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Register a new userChirality-driven ferroelectricity in LiCuVO$_4$
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Chirality or the handedness of objects is of prime importance in life science, biology, chemistry and physics. It is also a major symmetry ingredient in frustrated magnets revealing spin-spiral ground states. Vector chiral phases, with the twist (either clock- or counter clock-wise) between neighbouring spins being ordered, but with disorder with respect to the angles between adjacent spins, have been predicted almost five decades ago. Experimental proofs, however, are rare and controversial. Here, we provide experimental evidence for such a phase in LiCuVO$_4$, a one-dimensional quantum magnet with competing ferromagnetic and antiferromagnetic interactions. The vector chiral state is identified via a finite ferroelectric polarization arising at temperatures well above the multiferroic phase exhibiting long-range three-dimensional spin-spiral and polar order. On increasing temperatures, spin order becomes suppressed at TN, while chiral long-range order still exist, leaving a temperature window with chirality-driven ferroelectricity in the presence of an external magnetic field.
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We study field induced quantum phase in weakly-coupled ferromagnetic frustrated chain LiCuVO$_4$ by neutron diffraction technique. A new incommensurate magnetic peak is observed at $H ge 8.5$ T. The field dependent propagation vector is identified with the spin density wave correlation in the theoretically predicted magnetic quadrupole order. Quantum fluctuation, geometrical frustration, and interchain interaction induce the exotic spin density wave long-range order in the insulating magnet.
We present the results of broadband dielectric spectroscopy of GaMo$_4$S$_8$, a lacunar spinel system that recently was shown to exhibit non-canonical, orbitally-driven ferroelectricity. Our study reveals complex relaxation dynamics of this multiferroic material, both above and below its Jahn-Teller transition at T$_{textrm{JT}}=47$ K. Above T$_{textrm{JT}}$, two types of coupled dipolar-orbital dynamics seem to compete: relaxations within cluster-like regions with short-range polar order like in relaxor ferroelectrics and critical fluctuations of only weakly interacting dipoles, the latter resembling the typical dynamics of order-disorder type ferroelectrics. Below the Jahn-Teller transition, the onset of orbital order drives the system into long-range ferroelectric order and dipolar dynamics within the ferroelectric domains is observed. The coupled dipolar and orbital relaxation behavior of GaMo$_4$S$_8$ above the Jahn-Teller transition markedly differs from that of the skyrmion host GaV$_4$S$_8$, which seems to be linked to differences in the structural distortions of the two systems on the unit-cell level.
We report on NMR studies of the quasi one--dimensional (1D) antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$ in magnetic fields $H$ up to $mu_0H$ = 30 T ($approx 70$% of the saturation field $H_{rm sat}$). NMR spectra in fields higher than $H_{rm c2}$ ($mu_0H_{rm c2} approx 7.5$ T) and temperatures $T<T_{rm N}$ can be described within the model of a spin-modulated phase in which the magnetic moments are aligned parallel to the applied field $H$ and their values alternate sinusoidally along the magnetic chains. Based on theoretical concepts about magnetically frustrated 1D chains, the field dependence of the modulation strength of the magnetic structure is deduced from our experiments. Relaxation time $T_2$ measurements of the $^{51}$V nuclei show that $T_2$ depends on the particular position of the probing $^{51}$V nucleus with respect to the magnetic copper moments within the 1D chains: the largest $T_2$ value is observed for the vanadium nuclei which are very next to the magnetic Cu$^{2+}$ ion with largest ordered magnetic moment. This observation is in agreement with the expectation for the spin-modulated magnetic structure. The $(H,T)$ magnetic phase diagram of LiCuVO$_4$ is discussed.
We investigated the magnetoelastic properties of the quasi-one-dimensional spin-1/2 frustrated magnet LiCuVO$_4$. Longitudinal-magnetostriction experiments were performed at 1.5 K in high magnetic fields of up to 60 T applied along the $b$ axis, i.e., the spin-chain direction. The magnetostriction data qualitatively resemble the magnetization results, and saturate at $H_{text{sat}} approx 54$ T, with a relative change in sample length of $Delta L/L approx 1.8times10^{-4}$. Remarkably, both the magnetostriction and the magnetization evolve gradually between $H_{text{c3}} approx 48$ T and $H_{text{sat}}$, indicating that the two quantities consistently detect the spin-nematic phase just below the saturation. Numerical analyses for a weakly coupled spin-chain model reveal that the observed magnetostriction can overall be understood within an exchange-striction mechanism. Small deviations found may indicate nontrivial changes in local correlations associated with the field-induced phase transitions.
We report on a heat capacity study of high quality single crystal samples of lcvo -- a frustrated spin $S=1/2$ chain system -- in magnetic field amounting to 3/4 of the saturation field. At low fields up to about 7~T, a linear temperature dependence of the specific heat, $C_ppropto T$, resulting from 1D magnetic correlations in the spin chains is followed upon cooling by a sharp lambda anomaly of the transition into a 3D helical phase with $C_ppropto T^3$ behavior at low temperature. The transition from a spin liquid into a spin-modulated (SM) phase at higher fields occurs via a hump-like anomaly which, as the temperature decreases further turns into a $C_ppropto T^2$ law distinctive for a quasi-2D system. We suggest an explanation for how nonmagnetic defects in the Cu$^{2+}$ chains can suppress 3D long-range ordering in the SM phase and leave it undisturbed in a helical phase.
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