Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session X43: Low Dimensional Magnetism: Wires, Chains, and 1-D |
Hide Abstracts |
Sponsoring Units: GMAG Chair: J. P. Hill, Brookhaven Room: LACC 150C |
Friday, March 25, 2005 8:00AM - 8:12AM |
X43.00001: Emerging magnetism in transition metal nanowires Anna Delin, Erio Tosatti In this talk, I will present our recent research on magnetism in nanowires of 4d and 5d transition metals (e.g. palladium and platinum). We have used a computational approach based on density functional theory. Magnetism in nanosystems is particularly interesting since magnetic phenomena appear to be very sensitive to the dimensionality of the system. This means that a metal that normally shows no magnetism (not even as a free atom) can be provoked to spin-polarize if it is, for example, grown as a very thin surface, or formed into an extremely thin wire, a so-called nanowire. I will explain why magnetism appears in nanosystems, and how it affects other physical phenomena, particularly the quantized conductance through the nanowire. References: A. Delin, E. Tosatti, and R. Weht, Phys. Rev. Lett. 92, 057201 (2004); A. Delin and E. Tosatti, Phys. Rev. B 68, 144434 (2003). [Preview Abstract] |
Friday, March 25, 2005 8:12AM - 8:24AM |
X43.00002: Magnetic Structure of Domain Walls in Atomic-Size Ferromagnetic Wires J.D. Burton*, R. Sabirianov, S.S. Jaswal*, O. Mryasov, E. Tsymbal The magnetic structure of domain walls in confined geometries is different from that in bulk ferromagnets. In nanowires the domain wall width is generally strongly reduced and can be controlled by pinning. We perform \textit{ab initio} electronic structure calculations for nanowires of Co and Ni using a tight-binding LMTO method in real space. We consider monatomic wires and wires of 4-fold-symmetry and investigate different types of the domain walls, i.e. Bloch, Neel and linear walls. The domain walls are modeled using constrained geometry with the fixed width of a noncolliner region between two semiinfinite ferromagnetic wires of uniform magnetization. The self-consistent calculations are performed assuming fixed directions of magnetic moments. We show that the local magnetic moments of Co and Ni atoms in nanowires of uniform magnetization are larger than those in the bulk, but are slightly reduced in the presence of the domain wall. Our attempt to model a collinear domain wall in Co nanowires shows a tendency to create an abrupt interface between regions of well defined magnetic moments with opposite orientations. Ni wires have a tendency to create a thin linear DW with substantially reduced local magnetic moments. This work is supported by Seagate Research Center, National Science Foundation and Nebraska Research Initiative. [Preview Abstract] |
Friday, March 25, 2005 8:24AM - 8:36AM |
X43.00003: Energy gap of 3-leg spin tube Toru Sakai, Munehisa Matsumoto, Koichi Okunishi, Kiyomi Okamoto, Masahiro Sato Recently some quantum spin systems on tube lattices, so called spin nanotubes, have been synthesized. They are expected to be interesting low-dimensional systems like the carbon nanotubes. As the first step of theoretical study on the spin nanotube, we investigate the S=1/2 three-leg spin tube, which is the simplest one, using numerical analyses of finite clusters and a finite-size scaling technique. The spin gap, which is one of the most interesting macroscopic quantum effects, was revealed to be open for any finite rung exchange couplings, in contrast to the three-leg spin ladder system which is gapless. It is consistent with the previous effective Hamiltonian approach.[1] We also found a new quantum phase transition caused by an asymmetric rung interaction. When one of the three rung coupling constants is changed, the spin gap would vanish. The present size scaling analysis indicated that the region of the ratio of two rung couplings where the spin gap is open, decreases abruptly with increasing system size. It implies that this novel and exotic quantum phase transition would be observed in some nano-scale systems. [1] K. Kawano and M. Takahashi, J. Phys. Soc. Jpn. 66 (1997) 4001. [Preview Abstract] |
Friday, March 25, 2005 8:36AM - 8:48AM |
