arXiv subjects

Sara Haravifard

Publications and source records attributed to Sara Haravifard.

18 recordsLinked to original sources

arXiv2026

Observation of a gapped phase in the one-dimensional $S = {\frac{1}{2}}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr

Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C$_6$H$_8$N$_2$ = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^1$H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation ($μ$SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu$^{2+}$ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu$^{2+}$ spins as reflected by a Curie-Weiss temperature of about $-9$ K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the $^1$H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field $μ$SR evidences lack of any static magnetism.

cond-mat.str-el↗

arXiv2026

Scaling Breakdown as a Signature of Spinon-Gauge Interaction in the Quantum Spin Liquid YbZn$_2$GaO$_5$

Scaling behavior in magnetization has been reported in a wide range of quantum spin liquid (QSL) candidates and is often interpreted as evidence for scale-free spin liquid physics. Here we present a comprehensive scaling analysis of high-field magnetization measurements on the QSL material YbZn$_2$GaO$_5$. Between 5 K and 70 K, $M(H)$ displays scale invariance resembling that of a zero-field quantum critical point. Below 3 K, we observe a breakdown of this scale invariance that cannot be recovered by simply changing the critical exponents. This temperature coincides with the onset of enhanced spin correlations observed in $μ$SR measurements. Moreover, the form of the deviation from scaling is consistent with collective spinon excitations coupled via emergent gauge interactions. These results indicate that the breakdown of scaling reflects the emergence of intrinsic low-energy excitations upon entering the QSL regime. Our work clarifies that magnetic scaling is associated with quantum critical fluctuations rather than with the spin liquid phase itself, and establishes magnetization scaling as a sensitive thermodynamic probe of emergent energy scales in QSL systems.

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arXiv2026

Weyl Magnons in the Non-Coplanar Antiferromagnet MnTe$_2$

Using a combination of band representation analysis, inelastic neutron scattering (INS), magneto-Raman spectroscopy measurements, and linear spin wave theory, we establish that the non-coplanar antiferromagnet MnTe$_2$ is a tunable Weyl magnon material, hosting symmetry-protected topological nodal lines in its magnon band structure, protected by the the non-coplanar nature of the antiferromagnetic ordering, that transition into Weyl magnons upon the application of symmetry-breaking perturbations using an external magnetic field. By constructing a spin model that reproduces the observed INS magnon spectra and field-dependence of the Raman $Γ$-magnons, we directly probe the topological magnon nodal lines and observe their associated signature of non-trivial topology through the pseudo-spin winding of the scattering intensity in angular scans near the nodal lines. Finally, we discuss how to induce Weyl magnons in the spectrum through an external magnetic field, shedding light on future in-field INS and thermal Hall experiments. This work establishes a clear magnonic analog to Weyl electrons, enabling further exploration of topological behavior in bosonic systems and highlighting the interplay between magnetic order and band topology in non-coplanar antiferromagnets.

cond-mat.mtrl-sci↗

arXiv2025

Observation of Unprecedented Fractional Magnetization Plateaus in a New Shastry-Sutherland Ising Compound

Geometrically frustrated magnetic systems, such as those based on the Shastry-Sutherland lattice (SSL), offer a rich playground for exploring unconventional magnetic states. The delicate balance between competing interactions in these systems leads to the emergence of novel phases. We present the characterization of Er2Be2GeO7, an SSL compound with Er3+ ions forming orthogonal dimers separated by non-magnetic layers whose structure is invariant under the P-421m space group. Neutron scattering reveals an antiferromagnetic dimer structure at zero field, typical of Ising spins on that lattice and consistent with the anisotropic magnetization observed. However, magnetization measurements exhibit fractional plateaus at 1/4 and 1/2 of saturation, in contrast to the expected 1/3 plateau of the SSL Ising model. By comparing the energy of candidate states with ground-state lower bounds we show that this behavior requires spatially anisotropic interactions, leading to an anisotropic Shastry-Sutherland Ising Model (ASSLIM) symmetric under the Cmm2 space group. This anisotropy is consistent with the small orthorhombic distortion observed with single-crystal neutron diffraction. The other properties, including thermodynamics, which have been investigated theoretically using tensor networks, point to small residual interactions, potentially due to further couplings and quantum fluctuations. This study highlights Er2Be2GeO7 as a promising platform for investigating exotic magnetic phenomena.

