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Register a new userTopological quantum phase transition in the Ising-like antiferromagnetic spin chain BaCo$_2$V$_2$O$_8$
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B\\'eatrice Grenier
Publication date
2017
fields
Physics
and research's language is
English
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Since the seminal ideas of Berezinskii, Kosterlitz and Thouless, topological excitations are at the heart of our understanding of a whole novel class of phase transitions. In most of the cases, those transitions are controlled by a single type of topological objects. There are however some situations, still poorly understood, where two dual topological excitations fight to control the phase diagram and the transition. Finding experimental realization of such cases is thus of considerable interest. We show here that this situation occurs in BaCo$_2$V$_2$O$_8$, a spin-1/2 Ising-like quasi-one dimensional antiferromagnet when subjected to a uniform magnetic field transverse to the Ising axis. Using neutron scattering experiments, we measure a drastic modification of the quantum excitations beyond a critical value of the magnetic field. This quantum phase transition is identified, through a comparison with theoretical calculations, to be a transition between two different types of solitonic topological objects, which are captured by different components of the dynamical structure factor.
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Combining inelastic neutron scattering and numerical simulations, we study the quasi-one dimensional Ising anisotropic quantum antiferromagnet bacovo in a longitudinal magnetic field. This material shows a quantum phase transition from a Neel ordered phase at zero field to a longitudinal incommensurate spin density wave at a critical magnetic field of 3.8 T. Concomitantly the excitation gap almost closes and a fundamental reconfiguration of the spin dynamics occurs. These experimental results are well described by the universal Tomonaga-Luttinger liquid theory developed for interacting spinless fermions in one dimension. We especially observe the rise of mainly longitudinal excitations, a hallmark of the unconventional low-field regime in Ising-like quantum antiferromagnet chains.
Large single crystals of the new compound SrMn$_2$V$_2$O$_8$ have been grown by the floating-zone method. This transition-metal based oxide is isostructural to SrNi$_2$V$_2$O$_8$, described by the tetragonal space group $I4_1cd$. Magnetic properties were investigated by means of susceptibility, magnetization, and specific heat measurements. The title compound behaves like a one-dimensional magnetic system above the ordering temperature ($T_N$ = 43 K). The magnetic ground state can be described as a classical long-range ordered antiferromagnet with weak anisotropy.
BaCo$_2$V$_2$O$_8$ is a one-dimensional antiferromagnetic spin-1/2 chain system with pronounced Ising anisotropy of the magnetic exchange. Due to finite interchain interactions long-range antiferromagnetic order develops below $T_{rm N} simeq 5.5$ K, which is accompanied by a structural distortion in order to lift magnetic frustration effects. The corresponding temperature $vs. $ magnetic-field phase diagram is highly anisotropic with respect to the magnetic-field direction and various details are still under vivid discussion. Here, we report the influence of several substitutions on the magnetic properties and the phase diagrams of BaCo$_2$V$_2$O$_8$. We investigate the substitution series Ba$_{text{1-x}}$Sr$_{text{x}}$Co$_{text{2}}$V$_{text{2}}$O$_{text{8}}$ over the full range $0le x le 1$ as well as the influence of a partial substitution of the magnetic Co$^{2+}$ by small amounts of other magnetic transition metals or by non-magnetic magnesium. In all cases, the phase diagrams were obtained on single crystals from magnetization data and/or high-resolution studies of the thermal expansion and magnetostriction.
We report $^{51}$V nuclear magnetic resonance (NMR) and inelastic neutron scattering (INS) measurements on a quasi-1D antiferromagnet BaCo$_2$V$_2$O$_8$ under transverse field along the [010] direction. The scaling behavior of the spin-lattice relaxation rate above the N{e}el temperatures unveils a 1D quantum critical point (QCP) at $H_c^{1D}approx 4.7$ T, which is masked by the 3D magnetic order. With the aid of accurate analytical analysis and numerical calculations, we show that the zone center INS spectrum at $H_c^{1D}$ is precisely described by the pattern of the 1D quantum Ising model in a magnetic field, a class of universality described in terms of the exceptional $E_8$ Lie algebra. These excitations keep to be non-diffusive over a certain field range when the system is away from the 1D QCP. Our results provide an unambiguous experimental realization of the massive $E_8$ phase in the compound, and open new experimental route for exploring the dynamics of quantum integrable systems as well as physics beyond integrability.
We report a heat switch effect in single crystals of an antiferromagnet Co$_3$V$_2$O$_8$, that is, the thermal conductivity ($kappa$) can be changed with magnetic field in an extremely large scale. Due to successive magnetic phase transitions at 12--6 K, the zero-field $kappa(T)$ displays a deep minimum at 6.7 K and rather small magnitude at low temperatures. Both the temperature and field dependencies of $kappa$ demonstrate that the phonons are strongly scattered at the regime of magnetic phase transitions. Magnetic field can suppress magnetic scattering effect and significantly recover the phonon thermal conductivity. In particular, a 14 T field along the $a$ axis increases the $kappa$ at 7.5 K up to 100 times. For $H parallel c$, the magnitude of $kappa$ can be suppressed down to $sim$ 8% at some field-induced transition and can be enhanced up to 20 times at 14 T. The present results demonstrate that it is possible to design a kind of heat switch in the family of magnetic materials.
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