)(BO3)2 and Sr(1-x)La(x)Cu2(BO3)2. Sara Haravifard, Sarah Dunsiger, Bruce Gaulin, Hanna Dabkowska, Mark Telling, Toby Perring, Samir El Shawish, Janez Bonca SrCu2(BO3)2 is a quasi-2D quantum spin system known to possess a collective singlet ground state and a realization of the Shastry-Suthrland model. One aspect of the study of SrCu2(BO3)2 for which there is a little information is the influence of impurities on the nature of the singlet ground state. There is much interest in such studies due to the remarkable phenomena associated with doping other quasi-2D copper-oxide quantum magnets with high-T superconductivity. Here we report high resolution time-of-flight neutron scattering studies of single crystals of doped SrCu(2-x)Mg(x)(BO3)2 and Sr(1-x)La(x)Cu2(BO3) 2 and compare these results to the pure SrCu2(BO3)2 neutron scattering measurements. Particular emphasis is placed on the lifetimes of one-triplet excitations as well as the existence of in-gap spin excitations in the presence of Mg and La impurities. [Preview Abstract] |
Tuesday, March 17, 2009 9:12AM - 9:24AM |
H15.00007: Entanglement Perturbation Theory for Antiferromagnetic Heisenberg Chains. Lihua Wang, Sung Chung We use a novel method, Entanglement Perturbation Theory (EPT) to solve the Heisenberg chain comprehensively for both spin 1/2 and spin 1. A variety of quantities for xxx model and xxz model are calculated, including the ground state energies, the spin-spin correlation functions, and the first excited state energies relevant to a phase transition. EPT allows us to calculate systems with nearly one thousand sites and obtain spin-spin correlation functions over hundreds of sites with unprecedented accuracy. The successful application of EPT to the Heisenberg model shows that it is simple, general and exact for macroscopic quantum systems with translational symmetry. [Preview Abstract] |
Tuesday, March 17, 2009 9:24AM - 9:36AM |
H15.00008: Magnetoelectric Coupling in a Quantum Spin Ladder Jessica White, Janice Musfeldt, Scott Crooker, John Singleton, Ross MacDonald, Chris Landee, Mark Turnbull, Hans Christen We investigated the optical properties and magnetization of (2,3-dmpyH)$_2$CuBr$_4$, an antiferromagnetic quantum spin ladder with strong rail interactions. It is similar to the copper oxides, yet its modest exchange interactions allows it to be saturated at 29 T, whereas copper oxides require a much higher field. In the end, we were able to see that the field dependent integrated absorption difference tracks the magnetization, demonstrating that the structure is sensitive to the ferromagnetic transition. [Preview Abstract] |
Tuesday, March 17, 2009 9:36AM - 9:48AM |
H15.00009: Valence-Bond Monte Carlo Study of Random-Singlet Phase Formation Huan Tran, Nicholas Bonesteel In valence-bond Monte Carlo (VBMC)\footnote{A. Sandvik, PRL {\bf 95}, 207203 (2005).} the ground state of a quantum spin system is sampled directly from the valence-bond (VB) basis --- a useful basis for visualizing the properties of singlet ground states. For example, the ground state of the uniform AFM spin-$\frac{1}{2}$ Heisenberg chain is characterized by strongly fluctuating bonds with power-law length distribution, while in the random-singlet phase (RSP) of a {\it random} Heisenberg chain these bonds, while still having a power-law length distribution, lock into a particular VB state on long length scales.\footnote{D. S. Fisher, PRB \textbf{50}, 3799 (1994).} We use VBMC to directly probe the formation of a RSP by calculating both the average number of bonds $n_L$ leaving a block of $L$ spins (the VB entanglement entropy\footnote{F. Alet, et al., PRL \textbf{99}, 117204 (2007).}), and its {\it fluctuations}, $\sigma_{n_L}^2 = \left\langle\langle n_L^2\rangle- \langle n_L\rangle^2\right\rangle$. For the uniform chain they have been calculated exactly\footnote{J. L. Jacobsen and H. Saleur, PRL \textbf{100}, 087205 (2008).} and shown to grow logarithmically with $L$ --- signaling the strong bond fluctuations. For random chains while $n_L$ grows logarithmically with $L$, we find $\sigma_{n_L}^2$ {\it saturate} for large $L$, signaling the ``freezing" of the bonds into a particular random singlet state. [Preview Abstract] |
Tuesday, March 17, 2009 9:48AM - 10:00AM |
H15.00010: Quantum Monte Carlo simulations of dynamical properties for gapped spin chains Zhaoxin Xu, Juana Moreno, Mark Jarrell We study the dynamical properties of spin-1 antiferromagnetic chains and spin-1/2 ferromagnetic-antiferromagnetic bond alternating chains. We calculate their dynamical structure factors using quantum Monte Carlo simulations combined with the Maximum Entropy Method. We focus on the finite temperature dynamical behavior and impurity effects on these gapped spin chains. We also discuss the connection between our results and recent neutron scattering experiments. [Preview Abstract] |
Tuesday, March 17, 2009 10:00AM - 10:12AM |
