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We report a study on the specific heat and heat transport of Tb$_2$Ti$_{2-x}$Zr$_x$O$_7$ ($x =$ 0, 0.02, 0.1, 0.2, and 0.4) single crystals at low temperatures and in high magnetic fields. The magnetic specific heat can be described by the Schottky contribution from the crystal-electric-field (CEF) levels of Tb$^{3+}$, with introducing Gaussian distributions of the energy split of the ground-state doublet and the gap between the ground state and first excited level. These crystals has an extremely low phonon thermal conductivity in a broad temperature range that can be attributed to the scattering by the magnetic excitations, which are mainly associated with the CEF levels. There is strong magnetic field dependence of thermal conductivity, which is more likely related to the field-induced changes of phonon scattering by the CEF levels than magnetic transitions or spin excitations. For magnetic field along the [111] direction, there is large thermal Hall conductivity at low temperatures which displays a broad peak around 8 T. At high fields up to 14 T, the thermal Hall conductivity decreases to zero, which supports its origin from either the spinon transport or the phonon skew scattering by CEF levels. The thermal Hall effect is rather robust with Zr doping up to 0.2 but is strongly weakened in higher Zr-doped sample., Comment: 9 pages, 8 figures, accepted for publication in Phys. Rev. B

The interaction of a single-cycle THz electric field with the topological insulator $\mathrm{MnBi}_2\mathrm{Te}_4$ triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise non-interacting Raman-active $E_g$ and infrared (IR)-active $E_u$ phononic modes. Two-dimensional (2D) THz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multi-stage process underlying the excitation of the Raman-active $E_g$ phonon. In this process, the THz electric field first prepares a coherent IR-active $E_u$ phononic state and subsequently interacts with this state to efficiently excite the $E_g$ phonon., Comment: 27 pages, 20 figures

We report a systematic study of the structure, electric and magnetic properties of Ca$_3$Co$_{2-x}$Mn$_x$O$_6$ single crystals with $x =$ 0.72 and 0.26. The DC and AC magnetic susceptibilities display anomalies with characteristic of the spin freezing. The crystals show ferroelectric transition at 40 K and 35 K ($T_{FE}$) for $x =$ 0.72 and 0.26, respectively, with a large value of 1400 $\mu$C/m$^2$ at 8 K for electric polarization ($P_c$) along the spin-chain ($c$-axis) direction. Interestingly, the electric polarization perpendicular to the chain direction ($P_{ab}$) can also be detected and has value of 450 and 500 $\mu$C/m$^2$ at 8 K for the $x =$ 0.72 and 0.26 samples, respectively. The specific heat and magnetic susceptibility show no anomaly around $T_{FE}$, which means that the electric polarization of these samples has no direct relationship with the magnetism. The X-ray diffraction and the Raman spectroscopy indicate that these samples may undergo Jahn-Teller distortions that could be the reason of electric polarization., Comment: 11 pages, 16 figures, accepted for publication in Phys. Rev. B

We report a study on the thermal conductivity of CuFe$_{1-x}$Ga$_x$O$_2$ ($x =$ 0--0.12) single crystals at temperatures down to 0.3 K and in magnetic fields up to 14 T. CuFeO$_2$ is a well-known geometrically frustrated triangular lattice antiferromagnet and can be made to display multiferroicity either by applying magnetic field along the $c$ axis or by doping nonmagnetic impurities, accompanied with rich behaviors of magnetic phase transitions. The main experimental findings of this work are: (i) the thermal conductivities ($\kappa_a$ and $\kappa_c$) show drastic anomalies at temperature- or field-induced magnetic transitions; (ii) the low-$T$ $\kappa(H)$ isotherms exhibit irreversibility in a broad region of magnetic fields; (iii) there are phonon scattering effect caused by magnetic fluctuations at very low temperatures. These results demonstrate strong spin-phonon coupling in this material and reveal the non-negligible magnetic fluctuations in the "ground state" of pure and Ga-doped samples., Comment: 11 pages, 12 figures, accepted for publication in Phys. Rev. B

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., Comment: 5 pages, 4 figures, accepted for publication in Appl. Phys. Lett

Thermal conductivity (\kappa) of a distorted spin diamond-chain system, Cu_3(CO_3)_2(OH)_2, is studied at low temperatures down to 0.3 K and in magnetic fields up to 14 T. In zero field, the \kappa(T) curve with heat current along the chain direction has very small magnitudes and shows a pronounced three-peak structure. The magnetic fields along and perpendicular to the chains change the \kappa strongly in a way having good correspondence to the changes of magnetic specific heat in fields. The data analysis based on the Debye model for phononic thermal conductivity indicates that the heat transport is due to phonons and the three-peak structure is caused by two resonant scattering processes by the magnetic excitations. In particular, the spin excitations of the chain subsystem are strongly scattering phonons rather than transporting heat., Comment: 15 pages, 6 figures, accepted for publication in J. Phys.: Condens. Matt