cond-mat.str-el↗

arXiv2025

Emergent Hidden Multipolar State in the Triangular Lattice Magnet TmZn2GaO5

TmZn2GaO5 is a newly synthesized triangular lattice magnet that exhibits a unique quantum phase characterized by strong Ising anisotropy, a pseudo-doublet crystal electric field ground state, and a low-energy gapped excitation at the K point. Unlike its well-known counterparts, TmMgGaO4 and YbMgGaO4, this material crystallizes in a distinct hexagonal structure, leading to a cleaner platform for investigating frustrated magnetism. Magnetic susceptibility, heat capacity, and inelastic neutron scattering measurements confirm the absence of long-range magnetic order down to 50 mK, placing TmZn2GaO5 in a distinct region of the transverse-field Ising model phase diagram. Theoretical calculations based on spin-wave theory and mean-field modeling reproduce key experimental observations, reinforcing the material's placement in a quantum disordered/multipolar state. These results highlight its potential for exploring quantum disordered states, anisotropic excitations, and exotic quantum phases in frustrated spin systems.

cond-mat.str-el↗

arXiv2024

Evidence of Dirac Quantum Spin Liquid in YbZn2GaO5

The emergence of a quantum spin liquid (QSL), a state of matter that can result when electron spins are highly correlated but do not become ordered, has been the subject of a considerable body of research in condensed matter physics. Spin liquid states have been proposed as hosts for high-temperature superconductivity and can host topological properties with potential applications in quantum information science. The excitations of most quantum spin liquids are not conventional spin waves but rather quasiparticles known as spinons, whose existence is well established experimentally only in one-dimensional systems; the unambiguous experimental realization of QSL behavior in higher dimensions remains challenging. Here, we investigate the novel compound YbZn2GaO5, which hosts an ideal triangular lattice of effective spin-1/2 moments with no detectable inherent chemical disorder. Thermodynamic and inelastic neutron scattering measurements performed on high-quality single crystal samples of YbZn2GaO5 exclude the possibility of long-range magnetic ordering down to 0.06 K, demonstrate a quadratic power law for the specific heat and reveal a continuum of magnetic excitations in parts of the Brillouin zone. Both low-temperature thermodynamics and inelastic neutron scattering spectra suggest that YbZn2GaO5 is a U(1) Dirac QSL with spinon excitations concentrated at certain points in the Brillouin zone. We advanced these results by performing additional specific heat measurements under finite fields, further confirming the theoretical expectations for a Dirac QSL on the triangular lattice of YbZn2GaO5.

cond-mat.str-el↗

arXiv2024

Synthesis and characterization of the novel breathing pyrochlore compound Ba3Tm2Zn5O11

In this study, a novel material from the rare-earth based breathing pyrochlore family, Ba3Tm2Zn5O11, was successfully synthesized. Powder x-ray diffraction and high-resolution powder neutron diffraction confirmed phase purity and the F-43m breathing pyrochlore crystal structure, while thermogravimetric analysis revealed incongruent melting behavior compared to its counterpart, Ba3Yb2Zn5O11. High-quality single crystals of Ba3Tm2Zn5O11 were grown using the traveling solvent floating zone technique and assessed using Laue x-ray diffraction and single crystal x-ray diffraction. Thermodynamic characterization indicated paramagnetic behavior down to 0.05 K, and inelastic neutron scattering measurements identified distinct dispersionless crystal electric field energy bands, with the fitted crystal electric field model predicting a single-ion singlet ground state and an energy gap of ~9 meV separating it from the first excited (singlet) state. Additional low-energy excitation studies on single crystals revealed dispersionless bands at 0.8 and 1 meV. Computed phonon dispersions from first-principles calculations ruled out phonons as the origin of these modes, further illustrating the puzzling and unique properties of Ba3Tm2Zn5O11.