X43.00004: A One-Dimensional Magnetic System; Terbium Borosilicide Takao Mori Boron-rich compounds such as REB$_{66}$ are well known, but the main interest has tended to be of structural or mechanical nature. However, recently interesting magnetic behavior has been discovered in B$_{12}$ icosahedra-containing dilute semiconducting f-electron compounds [1]. For example, the 3D long range order of GdB$_{18}$Si$_{5}$ and 2D spin glass behavior of a homologous series of higher borides like REB$_{17}$CN. An interesting aspect of these transitions is the indication that B$_{12}$ icosahedral clusters mediate the magnetic interaction, possibly through a superexchange mechanism. Results obtained for neutron diffraction and doping effects on a TbB$_{50}$-type compound TbB$_{44}$Si$_{2}$ are presented. It is indicated that this is a 1D system with extremely short range interaction. [1] e.g. T. Mori and H. Mamiya, Phys. Rev. B, {\bf 68}, 214422 (2003), T. Mori, J. Appl. Phys., {\bf 95}, 7204 (2004). [Preview Abstract] |
Friday, March 25, 2005 8:48AM - 9:00AM |
X43.00005: Green's function for magnetically incoherent interacting electrons in one dimension Gregory Fiete, Leon Balents Using a path integral approach and bosonization, we calculate the low energy asymptotics of the one particle Green's function for a ``magnetically incoherent'' one dimensional strongly interacting electron gas at temperatures much greater than the typical exchange energy but much lower than the Fermi energy. [G. A. Fiete and L. Balents, PRL {\bf 93,} 226401 (2004)] The Green's function exhibits features reminiscent of spin-charge separation, with exponential spatial decay and scaling behavior with interaction dependent anomalous exponents inconsistent with any unitary conformal field theory. We compute the tunneling density of states at low energies and find that it is a power law in energy with exponent $1/(4g)-1$, where $g$ is the Luttinger interaction parameter in the charge sector. The underlying physics is made transparent by the simplicity of the approach. Our results generalize those of Cheianov and Zvonarev [Phys. Rev. Lett. {\bf 92}, 176401 (2004)]. [Preview Abstract] |
Friday, March 25, 2005 9:00AM - 9:12AM |
X43.00006: Collective mode spectra and dynamical response functions of weakly coupled random spin chains Eddy Yusuf, Kun Yang We study the dynamics of weakly coupled random antiferromagnetic spin chains at low temperature. We use the celebrated real-space renormalization group technique to tackle the intrachain couplings, and treat the interchain couplings within the Mean-Field/Random Phase Approximation (RPA) framework. Earlier work based on mean-field treatment of the inter chain couplings[joshi] indicated the existence of disorder-induced Neel ordering in such systems and clarified their phase diagrams. In the present work we apply the RPA formalism to calculate the dynamical response functions in the low temperature ordered phase. We obtain the dynamical susceptibility, structure factor, and the spin-wave spectra. Comparisons will be made with inelastic neutron scattering experiment for quasi-one-dimensional spin systems such as CuGeO$_3$. \\ \\ \textbf{References} \newline [joshi] Anuvrat Joshi and Kun Yang, Phys. Rev. B \textbf{67}, 174403 (2003) [Preview Abstract] |
Friday, March 25, 2005 9:12AM - 9:24AM |
X43.00007: Thermal conductivity of quasi-one-dimensional antiferromagneticspin-chain materials Sasha Chernyshev, Alex Rozhkov We study heat transport in quasi-one-dimensional spin-chain systems by considering the model of one-dimensional bosonic spin excitations interacting with three-dimensional phonons and impurities in the limit of weak spin-lattice coupling and fast spin excitations. A combined effect of the phonon and impurity scatterings yields the following spin-boson thermal conductivity behavior: $\kappa_s\propto T^2$ at low, $\kappa_s\propto T^{-1}$ at intermediate, and $\kappa_s=const$ at higher temperatures. Our results agree very well with the existing experimental data for Sr$_2$CuO$_3$. We predict an unusual dependence on the impurity concentration for a number of observables and propose further experiments. [Preview Abstract] |
Friday, March 25, 2005 9:24AM - 9:36AM |
X43.00008: Far-infrared study of the CuO$_2$ chain layer magnetism in Sr$_{14}$Cu$_{24}$O$_{41}$ Toomas R{\~o}{\~o}m, D. H{\"u}vonen, U. Nagel, P. Haas, B. Gorshunov, M. Dressel, J. Akimitsu, T. Sasaki, T. Nagata Sr$_{14}$Cu$_{24}$O$_{41}$ consists of two-leg ladders and chains of copper ions. Planes of ladders and chains alternate in $b$ axis direction. Oxygen holes, present in stochiometric Sr$_{14}$Cu$_{24}$O$_{41}$, and Cu$^{2+}$ spins in the chain are coupled into Zhang-Rice spin singlet. Chain Cu$^{2+}$ spins that are separated by Zhang-Rice singlets form spin dimers. It is known from inelastic neutron scattering studies that because of inter-chain coupling there are two weakly dispersing triplet modes in the chain layer. We found that these triplet modes are optically active. Light is absorbed at 77.80 and 87.54\,cm$^{-1}$ in zero magnetic field and $T=4$\,K. When magnetic field is applied along the $c$ axis both resonances split. The triplet state g-factor is $2.048\pm0.002$ and $2.063\pm0.006$ for the lower and upper triplet. The results on the absorption line intensities and frequencies as a function of $T$, $B_0$, and light polarization are presented. [Preview Abstract] |