We study the low-temperature thermal conductivity (\kappa) of Dy_2Ti_2O_7 and Yb_2Ti_2O_7 single crystals in magnetic fields up to 14 T along the [111], [100] and [110] directions. The main experimental findings for Dy_2Ti_2O_7 are: (i) the low-T \kappa(H) isotherms exhibit not only the step-like decreases at the low-field (< 2 T) magnetic transitions but also obvious field dependencies in high fields (> 7 T); (ii) at T \le 0.5 K, the \kappa(H) curves show anisotropic irreversibility in low fields, that is, the \kappa(H) hysteresis locates at the first-order transition with H \parallel [100] and [110], while it locates between two successive transitions with H \parallel [111]; (iii) the \kappa in the hysteresis loops for H \parallel [100] and [110] show an extremely slow relaxation with the time constant of \sim 1000 min. The main experimental findings for Yb_2Ti_2O_7 are: (i) the zero-field \kappa(T) show a kink-like decrease at the first-order transition (\sim 200 mK) with decreasing temperature; (ii) the low-T \kappa(H) isotherms show a decrease in low field and a large enhancement in high fields; (iii) the low-T \kappa(H) curves show a sharp minimum at 0.5 T for H \parallel [110] and [111]. The roles of monopole excitations, field-induced transitions, spin fluctuations and magnetoelastic coupling are discussed., Comment: 15 pages, 11 figures, accepted for publication in Phys. Rev. B

The heat transport of the spin-gapped material (CH_3)_2CHNH_3CuCl_3 (IPA-CuCl_3), a candidate quantum magnet with Bose-Einstein condensation (BEC), is studied at ultra-low temperatures and in high magnetic fields. Due to the presence of the spin gap, the zero-field thermal conductivity (\kappa) is purely phononic and shows a ballistic behavior at T < 1 K. When the gap is closed by magnetic field at H = H_{c1}, where a long-range antiferromanetic (AF) order of Cu^{2+} moments is developed, the magnons contribute significantly to heat transport and exhibit a ballistic T^3 behavior at T < 600 mK. In addition, the low-T \kappa(H) isotherms show sharp peaks at H_{c1}, which indicates a gap re-opening in the AF state (H > H_{c1}) and demonstrates limited applicability of the BEC model to IPA-CuCl_3., Comment: 9 pages, 5 figures, accepted for publication in Phys. Rev. B

We consider the excitation of large amplitude gyrotropic vortex core precession in a Permalloy nanodisk by the torques originating from the in-plane microwave current flowing along the interface of the Permalloy/Bi$_2$Se$_3$ heterostructures, in which the huge charge-to-spin conversion ratio is observed \cite{Mellnik-2014}. We consider analytically and by micromagnetic modelling the dependence of this excitation on the frequency and magnitude of the microwave current. The analogies of the vortex dynamics and the Landau phase transitions theory is demonstrated. These findings open the possibility to excite gyrotropic vortex motion with the current densities far lower than by any other means., Comment: 6 pages, 2 figures

A nonequilibrium Green's function technique combined with density functional theory is used to study the spin-dependent electronic band structure and transport properties of zigzag silicene nanoribbons (ZSiNRs) doped with aluminum (Al) or phosphorus (P) atoms. The presence of a single Al or P atom induces quasibound states in ZSiNRs that can be observed as new dips in the electron conductance. The Al atom acts as an acceptor whereas the P atom acts as a donor when it is placed at the center of the ribbon. This behavior is reversed when the dopant is placed on the edges. Accordingly, an acceptor-donor transition is observed in ZSiNRs upon changing the dopant's position. Similar results are obtained when two silicon atoms are replaced by two impurities (Al or P atoms) but the conductance is generally modified due to the impurity-impurity interaction. When the doping breaks the two-fold rotational symmetry about the central line, the transport becomes spin dependent.

We report a study of the Ce doping effect on the thermal conductivity (\kappa) of Nd_{2-x}Ce_{x}CuO_{4} (NCCO) at low temperatures down to 0.3 K and in magnetic fields up to 14 T. It is found that with Ce doping, the electronic thermal conductivity increases; at the same time, the a-axis field induced changes in \kappa(H), associated with the spin flop and spin polarization of Nd^{3+} sublattice, and the spin flop of Cu^{2+} sublattice, gradually disappear. These are clearly due to the electron doping and the destruction of the antiferromagnetic orders. In the superconducting NCCO with x = 0.14 and 0.18, although the electronic thermal conductivity shows sizable field dependencies with H // c, the paramagnetic scattering of phonons is still playing the dominant role in the heat transport, which is different from many other cuprates. In the lightly doped samples (x = 0.03 and 0.06), the low-T \kappa(H) isotherms with H // c show a step-like anomaly and is likely related to the spin/charge stripes., Comment: 10 pages, 7 figures, accepted for publication in Phys. Rev. B

We report a study on the crystal growth of the hexagonal manganites $R$MnO$_3$ ($R$ = Y, Lu, Ho, Er, and Tm) by using an optical floating-zone method. It was found that high-quality single crystals of $R$ = Y, Lu, and Ho could be easily grown with essentially the same conditions as those reported in literature, that is, with an atmosphere of normal pressure Ar and oxygen mixture and a growth rate of 2--4 mm/h. However, these conditions were not feasible for growing good crystals of $R$ = Er and Tm. The chemical analysis indicated that it was due to an off-stoichiometric phenomenon in the formed single crystals. We used an effective and simple way to resolve this problem by adjusting the nominal compositions of the polycrystal feed rods to be 1--$2\%$ rare-earth excess. The structures and physical properties were characterized by X-ray diffraction, magnetic susceptibility, specific heat, resistivity, and dielectric constant measurements., Comment: 21 pages, 8 figures, accepted for publication in Journal of Crystal Growth