Session Y18: Focus Session: Low D/Frustrated Magnetism - More Frustrated Magnets

Field-induced thermal transport in BEC antiferromagnets

Recent experiments in BEC quantum magnets exhibit a dramatic evolution of the thermal conductivity of these materials in magnetic field. By considering various relaxation mechanisms of bosonic excitations in the vicinity of the BEC quantum-critical point at finite temperature we provide a detailed explanation of several unusual features of the data. We identify the leading impurity-scattering interaction and demonstrate that its renormalization due to quantum fluctuations of the paramagnetic state compensates the related mass renormalization effect. This explains the enigmatic absence of the asymmetry between the two critical points in the low-$T$ thermal conductivity data, while such an asymmetry is prominent in many other physical quantities. The observed characteristic ``migration'' of the peak in thermal conductivity away from the transition points as a function of temperature is explained as due to a competition between an increase in the number of heat carriers and an enhancement of their mutual scattering. An important role of the three-boson scattering processes within the ordered phase of these systems is also discussed. Other qualitative and quantitative features of the experiment are clarified and the future directions are sketched.   

Investigation of the magnetic susceptibility of the disordered BEC system NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ at ultralow-temperatures

We report measurements of the magnetic susceptibility of a disordered BEC system of magnons for single crystals of NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ (with 15{\%} Cl atoms replaced by Br). NiCl$_{0.85}$Br$_{0.15}$-4SC(NH$_{2})_{2}$ is a potential candidate for a Bose glass (BG) phase of the spins adjacent to a region of Bose-Einstein condensation (BEC). The BG to BEC phase is the bosonic analog of a metal-insulator transition for fermions. The measurements were carried out for temperatures down to 1mK and for applied magnetic fields up to 14.5T. The results show that the critical fields $H_{c}$ do not obey the conventional 3D universality class for a BEC, $H_{c}(T)$ -- $H_{c}(0) \quad \sim $ $T^{\alpha }$, where $\alpha $ = 1.5 [1]. The values of $\alpha $ changes from $\alpha $ = 0.52 for T $>$ 300 mK to $\alpha $ = 0.91 for T $<$ 250 mK and then again at 70$\sim $90mK to $\alpha $ = 0.48 for T$<$ 70 mK, indicating a crossover to possible BG behavior.   

Bose-Einstein Condensation in Han Purple - a NMR Study

NMR study of the two quasi-2D coupled spin-1/2 dimer compound, BaCuSi$_2$O$_6$ (Han Purple) [1], is presented. $T_{\rm{BEC}}$ varies as $(H-H_{c1})^{2/d}$, where $d$ is the dimensionality of the system, and $H_{c1}$ the critical field which closes the gap. BaCuSi$_2$O$_6$ was claimed to exhibit an reduction of d from 3D to 2D at low T [2]. However, due to a structural transformation at 90 K, different intradimer exchange couplings and different gaps ($\Delta_{\rm{B}}$/$\Delta_{\rm{A}}$=1.16) exist in every second plane along the c axis [3]. In our first NMR experiments [3], we have shown that the population of bosons in the B planes $n_{\rm{B}}$ was much smaller than $n_{\rm{A}}$, but finite in the field range $\Delta_{\rm{A}}/g\mu_{\rm{B}} < H < \Delta_{\rm{B}}/g\mu_{\rm{B}}$ where $n_{\rm{B}}$ = 0 is expected in a naive model of uncoupled planes. Recently, a new model has been presented [4] which takes into account both frustration and quantum fluctuations. This leads to a non-zero population $n_{\rm{B}}$ of \emph{uncondensed} bosons in the $B$ plane, increasing quadratically with $(H-H_{c1})$, as compared to the linear dependence of $n_{\rm{A}}$. We compare our new NMR results to these predictions. $[$1$]$ M. Jaime \textit{et al.}, PRL \textbf{93},087203 (2004). $[$2$]$ S. E. Sebastian \textit{et al.}, Nature \textbf{441}, 617 (2006). $[$3$]$ S. Kr\"{a}mer \textit{et al.}, PRB \textbf{76}, 100406(R) (2007). $[$4$]$ N. Laflorencie and F. Mila, PRL \textbf{102}, 060602 (2009).   