H15.00011: Exploring Possible Magnetic Properties of Ordered Manganese Monolayer on Wurtzite GaN A.R. Smith, A. Chinchore, K. Wang, W. Lin, T. Chen, Y. Liu, J. Pak For future spin-based device technologies, it is crucial to investigate magnetic material systems using techniques having high magnetic resolution. Spin-polarized scanning tunneling microscopy has proven to be extremely powerful for resolving magnetic structure down to even the atomic scale. Of great interest lately are transition-metal-on-semiconductor systems. We have recently discovered a well-ordered Mn monolayer having $\surd $3$\times \surd $3-R30$^{o}$ structure, formed on wurtzite gallium nitride.[1] It is intriguing to explore the possibility of atomic-scale magnetic ordering in this system. For this purpose, we have designed a new SP-STM system combined with a highly flexible, epitaxial nitride growth facility. The new SP-STM has been designed for variable-low-temperature operation within an applied magnetic field of up to +/- 4.5 Tesla. Initial results with the new system are expected within the very near future. Work is supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0730257). Equipment support from ONR is also acknowledged. [1] A. Chinchore et al., Appl. Phys. Lett. \textbf{93(18)}, 181908 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 10:12AM - 10:24AM |
H15.00012: Manganese Atom Ordered Monolayer on Wurtzite Gallium Nitride Abhijit Chinchore, Kangkang Wang, Wenzhi Lin, Jeongihm Pak, Yinghao Liu, Arthur Smith While transition-metal-doped gallium nitride (GaN) thin films have been explored as potential dilute magnetic semiconductor bulk layers, the structural and magnetic effects of various transition metal adatoms on GaN surfaces are not even well understood. In this work, we investigate the sub-monolayer deposition of manganese (Mn) onto the N-polar wurtzite GaN (000-1) 1$\times $1 surface. The growth is monitored in-situ using reflection high energy electron diffraction (RHEED). A fresh GaN(000-1) 1$\times $1 surface is prepared by rf nitrogen plasma-assisted MBE followed by annealing to remove excess gallium adatoms. The atomically flat GaN surface, held at 200$^{o}$ C, is then exposed to submonolayer doses of Mn. The deposition rate is maintained at 0.007 ML per second, and a 3$\times $ pattern develops along [10-10]; whereas, only 1$\times $ is seen along [11-20]. Analysis of the RHEED pattern and subsequent modeling indicates a $\surd $3 $\times \surd $3 R 30$^{o}$ structure consisting of 2/3 ML Mn atoms in a row-like arrangement having spacing $\surd $3a/2 along rows and 3a/2 between rows. Scanning tunneling microscopy/spectroscopy studies are currently underway to explore this surface further. This work is supported by DOE (Grant No.DE-FG02-06ER46317) and NSF (Grant No. 0730257). [Preview Abstract] |
Tuesday, March 17, 2009 10:24AM - 10:36AM |
H15.00013: Real-space pseudopotential method for noncollinear magnetism within density functional theory Doron Naveh, Leeor Kronik We present a real-space pseudopotential method for first principles calculations of noncollinear magnetic phenomena within density functional theory. We demonstrate the validity of the method using the test cases of the Cr$_3$ the a Cr monolayer. The approach retains all the typical benefits of the real-space approach, notably massive parallelization. It can be employed with arbitrary boundary conditions and can be combined with the computation of pseudopotential-based spin-orbit coupling effects. [Preview Abstract] |
Tuesday, March 17, 2009 10:36AM - 10:48AM |
H15.00014: Properties of weakly coupled 1/2 spin ladders in a magnetic field Pierre Bouillot, Corinna Kollath, Andreas Lauchli, Edmond Orignac, Roberta Citro, Thierry Giamarchi Weakly coupled $1/2$-spin ladders have a very rich physics that can be observed in their $(T,H)$ phase diagram. One of the most interesting particularity, is the possibility to explore the entirety of their Luttinger-Liquid phase by controlling the exponent through the applied magnetic field. We investigate this system that is closely related to the recent experiments on the compound $(Hpip)_2CuBr_4$. From a bosonization interpretation of zero temperature DMRG calculations, we determine the related Luttinger liquid parameters in the incommensurate phase and use them to obtain the transition temperature between the Luttinger-Liquid and the Neel phase. In the latter, we determine the order parameter at zero temperature. From temperature dependent DMRG computations, we obtain the magnetic field dependence of the specific heat and the magnetization. A very good agreement is found between our numerical results and the measurements in the coumpound $(Hpip)_2CuBr_4$ intensively explored in [Phys. Rev. Lett. 101, 137207 (2008)], [arXiv:0808.2715 (PRL in press.)] and [arXiv:0809.0440]. [Preview Abstract] |
Tuesday, March 17, 2009 10:48AM - 11:00AM |
H15.00015: Spin and exchange coupling in Ti atom and Ti dimers Pushpa Raghani, Jesus Cruz, Barbara Jones It is very important to know the spins on magnetic atoms embedded in a molecular network. Single or multiple magnetic atoms with a large spin can be used as molecular magnets for magnetic storage devices. We use denisty functional theory (DFT) with pseudopotentials and GGA+U to calculate spin on Ti atoms adsorbed on CuN/Cu(100) surface; and compare this spin with that obtained from scanning tunneling microscopy experiments. Then we calculate the exchange coupling for a complete layer of Ti, as well as dimers of Ti on the same CuN/Cu(100) surface. [Preview Abstract] |
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