arXiv subjects

Rafael S. Freitas

Publications and source records attributed to Rafael S. Freitas.

5 recordsLinked to original sources

arXiv2026

Terahertz Lattice Imprint of a Hidden Multipolar Ordering Transition in Gd$_2$Ru$_2$O$_7$

Magnetic order carried by higher-rank multipoles produces no net moment and couples only weakly to conventional dipolar probes. Recently, the lattice has been proposed as an alternative detection channel for the associated ordering transitions. Here we report magneto-terahertz time-domain spectroscopy of the pyrochlore ruthenate Gd$_2$Ru$_2$O$_7$, in which a single spectrum resolves an exchange-split Gd$^{3+}$ transition and an infrared-active optical phonon. The temperature and field dependence of the magnetic mode give two independent determinations of the internal exchange field at the rare-earth site, which yield a magnitude of 5-6 T and a free-ion $g$-factor. The optical phonon, sensitive to spin-disorder scattering rather than to the static exchange field, exhibits a sharp anomaly in frequency and lifetime at 10 K that shifts to 30 K under 7 T, which we identify as an ordering transition of the Gd sublattice hidden from bulk thermodynamic probes. These results identify spin-phonon coupling as a route to rare-earth ordering transitions in multipolar magnets, one that is applicable to polycrystalline samples.

cond-mat.str-el↗

arXiv2021

Magnetic field induced quantum spin liquid in the two coupled trillium lattices of K$_2$Ni$_2$(SO$_4$)$_3$

Quantum spin liquids are exotic states of matter which form when strongly frustrated magnetic interactions induce a highly entangled quantum paramagnet far below the energy scale of the magnetic interactions. Three-dimensional cases are especially challenging due to the significant reduction of the influence of quantum fluctuations. Here, we report the magnetic characterization of {\kni} forming a three dimensional network of Ni$^{2+}$ spins. Using density functional theory calculations we show that this network consists of two interconnected spin-1 trillium lattices. In the absence of a magnetic field, magnetization, specific heat, neutron scattering and muon spin relaxation experiments demonstrate a highly correlated and dynamic state, coexisting with a peculiar, very small static component exhibiting a strongly renormalized moment. A magnetic field $B \gtrsim 4$ T diminishes the ordered component and drives the system in a pure quantum spin liquid state. This shows that a system of interconnected $S=1$ trillium lattices exhibit a significantly elevated level of geometrical frustration.

cond-mat.str-el↗

arXiv2017

Doping-induced quantum cross-over in Er$_2$Ti$_{2-x}$Sn$_x$O$_7$

We present the results of the investigation of magnetic properties of the Er$_2$Ti$_{2-x}$Sn$_x$O$_7$ series. For small doping values the ordering temperature decreases linearly with $x$ while the moment configuration remains the same as in the $x = 0$ parent compound. Around $x = 1.7$ doping level we observe a change in the behavior, where the ordering temperature starts to increase and new magnetic Bragg peaks appear. For the first time we present evidence of a long-range order (LRO) in Er$_2$Sn$_2$O$_7$ ($x = 2.0$) below $T_N = 130$ mK. It is revealed that the moment configuration corresponds to a Palmer-Chalker type with a value of the magnetic moment significantly renormalized compared to $x = 0$. We discuss our results in the framework of a possible quantum phase transition occurring close to $x = 1.7$.

cond-mat.str-el↗

arXiv2016

Critical Field Determination in Organic Antiferromagnet F4BImNN

A combination of magnetic heat capacity measurements versus temperature under multiple fixed external fields using a polycrystalline sample, plus easy and hard axis aligned magnetization versus field measurements using a single crystal of the radical 2-(4,5,6,7-tetrafluorobenzimidazol-2-yl)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazole-3-oxide-1-oxyl) at multiple fixed temperatures below 0.75 K, confirmed a zero-field Neel temperature of TN(0) = 0.73 K, with extrapolated 0 K critical field parameters of Hc(parallel) = 170 Oe and Hc(perpendicular) = 306 Oe. Assuming the previously determined 1D intrachain ferromagnetic exchange interaction of J = +22 K between radicals (Hamiltonian H = -2J ΣSi . Sj), , the interchain exchange in antiferromagnetic with 2zJinter = -0.09 K.

cond-mat.mtrl-sci↗

arXiv2007

Existence of two electronic states in Sr4Ru3O10 at low temperatures

We report measurements on in-plane resistivity, thermopower, and magnetization as a function of temperature and magnetic fields on single crystalline Sr4Ru3O10 grown by the floating zone method. As the temperature was lowered to below around 30 K, the in-plane and c-axis resistivities and the thermopower were found to exhibit a step feature accompanied by hysteresis behavior when the in-plane field was swept up and down from below 10 kOe to above 20 kOe. The sharp increase in the thermopower with increasing in-plane magnetic field at low temperatures has not been observed previously in layered transition metal oxides. Comparing with magnetization data, we propose that the step feature marks the transition between the two different electronic states in Sr4Ru3O10. We propose that the alignment of domains by the in-plane magnetic field is responsible to the emergence of the new electronic states in high applied in-plane magnetic field.

cond-mat.str-el↗