Magnetic neutron scattering of a Prussian blue analogue photomagnet

Since the discovery of photoinduced magnetism in cobalt hexacyanoferrate (CoFe) Prussian blue analogues (PBAs) in 1996,$^1$ there have been many, multifarious studies that elucidated the nature of the photoeffect. However, the magnetization in CoFe has proven difficult to model quantitatively using macroscopic data due to the presence of multiple magnetic species, magnetic bistability, superexchange, and unquenched orbital angular momentum. To investigate the ordered magnetization directly, we have studied dueterated powders of CoFe using unpolarized and polarized neutron diffraction, and observed magnetic neutron scattering for the first time in this compound. A model for the magnetic structure based upon neutron diffraction, elemental analysis, infrared spectroscopy, and macroscopic magnetization will be presented. \newline [1] O. Sato, et al., Science 272, 704 (1996).   

Photoinduced Magnetism in Nanoscale Heterostructures of Prussian Blue Analogues*

Nanometer-scale cubic heterostructures of two Prussian blue analogues, ferromagnetic K$_j$Ni$_k$[Cr(CN)$_6$]$_l \cdot$\emph{n}H$_2$O (\textbf{A}) with $T_c\sim70$~K and photo-active ferrimagnetic Rb$_a$Co$_b$[Fe(CN)$_6$]$_c \cdot$\emph{m}H$_2$O (\textbf{B}) with $T_c\sim20$~K, have been studied.\footnote{M. F. Dumont \emph{et al.}, Inorg. Chem., submitted.} These samples exhibit a persistent photoinduced decrease in magnetization at temperatures up to $T_c\sim70$~K of the \textbf{A} constituent, resembling results from analogous \textbf{ABA} heterostructured films.\footnote{D. M. Pajerowski \emph{et al.}, J. Am. Chem. Soc. \textbf{132}, 4058 (2010).} This net decrease suggests that the photoinduced structural transition in the \textbf{B} layer generates a strain-induced decrease in the magnetization of the \textbf{A} layer, similar to a pressure-induced decrease previously observed in the pure \textbf{A} material.\footnote{M. Zentkov\'{a} \emph{et al.}, J. Phys.: Condens. Matter \textbf{19}, 266217 (2007).} Core-shell and core-shell-shell configurations \textbf{AB}, \textbf{BA}, \textbf{ABA}, and \textbf{BAB} have been characterized by TEM, FTIR, XRD, and SQUID magnetometry.\\[4pt]*Supported, in part, by NSF DMR-0701400 (MWM) and DMR-1005581 (DRT), NSERC, CFI, the NHMFL, and the State of Florida.   

Structural and Magnetic Interplay in Molecule-based Magnets with Photocontrollable Properties

Understanding the cooperative effects, such as electron-lattice interactions, in molecule-based magnetic coordination complexes possessing photoinduced phase transitions is an important step to being able to rationally tune the variables governing the process.\footnote{H.~Watanabe \emph{et al.}, Phys.~Rev.~B {\bf 79} (2009) 180405.} Specifically, variable temperature FTIR spectroscopy and magnetometry have been used to explore the temperature and photocontrollable spin transitions in Co-Fe Prussian blue analogues, $A_j$Co$_k$[Fe(CN)$_6$]$_{\ell}\cdot n$H$_2$O, where $A$ is an alkali ion, and in new Fe spin-crossover complexes. By studying nanoparticles\footnote{M.F.~Dumont \emph{et al.}, Inorg.~Chem., submitted.} and heterostructures,\footnote{D.M.~Pajerowski \emph{et al.}, J.~Am.~Chem.~Soc.~{\bf 132} (2010) 4058.} the data provide insight into the roles played by restricted lattice geometries and strain-pressure effects.   

Interplay between charge fluctuations and magnetic order in a stacked triangular-Kagome lattice : Applications to FeCrAs

The recently studied antiferromagnet FeCrAs [Wu et al, EPL, 85 17009 (2009)] exhibits a surprising combination of experimental signatures, with Fermi liquid like specific heat but resistivity showing strong non-Fermi liquid character. From the material properties we motivate a minimal model for the low energy degrees of freedom, and study its properties using slave-rotor mean field theory. Using this approach we find a variety of exotic phases and propose that the features of FeCrAs can be qualitatively explained by a spin liquid proximate to a metal-insulator transition.   

