Daniel Flavián Blasco


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Flavián Blasco

First Name

Daniel

ORCID

0000-0001-5003-0425

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2021 - 20257
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Publications 1 - 7 of 7
  • Dielectric Relaxation by Quantum Critical Magnons
    Item type: Journal Article
    Flavián Blasco, Daniel; Volkov, Pavel A.; Hayashida, Shohei; et al. (2023)
    Physical Review Letters
    We report the experimental observation of dielectric relaxation by quantum critical magnons. Complex capacitance measurements reveal a dissipative feature with a temperature-dependent amplitude due to low-energy lattice excitations and an activation behavior of the relaxation time. The activation energy softens close to a field-tuned magnetic quantum critical point at H=Hc and follows single-magnon energy for H>Hc, showing its magnetic origin. Our study demonstrates the electrical activity of coupled low-energy spin and lattice excitations, an example of quantum multiferroic behavior.
  • Braided Ising spin-tube physics in a purported kagome magnet
    Item type: Journal Article
    Nagl, Jakob; Flavián Blasco, Daniel; Duncan, Benjamin; et al. (2025)
    Physical Review B
    The magnetic insulator Nd3BWO9 has been previously proposed to realize the highly frustrated breathing kagome lattice model. We report a combination of single-crystal neutron scattering studies and numerical simulations that debunk this interpretation. We show that it is the interplane couplings that determine the physics. To explain the exotic magnetism, we derive a simple one-dimensional Ising model composed of twisted triangular spin tubes, i.e., triple braids of Ising spin chains with almost-orthogonal anisotropy frames and competing ferro-antiferromagnetic interactions. This model can account for the ground state, excitations, the numerous field-induced fractional magnetization plateau phases, and incommensurate magnetic correlations at elevated temperatures. Nd3BWO9 constitutes a rare example where rich magnetic phenomena can be understood and simulated quantitatively in terms of a simple classical Hamiltonian.
  • Dielectric relaxation in the quantum multiferroics Rb₂Cu₂Mo₃ O₁₂ and Cs₂Cu₂Mo₃O₁₂
    Item type: Journal Article
    Flavián Blasco, Daniel; Volkov, Pavel A.; Hayashida, Shohei; et al. (2024)
    Physical Review B
    Motivated by by the recent discovery of dielectric relaxation by quantum critical magnons in Cs₂Cu₂Mo₃O₁₂, we conduct a detailed analysis of its dielectric response and compare it to that in the isostructural compound Rb₂Cu₂Mo₃O₁₂. Measurements in the vicinity of the field-induced magnon softening show that its description in terms of a three-dimensional Bose-Einstein condensation quantum critical point is unaltered by the presence of dielectric relaxation. We also demonstrate the existence of dielectric relaxation anomalies at 19 K in Rb₂Cu₂Mo₃O₁₂ and discuss the implications for the microscopic origin of dielectric activity in two compounds.
  • Excitation spectrum and spin Hamiltonian of the frustrated quantum Ising magnet Pr₃BWO₉
    Item type: Journal Article
    Nagl, Jakob; Flavián Blasco, Daniel; Hayashida, Shohei; et al. (2024)
    Physical Review Research
    We present a thorough experimental investigation on single crystals of the rare-earth based frustrated quantum antiferromagnet Pr₃BWO₉, a purported spin-liquid candidate on the breathing kagome lattice. This material possesses a disordered ground state with an unusual excitation spectrum involving a coexistence of sharp spin waves and broad continuum excitations. Nevertheless, we show through a combination of thermodynamic, magnetometric, and spectroscopic probes with detailed theoretical modeling that it should be understood in a completely different framework. The crystal field splits the lowest quasidoublet states into two singlets moderately coupled through frustrated superexchange, resulting in a simple effective Hamiltonian of an Ising model in a transverse magnetic field. While our neutron spectroscopy data do point to significant correlations within the kagome planes, the dominant interactions are out-of-plane, forming frustrated triangular spin-tubes through two competing ferro-antiferromagnetic bonds. The resulting ground state is a simple quantum paramagnet, where the presence of strongly hyperfine-coupled nuclear moments and weak structural disorder causes significant modifications to both thermodynamic and dynamic properties.
  • Magnetic phase diagram of the breathing-kagome antiferromagnet Nd₃BWO₉
    Item type: Journal Article
    Flavián Blasco, Daniel; Nagl, Jakob; Hayashida, Shohei; et al. (2023)
    Physical Review B
    The highly frustrated rare-earth-based magnet Nd₃BWO₉ is a promising candidate in the search for proximate spin-liquid physics. We present a thorough investigation on single crystals of this material using bulk and microscopic techniques. Magnetization data reveal a fractional magnetization plateau for three different investigated field directions. The magnetic phase diagram is mapped out from calorimetric data and exhibits several domes of magnetic order below 0.3 K. Propagation vectors for all ordered phases are presented. The results suggest complex ordering in this material, and unveil the existence of a commensuration transition of the propagation vector at zero magnetic field. A scenario where interplane exchange interactions are essential to a magnetic model of Nd₃BWO₉ is discussed.
