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Added by
Steffen S\\\"aubert
Publication date
2019
fields
Physics
and research's language is
English
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In the quest to realize a quantum spin liquid (QSL), magnetic long-range order is hardly welcome. Yet it can offer deep insights into a complex world of strong correlations and fluctuations. Much hope was placed in the cubic pyrochlore Yb$_2$Ti$_2$O$_7$ as a putative U(1) QSL but a new class of ultra-pure single crystals make it abundantly clear the stoichiometric compound is a ferromagnet. Here we present a detailed experimental and theoretical study of the corresponding field-temperature phase diagram. We find it to be richly anisotropic with a critical endpoint for $vec{B},parallel,langle 100rangle$, while field parallel to $langle 110 rangle$ and $langle 111 rangle$ enhances the critical temperature by up to a factor of two and shifts the onset of the field-polarized state to finite fields. Landau theory shows that Yb$_2$Ti$_2$O$_7$ in some ways is remarkably similar to pure iron. However, it also pinpoints anomalies that cannot be accounted for at the classical mean-field level including a dramatic enhancement of $T_{mathrm{C}}$ and reentrant phase boundary by fields with a component transverse to the easy axes, as well as the anisotropy of the upper critical field in the quantum limit.
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We report low temperature specific heat and muon spin relaxation/rotation ($mu$SR) measurements on both polycrystalline and single crystal samples of the pyrochlore magnet Yb$_2$Ti$_2$O$_7$. This system is believed to possess a spin Hamiltonian supporting a Quantum Spin Ice (QSI) ground state and to display sample variation in its low temperature heat capacity. Our two samples exhibit extremes of this sample variation, yet our $mu$SR measurements indicate a similar disordered low temperature state down to 16 mK in both. We report little temperature dependence to the spin relaxation and no evidence for ferromagnetic order, in contrast to recent reports by Chang emph{et al.} (Nat. Comm. {bf 3}, 992 (2012)). Transverse field (TF) $mu$SR measurements show changes in the temperature dependence of the muon Knight shift which coincide with heat capacity anomalies. We are therefore led to propose that Yb$_2$Ti$_2$O$_7$ enters a hidden order ground state below $T_csim265$ mK where the nature of the ordered state is unknown but distinct from simple long range order.
The very nature of the ground state of the pyrochlore compound Yb$_2$Ti$_2$O$_7$ is much debated, as experimental results demonstrate evidence for both a disordered or a long-range ordered ground state. Indeed, the delicate balance of exchange interactions and anisotropy is believed to lead to competing states, such as a Quantum Spin Liquid state or a ferromagnetic state which may originate from an Anderson-Higgs transition. We present a detailed magnetization study demonstrating a first order ferromagnetic transition at 245 mK and 150 mK in a powder and a single crystal sample respectively. Its first-order character is preserved up to applied fields of $sim$ 200 Oe. The transition stabilizes a ferromagnetic component and involves slow dynamics in the magnetization. Residual fluctuations are also evidenced, the presence of which might explain some of the discrepancies between previously published data for Yb$_2$Ti$_2$O$_7$.
We report on measurements of the sound velocity and attenuation in a single crystal of the candidate quantum- spin-ice material Yb$_2$Ti$_2$O$_7$ as a function of temperature and magnetic field. The acoustic modes couple to the spins magneto-elastically and, hence, carry information about the spin correlations that sheds light on the intricate magnetic phase diagram of Yb$_2$Ti$_2$O$_7$ and the nature of spin dynamics in the material. Particularly, we find a pronounced thermal hysteresis in the acoustic data with a concomitant peak in the specific heat indicating a possible first-order phase transition at about $0.17$ K. At low temperatures, the acoustic response to magnetic field saturates hinting at the development of magnetic order. Furthermore, mean-field calculations suggest that Yb$_2$Ti$_2$O$_7$ undergoes a first-order phase transition from a cooperative paramagnetic phase to a ferromagnet below $Tapprox 0.17$ K.
We explore the field-temperature phase diagram of the XY pyrochlore antiferromagnet Er$_2$Ti$_2$O$_7$, by means of magnetization and neutron diffraction experiments. Depending on the field strength and direction relative to the high symmetry cubic directions $[001], [1bar{1}0]$ and $[111]$, the refined field induced magnetic structures are derived from the zero field $psi_2$ and $psi_3$ states of the $Gamma_5$ irreducible representation which describes the ground state of XY pyrochlore antiferromagnets. At low field, domain selection effects are systematically at play. In addition, for $[001]$, a phase transition is reported towards a $psi_3$ structure at a characteristic field $H_c^{001}=$ 43 mT. For $[1bar{1}0]$ and $[111]$, the spins are continuously tilted by the field from the $psi_2$ state, and no phase transition is found while domain selection gives rise to sharp anomalies in the field dependence of the Bragg peaks intensity. For $[1bar{1}0]$, these results are confirmed by high resolution inelastic neutron scattering experiments, which in addition allow us to determine the field dependence of the spin gap. This study agrees qualitatively with the scenario proposed theoretically by Maryasin {it et al.} [Phys. Rev. B {bf 93}, 100406(R) (2016)], yet the strength of the field induced anisotropies is significantly different from theory.
The elementary excitations of the spin-ice materials Ho$_2$Ti$_2$O$_7$ and Dy$_2$Ti$_2$O$_7$ in zero field can be described as independent magnetic monopoles. We investigate the influence of these exotic excitations on the heat transport by measuring the magnetic-field dependent thermal conductivity $kappa $. Additional measurements on the highly dilute reference compounds HoYTi$_2$O$_7$ and DyYTi$_2$O$_7$ enable us to separate $kappa $ into a sum of phononic ($kappa_{ph}$) and magnetic ($kappa_{mag}$) contributions. For both spin-ice materials, we derive significant zero-field contributions $kappa_{mag}$, which are rapidly suppressed in finite magnetic fields. Moreover, $kappa_{mag}$ sensitively depends on the scattering of phonons by magnetic excitations, which is rather different for the Ho- and the Dy-based materials and, as a further consequence, the respective magnetic-field dependent changes $kappa_{ph}(B)$ are even of opposite signs.
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