Giant Anomalous Hall Effect in (Ba,Sr)T$_{2+x}$Ru$_{4-x}$O$_{11}$ (T=Fe,Co,Mn) Ferrites

Hexagonal R-type ferrites (Ba,Sr)T$_{2+x}$Ru$_{4-x}$O$_{11}$ are promising spintronic materials that exhibit collinear ferrimagnetic order at unusually high critical temperatures T$_{C} \quad \le $ 490 K for Fe-bearing compositions, and an in-plane, ``all-in/all-out'' order at T$_{C}$'s $<<$ 300 K due to frustrated antiferromagnetic interactions within the Kagome basal plane in metallic Co or Mn compositions. A strong, nonmonotonic field dependence of the anomalous Hall effect is observed in metallic ferrites, which is generated by non-zero scalar spin chirality and the Berry phase acquired by carriers moving in the ``topologically nontrivial'' spin background of the Kagome plane. The FM semiconductor BaFe$_{3.4}$Ru$_{2.6}$O$_{11}$ (T$_{C}$ = 440 K) exhibits a giant Hall resistivity = 77 $\mu \Omega $-cm at 300 K, with a low-temperature sign change and monotonic field dependence that are consistent with a strong Berry phase curvature (gauge field) acquired by carriers in momentum space.   

Coexistence of ferromagnetic and antiferromagnetic orders in Ba-doped cobalt perovskites studied by neutron scattering

Cobalt-containing oxide compounds have attracted a great deal of interest in recent years due to the variety of magnetic and electrical properties. We performed single crystal neutron diffraction on 6T2 at~the~LLB in France and~the~HB3A four-circle diffractometor at~the~HFIR of ORNL. The Ba-doped cobalt perovskite (La$_{0.8}$Ba$_{0.2}$CoO$_{3})$ crystal was measured in the temperature range of 2-250 K. At temperature $T<$ 200 K, a set of ferromagnetic peaks ($k_{1}$ = 0) onsets and then antiferromagnetic peaks with $k_{2}$* = (1/2 0 1/2) and (0 0 3/2) join in at $T<$ 100 K. Both ferromagnetic and antiferromagnetic peaks saturate at $T\approx $ 40 K. By refining the peaks collected for $k_{1}$ and $k_{2}$ sets, magnetic structures were determined.   

Density matrix renormalization group study of optical conductivity in the one-dimensional Mott insulator Sr$_2$CuO$_3$

We examine the optical conductivity of Sr$_2$CuO$_3$ by using zero and finite temperature dynamical density matrix renormalization group (DMRG) methods. Employing a Hubbard- Holstein model containing Holstein-type coupling of electron to the Einstein phonons, we reproduce both the Mott-gap excitation and phonon-assisted spin excitation observed experimentally [1,2] by using the dynamical DMRG method combined with a regulated polynomial expansion [3]. We find a parameter set describing Sr$_2$CuO$_3$. Furthermore, by using a low- temperature dynamical DMRG method which is recently developed by present authors [4], we examine the temperature effect of the Mott-gap excitation to clarify the effect of optical phonons on spectral shape at finite temperatures. We find that the presence of phonons induces the enhancement of the width of an excitonic peak in the optical conductivity. [1] M. Ono, K. Miura, A. Maeda, H. Matsuzaki, H. Kishida, Y. Taguchi, Y. Tokura, M. Yamashita, and H. Okamoto, Phys. Rev. B {\bf 70}, 085101 (2004). [2] H. Suzuura, H. Yasuhara, A. Furusaki, N. Nagaosa, Y. Tokura: Phys. Rev. Lett. {\bf 76}, 2579 (1996). [3] S. Sota and M. Itoh, J. Phys. Soc. Jpn. {\bf 76}, 054004 (2007). [4] S. Sota and T. Tohyama, Phys. Rev. B {\bf 78}, 113101 (2008).   