  • Dielectric phenomena at magnetic quantum critical points
    Item type: Doctoral Thesis
    Flavián Blasco, Daniel (2024)
    The interplay between magnetic and dielectric degrees of freedom gives rise to novel emergent phases in condensed matter systems. In the proximity to quantum critical points, exotic phenomena are stabilized by quantum fluctuations. In this work, we investigate experimentally dielectric phenomena at magnetic quantum critical points. Our focus is the highly frustrated quasi-one-dimensional quantum family of magnets: Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12. These systems offer several experimentally accessible field-induced quantum critical points, all of which can be described as a Bose-Einstein Condensation (BEC) of magnons. Electrometry is an extremely sensitive technique to detect phase transitions and map out the magnetic phase diagrams on both materials. Our data uncovers yet unnoticed presaturation phases in Rb2Cu2Mo3O12. More importantly, we demonstrate how magneto-electric coupling provides new perspectives on quantum criticality. A comprehensive set of dielectric permittivity data shows that the long-range order domes are multiferroic. The bilinear coupling arising at the spin saturation transition between ferroelectric polarization and staggered magnetization implies that both quantities are primary order parameters at the quantum phase transition. As a result, the critical susceptibility of this BEC quantum critical point can be unprecedentedly accessed via the dielectric channel and measured directly in a probe-response experiment. Our results in both Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12 agree with long-standing predictions of BEC criticality. The main observation of this work is the emergence of a new magneto-electric phenomenon in the dielectric response of Cs2Cu2Mo3O12: the relaxation of electric dipoles mediated by quantum critical magnons. Complex capacitance measurements in the millikelvin regime reveal a distinct dissipative feature, with its temperature-dependent amplitude due to low-energy lattice excitations and an activation behavior of the relaxation time. The activation energy softens the field-tuned magnetic quantum critical point (H = H_c) and follows the single-magnon Zeeman energy for H > H_c, showing its magnetic origin. These measurements firmly prove the electrical activity of coupled low-energy spin and lattice excitations, an example of quantum multiferroic behavior. Dipolar relaxation is also found in Rb2Cu2Mo3O12, albeit mediated by optical phonons. This hints at the origin of the electric dipoles being specific vacancies in both materials, however, a microscopic model for the coupling to magnetic fluctuations is lacking. On a different note, this work also explores the magnetism of the highly frustrated, 2D kagome rare-earth magnet Nd3BWO9. The lack of intrinsic disorder in single crystals and the strong antiferromagnetic interactions without long-range order qualifies it as a prime candidate for realizing the long-sought quantum spin liquid. Yet our experimental investigation reveals a completely different scenario. A study of the crystal electric field shows that the low-energy physics can be described in terms of j_eff = 1/2 spins, with significant Ising anisotropy. Thermodynamic and magnetic measurements are combined to map out the magnetic phase diagram, finding several domes of long-range order below 300 mK. Strikingly, fractional magnetization plateaux are found for all field geometries, with different structures for each. Neutron diffraction elucidates the magnetic structures, revealing complex spin-density wave behavior. All the results highlight the strong frustration in Nd3BWO9 while showing that it is a network of weakly coupled 1D-spin tubes, perpendicular to the kagome planes.
  • Critical dielectric susceptibility at a magnetic BEC quantum critical point
    Item type: Journal Article
    Hayashida, Shohei; Huberich, Lysander; Flavián Blasco, Daniel; et al. (2021)
    Physical Review Research
    Magnetic-field-induced phase transitions are investigated in the frustrated gapped quantum paramagnet Rb2Cu2Mo3O12 through dielectric and calorimetric measurements on single-crystal samples. It is clarified that the previously reported dielectric anomaly at 8 K in powder samples is not due to a chiral spin liquid state as has been suggested, but rather to a tiny amount of a ferroelectric impurity phase. Two field-induced quantum phase transitions between paraelectric and paramagnetic and ferroelectric and magnetically ordered states are clearly observed. It is shown that the electric polarization is a secondary order parameter at the lower-field (gap closure) quantum critical point but a primary one at the saturation transition. Having clearly identified the magnetic Bose-Einstein condensation (BEC) nature of the latter, we use the dielectric channel to directly measure the critical divergence of BEC susceptibility. The observed power-law behavior is in very good agreement with theoretical expectations for three-dimensional BEC. Finally, dielectric data reveal magnetic presaturation phases in this compound that may feature exotic order with unconventional broken symmetries.
Publications 1 - 7 of 7