Friday, March 25, 2005 9:36AM - 9:48AM |
X43.00009: Na$_{2}$Cu$_{2}$Si$_{4}$O$_{11}\cdot $H$_{2}$O: A new S=1/2 chain material with microporous structure Antonio Moreira dos Santos, Paula Brandao, Filipe Almeida Paz, Victor Amaral, Joao Rocha, Margarida Godinho, Alexander Vasiliev We report the first S=1/2 chain microporous silicate, Na$_{2}$Cu$_{2}$Si$_{4}$O$_{11}\cdot $H$_{2}$O. This compound was synthesized hydrothermally and its structure determined through single-crystal X-ray diffraction. It consists of pyroxene-type chains of distorted edge sharing CuO$_{6}$ octahedra separated by SiO$_{4}$ tetrahedra and running parallel to Na$^{+}$ filled channels. Susceptibility data was fitted using the S=1/2 alternating exchange parameters and J$_{1}$/K$_{b}$ and J$_{2}$/K$_{b}$ were determined to be 102 K and 16 K respectively. The spin gap $\Delta $ was extracted from the low temperature data and decreases with increasing magnetic field, also 2$\Delta $/K$_{b}$Tsp= 3.9 close to 3.53, the expected value for a Spin-Peierls transition. These observations seem to indicate that this system is a two step Spin-Peierls system where a structural dimerization, already present at room temperature, is followed by a magnetic transition at lower temperature. [Preview Abstract] |
Friday, March 25, 2005 9:48AM - 10:00AM |
X43.00010: Magnetic impurities in spin-1/2 chains Lara Benfatto, Thierry Giamarchi The problem of a magnetic impurity embedded in a spin-1/2 chain has attracted a lot of attention in the literature. In particular, it has been suggested that this problem can be mapped into the two-channel Kondo problem, with a resulting overscreened spin-impurity susceptibility. Here we propose a different analytical approach to analyze the low-energy field theory, which allows us to extend our understanding of the single-impurity case beyond the known results for the two-channel Kondo problem. [Preview Abstract] |
Friday, March 25, 2005 10:00AM - 10:12AM |
X43.00011: Condensation of bosons and fermions in spin chains and ladders Oleg Tchernyshyov, Jean-Baptiste Fouet, Frederic Mila, Paul Fendley The closing of the Haldane gap in spin chains in a strong magnetic field can be viewed as Bose condensation of magnons [1]. This analogy helps to obtain the universal properties of spin chains near a quantum critical point separating an {\em integer} (in the sense of Oshikawa {\em et al.} [2]) magnetization plateau from an incommensurate phase [3]. We find a different universal behavior at the quantum phase transitions out of a {\em half-integer} magnetization plateau. That transition can be described as condensation of spinons---domain walls carrying spin $s_z = \pm 1/2$ and best viewed as fermions. The difference between the two types of condensation is most pronounced in the presence of a staggered transverse magnetic field that is often induced in real materials by the staggering of the $g$-tensor or by the Dzyaloshinskii-Moriya interaction. [1] I. Affleck, Phys. Rev. B {\bf 43,} 3215 (1991). [2] M. Oshikawa, M. Yamanaka, and I. Affleck, Phys. Rev. Lett. {\bf 78,} 1984 (1997). [3] J.-B. Fouet, O. Tchernyshyov, and F. Mila, Phys. Rev. B {\bf 70,} 174427 (2004). [Preview Abstract] |
Friday, March 25, 2005 10:12AM - 10:24AM |
X43.00012: Magnetization patterns and wavefunctions in finite spin chains Sebastian Eggert, Ian Affleck, Fabrizio Anfuso The magnetization pattern along finite spin-1/2 chains is studied in great detail. For individual excited states we can identify the corresponding spinon wavefunctions, which characterize the strongly correlated state. For a finite applied staggered field we can calculate the detailed magnetization from the soliton wavefunction in the gapped phase. This in turn determines the distribution of magnetic moments in the ordered phase of spin chain compounds, which has recently been studied by muSR experiments. [Preview Abstract] |
Friday, March 25, 2005 10:24AM - 10:36AM |
