Giant directional dichroism of terahertz light in resonance with magnetic excitations of the multiferroic oxide BaCo2_2Ge2_2O7_7

We propose that concurrently magnetic and ferroelectric, i.e. multiferroic, compounds endowed with electrically-active magnetic excitations (electromagnons) provide a key to produce large directional dichroism for long wavelengths of light. By exploiting the control of ferroelectric polarization and magnetization in a multiferroic oxide Ba$_2$CoGe$_2$O$_7$, we demonstrate the realization of such a directional light-switch function at terahertz frequecies in resonance with the electromagnon absorption. Our results imply that this hidden potential is present in a broad variety of multiferroics

Terahertz spectroscopy of spin excitations in magnetoelectric LiFePO4 in high magnetic fields

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Spin excitations of magnetoelectric LiNiPO4 in multiple magnetic phases

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High-pressure infrared spectroscopy: Tuning of the low-energy excitations in correlated electron systems

We have extended the range of the high-pressure optical spectroscopy to the far-infrared region keeping the accuracy of ambient-pressure experiments. The developed method offers a powerful tool for the study of pressure-induced phase transitions and electronic-structural changes in correlated electron systems as the optical pressure cell, equipped with large free-aperture diamond window, allows the measurement of optical reflectivity down to omega approximate to 20-30 cm(-1) for hydrostatic pressures up to p approximate to 26 kbar. The efficiency of the technique is demonstrated by the investigation of the two-dimensional charge-density-wave 1T-TaS2 whose electronic structure shows high sensitivity to external pressure. The room-temperature semimetallic phase of 1T-TaS2 is effectively extended by application of pressure and stabilized as the ground state above p=14 kbar. The corresponding fully incoherent low-energy optical conductivity is almost temperature independent below T=300 K. For intermediate pressures, the onset of the low-temperature insulating phase is reflected by the sudden drop of the reflectivity and by the emergence of sharp phonon resonances

What is Minimal Model of 3He Adsorbed on Graphite? -Importance of Density Fluctuations in 4/7 Registered Solid -

We show theoretically that the second layer of 3He adsorbed on graphite and solidified at 4/7 of the first-layer density is close to the fluid-solid boundary with substantial density fluctuations on the third layer. The solid shows a translational symmetry breaking as in charge-ordered insulators of electronic systems. We construct a minimal model beyond the multiple-exchange Heisenberg model. An unexpectedly large magnetic field required for the measured saturation of magnetization is well explained by the density fluctuations. The emergence of quantum spin liquid is understood from the same mechanism as in the Hubbard model and in \kappa-(ET)_2Cu_2(CN)_3 near the Mott transitions.Comment: 9 pages, 5 figure

Optical Diode Effect at Spin-Wave Excitations of the Room-Temperature Multiferroic BiFeO3

Multiferroics permit the magnetic control of the electric polarization and the electric control of the magnetization. These static magnetoelectric (ME) effects are of enormous interest: The ability to read and write a magnetic state current-free by an electric voltage would provide a huge technological advantage. Dynamic or optical ME effects are equally interesting, because they give rise to unidirectional light propagation as recently observed in low-temperature multiferroics. This phenomenon, if realized at room temperature, would allow the development of optical diodes which transmit unpolarized light in one, but not in the opposite, direction. Here, we report strong unidirectional transmission in the room-temperature multiferroic BiFeO3 over the gigahertz-terahertz frequency range. The supporting theory attributes the observed unidirectional transmission to the spin-current-driven dynamic ME effect. These findings are an important step toward the realization of optical diodes, supplemented by the ability to switch the transmission direction with a magnetic or electric field.clos

Field and anisotropy driven transformations of spin spirals in cubic skyrmion hosts

We discuss distinctive features of spiral states in bulk chiral magnets such as MnSi and Cu2OSeO3 that stem from the effect of the cubic magnetocrystalline anisotropy. First of all, at both the helical-to-conical and theconical-to-ferromagnetic transitions, taking place at Hc1 and Hc2, respectively, the cubic anisotropy leads to reversible or irreversible jumplike reorientations of the spiral wave vectors. The subtle interplay between the easy and hard anisotropy axes gives rise to a phase transition between elliptically distorted conical states almost without any detectable change in the period. We show that the competition between on-site cubic and exchange anisotropy terms can also lead to oblique spiral states. Our work gives clear directions for further experimental studies to reveal theoretically predicted spiral states in cubic helimagnets beyond the aforementioned well-established states and thus it can help to understand the magnetic phase diagram of these archetypal skyrmion hosts. In addition, we show that properties of isolated skyrmions such as interskyrmion attraction, orientation, and/or nucleation are also rooted in the properties of host spiral states, in which skyrmions are stabilizedRST/Neutron and Positron Methods in Material

Exchange anisotropy in the skyrmion host GaV4S8

Using ferromagnetic resonance spectroscopy at 34 GHz we explored the magnetic anisotropy of single-crystalline GaV4S8 in the field-polarized magnetic state. We describe the data in terms of an easy-axis type uniaxial anisotropy with an anisotropy constant K-1 = 1.6 . 10(5) erg cm(-3) at 2 K, corresponding to a relative exchange anisotropy Delta J/J approximate to 5%, and about 1.0 . 10(5) erg cm(3) near 11 K, i.e. at temperatures where the skyrmion-lattice phase was recently discovered. The relatively large value of K-1 explains the confinement of the skyrmion tubes to the easy axes. A distinct set of resonances in the spectra is attributed to the co-existence of different rhombohedral domains. Complementary broadband spectroscopy demonstrates that non-collinear spin states may sensitively be detected by electron spin resonance techniques