cond-mat.str-el↗

arXiv2024

High-throughput search for topological magnon materials

Topological magnons give rise to possibilities for engineering novel spintronics devices with critical applications in quantum information and computation, due to its symmetry-protected robustness and low dissipation. However, to make reliable and systematic predictions about material realization of topological magnons has been a major challenge, due to the lack of neutron scattering data for most materials. In this work, we significantly advance the symmetry-based approach for identifying topological magnons through developing a fully automated algorithm, utilizing the theory of symmetry indicators, that enables a highly efficient and large-scale search for candidate materials hosting field-induced topological magnons. This progress not only streamlines the discovery process but also expands the scope of materials exploration beyond previous manual or traditional methods, offering a powerful tool for uncovering novel topological phases in magnetic systems. Performing a large-scale search over all 1649 magnetic materials in Bilbao Crystallographic Server with a commensurate magnetic order, we discover 387 candidate materials for topological magnons, significantly expanding the pool of topological magnon materials. We further discuss examples and experimental accessibility of the candidate materials, shedding light on future experimental realizations of topological magnons in magnetic materials.

cond-mat.mtrl-sci↗

arXiv2024

Experimental evidence of crystal-field, Zeeman splitting, and spin-phonon excitations in the quantum supersolid Na2BaCo(PO4)2

Drawing inspiration from the recent breakthroughs in the \ce{Na_{2}BaCo(PO_{4})_{2}} quantum magnet, renowned for its spin supersolidity phase and its potential for revolutionary cooling applications, our study delves into the intricate interplay among lattice, spin, and orbital degrees of freedom within this intriguing compound. Using meticulous temperature, field, and pressure-dependent Raman scattering techniques, we present compelling experimental evidence revealing pronounced crystal-electric field (CEF) excitations, alongside the interplay of CEF-phonon interactions. Notably, our experiments elucidate all electronic transitions from $j_{1 / 2}$ to $j_{3 / 2}$ and from $j_{1 / 2}$ to $j_{5 / 2}$, with energy level patterns closely aligned with theoretical predictions based on point-charge models. Furthermore, the application of a magnetic field and pressure reveals Zeeman splittings characterized by Landé-g factors as well as the CEF-phonon resonances. The anomalous shift in coupled peak at low temperatures originates from the hybridization of CEF and phonon excitations due to their close energy proximity. These findings constitute a significant step towards unraveling the fundamental properties of this exotic quantum material for future research in fundamental physics or engineering application.

cond-mat.mtrl-sci↗

arXiv2023

Realization of two-sublattice exchange physics in the triangular lattice compound Ba3Er(BO3)3

We have synthesized high-quality single crystal samples of the erbium-based triangular lattice compound Ba3Er(BO3)3. Thermal and magnetic measurements reveal large anisotropy and a possible phase transition at 100 mK. The low-temperature magnetic heat capacity can be understood from the two Wyckoff positions that the magnetic ions Er3+ occupy, which have distinct symmetry properties and crystal field environments. A point charge calculation for the crystal electric field levels is consistent with this understanding. Based on symmetry analysis and classical simulation, we argue that Ba3Er(BO3)3 realizes an interesting two-sublattice exchange physics, in which the honeycomb lattice spins develop ferromagnetic correlations due to the additional spins at the hexagon centers but eventually order antiferromagnetically. Additionally, our results suggest that quantum fluctuations need to be considered in order to fully explain the experimental observations.

cond-mat.str-el↗

arXiv2022

Beyond Single Tetrahedron Physics of Breathing Pyrochlore Compound Ba3Yb2Zn5O11

Recently a new class of quantum magnets, the so-called breathing pyrochlore spin systems, have attracted much attention due to their potential to host exotic emergent phenomena. Here, we present magnetometry, heat capacity, thermal conductivity, Muon-spin relaxation, and polarized inelastic neutron scattering measurements performed on high-quality single-crystal samples of breathing pyrochlore compound Ba3Yb2Zn5O11. We interpret these results using a simplified toy model and provide a new insight into the low-energy physics of this system beyond the single-tetrahedron physics proposed previously.