X43.00013: Sr$_2$Cu(PO$_4$)$_2$ - an unexpected one dimensional spin 1/2 Heisenberg system with isolated CuO$_4$ units Helge Rosner, Michelle Johannes, Johannes Richter, Stefan-Ludwig Drechsler Recently, Belik {\it et. al.} [1] reported synthesis and physical properties of the compound Sr$_2$Cu(PO$_4$)$_2$. The measured magnetic susceptibility [1] exhibits a broad maximum at 92 K characteristic for quasi-1D systems, but shows no long range magnetic ordering down to 0.45 K. Here, we present full potential electronic structure calculations within the local spin density approximation, followed by a subsequent mapping to a one-band tight-binding model and an extended Heisenberg model. Although the crystal structure of Sr$_2$Cu(PO$_4$)$_2$ is formed by unlinked CuO$_4$ units, we find a surprisingly pronounced one dimensional behaviour with substantial coupling between nearest neighbors (NN) only. The calculated NN exchange coupling J$_1$$\sim$180 K is in good agreement with the experimental estimate. It exceeds all other couplings by at least two orders of magnitude, placing the system in the forefront of 1D spin 1/2 model compounds. Model calculations using the derived exchange constants suggest that no long range magnetic ordering should be expected down to very low temperatures.\\[0pt] [1]Belik {\it et. al.}, J. of Sol. Stat. Chem. {\bf 177}, 883 (2004). [Preview Abstract] |
Friday, March 25, 2005 10:36AM - 10:48AM |
X43.00014: Numerical contractor renormalization method for quantum spin models Sylvain Capponi, Andreas Laeuchli, Matthieu Mambrini We demonstrate the utility of the numerical contractor renormalization (CORE) method for quantum spin systems by studying one- and two-dimensional model cases. Our approach consists of two steps: (i) building an effective Hamiltonian with longer ranged interactions up to a certain cutoff using the CORE algorithm and (ii) solving this new model numerically on finite clusters by exact diagonalization and performing finite-size extrapolations to obtain results in the thermodynamic limit. This approach, giving complementary information to analytical treatments of the CORE Hamiltonian, can be used as a semiquantitative numerical method. In two dimensions we consider the plaquette lattice and the kagomé lattice as nontrivial test cases for the method. As it becomes more difficult to extend the range of the effective interactions, we propose diagnostic tools (such as the density matrix of the local building block) to ascertain the validity of the basis truncation. On the plaquette lattice we have an excellent description of the system in both the disordered and the ordered phases, thereby showing that the CORE method is able to resolve quantum phase transitions. On the kagomé lattice we find that the previously proposed twofold degenerate S=1/2 basis can account for a large number of phenomena of the spin 1/2 kagomé system. [Preview Abstract] |
Friday, March 25, 2005 10:48AM - 11:00AM |
X43.00015: Dimerized phase of the spin-1/2 anisotropic triangular lattice antiferromagnet. Oleg Starykh, Leon Balents, Anton Burkov With Cs$_2$CuCl$_4$ in mind, we analyze quasi-one-dimensional limit of the quantum $S=1/2$ antiferromagnet on a triangular lattice. In this limit, the system can be viewed as a collection of parallel Heisenberg spin chains (exchange $J$) coupled by a weak zig-zag inter-chain exchange $J'$. We demonstrate, using renormalization group considerations and the Haldane-Shastry chain (which is equivalent to the Heisenberg one at low energies), that a marginally relevant ``twist" interaction between nearest chains generates a {\it relevant} interaction between staggered dimerizations on the next-nearest chains. This, previously overlooked, interaction appears to drive the system into a two-dimensional valence-bond solid phase with staggered ordering of dimerizations on next-nearest chains. The effect of weak further neighbor and Dzyaloshinskii-Moriya interactions in the anisotropic limit will also be discussed. [Preview Abstract] |
Follow Us |
Engage
Become an APS Member |
My APS
Renew Membership |
Information for |
About APSThe American Physical Society (APS) is a non-profit membership organization working to advance the knowledge of physics. |
© 2024 American Physical Society
| All rights reserved | Terms of Use
| Contact Us
Headquarters
1 Physics Ellipse, College Park, MD 20740-3844
(301) 209-3200
Editorial Office
100 Motor Pkwy, Suite 110, Hauppauge, NY 11788
(631) 591-4000
Office of Public Affairs
529 14th St NW, Suite 1050, Washington, D.C. 20045-2001
(202) 662-8700