A Compton scattering study of the magnetic structure of NbFe$_2$

NbFe$_2$ displays a diverse phase diagram over a narrow compositional range, possibly due to close proximity to a Quantum Critical Point. The ground state of NbFe$_2$ has been the subject of a number of recent theoretical investigations [1,2], but for near-stoichiometric compositions its exact nature remains ambiguous. We have probed the low temperature magnetic structure by performing Magnetic Compton Scattering (MCS) measurements on polycrystalline Nb$_{1+y}$Fe$_{2-y}$ samples with $y=-0.02,0.00$ and $0.03$. MCS is able to measure how the magnetic electrons within a sample are distributed in momentum space, and can be a useful tool for resolving site-specific contributions to the spin moment. The interpretation of these measurements was aided by electronic structure calculations, which favour a ferrimagnetic ground state. The results are presented with reference to the possible presence of spin fluctuations. \\[4pt] [1] Subedi and Singh, PRB {\bf 81}, 024422 (2010) \\[0pt] [2] Tompsett et al, PRB {\bf 82}, 155137 (2010)   

Impurity-entanglement in dimerized spin chains

To quantify the entanglement caused by an impurity in an $S=\frac{1}{2}$ dimerized $J_1-J_2$ quantum spin chain, several different entanglement-measures have been utilized. We present the results of variational calculations of the impurity entanglement entropy as well as the negativity for a chain with an impurity attached at one end. We compare the results for both of these measures.   

On the low-temperature behavior of a geometrically frustrated Heisenberg antiferromagnet

The thermodynamic behavior of the Heisenberg antiferromagnet on the kagome lattice and the effects of its geometrical frustration are widely understood [1]. At low temperatures planar spin configurations due to multiple zero-modes (so-called Weathervane loops) are favored entropically. These modes occur when spin clusters are bounded by spins pointing in a similar direction, so that the cluster spins revolve freely around this direction. However, it remains unclear if with decreasing temperature the number of these modes continues to increase and if this eventually leads to a highly ordered $\sqrt{3}\!\times\!\sqrt{3}$ state. In order to investigate this system we applied the simulated tempering method, combined with the Heatbath algorithm for single spins and a Metropolis loop-flip Monte Carlo move. We examined the thermodynamic properties for temperatures $\frac{k_BT}{J}\!\ge\!10^{-6}$; and found that once the planar state is attained, the out-of-plane excitations are reduced with decreasing temperature but no further order is established. Hence, the prevailing spin structure represents a temperature independent entropy maximum where any entropy gain produced by additional zero modes is neutralized by an entropy loss in the Weathervane loop structure.\\[4pt] [1] J.\ N.\ Reimers and A.\ J.\ Berlinsky, Phys. Rev. B {\bf 48}, 9539 (1993).   

Topological phases and quenches in spin-ladder systems

We show that a ladder version of Kitaev's honeycomb model can be directly mapped to a one-dimensional $p$-wave superconducting system. The ladder system is characterized by $Z_2$ vortices at every unit cell; the presence of vortices is encoded in the sign of the local chemical potential in the $p$-wave system. Compared to recently studied phases in topological superconductors, we show that certain vortex patterns in this ladder system can result in new topological phases and can alter the universality classes for associated phase transitions. We discuss the effect of performing time-dependent quenches in these new phases.   

Multi-spin exchange model for a quantum spin liquid on the honeycomb lattice

Recently, a possible quantum spin liquid (QSL) state has been found through quantum Monte Carlo studies of Hubbard model on the honeycomb lattice. The obtained QSL does not show long range correlation of any known type, which has a finite spin gap and a short range dimer-dimer correlation pattern resembling the short range resonant-valence-bond (RVB) state. Given the intensive current interest in such an exotic QSL, it is natural and timely to ask a question: what is the effective spin model to capture the essential low-energy physics near this QSL region? We report here a comparative numerical study based on finite-size exact diagonalizations (ED) of the Hubbard model, and a multi-spin exchange model with two-, four- and six-spin exchange terms. The latter model is derived from the strong coupling expansion of the former one. Through extensive ED calculations of low-energy spectra and ground-state correlation functions of both models, we try to establish connections between them, especially near the QSL region. Furthermore, the phase diagram of the multi-spin exchange model is explored in details.