cond-mat.str-el↗

arXiv2021

Realization of Quantum Dipoles in Triangular Lattice Crystal Ba3Yb(BO3)3

We investigate the thermodynamic properties of the ytterbium-based triangular lattice compound Ba3Yb(BO3)3. The results demonstrate the absence of any long-range ordering down to 56 mK. Analysis of the magnetization, susceptibility and specific heat measurements suggests that Ba3Yb(BO3)3 may realize a S = 1/2 quantum dipole lattice, in which the dominant interaction is the long range dipole-dipole coupling on the geometrically frustrated triangular lattice, and exchange interactions are subdominant or negligible.

cond-mat.str-el↗

arXiv2021

Towards understanding the magnetic properties of the breathing pyrochlore compound Ba3Yb2Zn5O11: A single crystal study

Ba3Yb2Zn5O11 is unique among breathing pyrochlore compounds for being in the nearly decoupled limit where inter-tetrahedron interactions are weak, hosting isolated clusters or "molecular magnet" like tetrahedra of magnetic ytterbium (Yb3+) ions. In this work, we present the first study carried out on single-crystal samples of the breathing pyrochlore Ba3Yb2Zn5O11, using a variety of magnetometry and neutron scattering techniques along with theoretical modeling. We employ inelastic neutron scattering to investigate the magnetic dynamics as a function of applied field (with respect to both magnitude and direction) down to a temperature of 70 mK, where inelastic scattering reveals dispersionless bands of excitations as found in earlier powder sample studies, in good agreement with a single-tetrahedron model. However, diffuse neutron scattering at zero field and dc-susceptibility at finite field exhibit features suggesting the presence of excitations at low-energy that are not captured by the single tetrahedron model. Analysis of the local structure down to 2 K via pair distribution function analysis finds no evidence of structural disorder. We conclude that effects beyond the single tetrahedron model are important in describing the low-energy, low temperature physics of Ba3Yb2Zn5O11, but their nature remains undetermined.

cond-mat.str-el↗

arXiv2021

Phase diagram of the Shastry-Sutherland Compound SrCu2(BO3)2 under extreme combined conditions of field and pressure

Motivated by the intriguing properties of the Shastry-Sutherland compound SrCu2(BO3)2 under pressure, with a still debated intermediate plaquette phase appearing at around 20 kbar and a possible deconfined critical point at higher pressure upon entering the antiferromagnetic phase, we have investigated its high-field properties in this pressure range using tunnel diode oscillator (TDO) measurements. The two main new phases revealed by these measurements are fully consistent with those identified by infinite Projected Entangled Pair states (iPEPS) calculations of the Shastry-Sutherland model, a 1/5 plateau and a 10 x 2 supersolid. Remarkably, these phases are descendants of the full-plaquette phase, the prominent candidate for the intermediate phase of SrCu2(BO3)2. The emerging picture for SrCu2(BO3)2 is shown to be that of a system dominated by a tendency to an orthorhombic distortion at intermediate pressure, an important constraint on any realistic description of the transition into the antiferromagnetic phase.

cond-mat.str-el↗

arXiv2021

Phase Diagram of YbZnGaO4 in Applied Magnetic Field

Recently, Yb-based triangular lattice antiferromagnets have garnered significant interest as possible quantum spin liquid candidates. One example is YbMgGaO4, which showed many promising spin liquid features, but also possesses a high degree of disorder owing to site-mixing between the non-magnetic cations. To further elucidate the role of chemical disorder and to explore the phase diagram of these materials in applied field, we present neutron scattering and sensitive magnetometry measurements of the closely related compound, YbZnGaO4. Our results suggest a difference in magnetic anisotropy between the two compounds, and we use key observations of the magnetic phase crossover to motivate an exploration of the field- and exchange parameter-dependent phase diagram, providing an expanded view of the available magnetic states in applied field. This enriched map of the phase space serves as a basis to restrict the values of parameters describing the magnetic Hamiltonian with broad application to recently discovered related materials.

cond-mat.str-el↗

arXiv2020

Incommensurate two-dimensional checkerboard charge density wave in the low dimensional superconductor Ta4Pd3Te16

We report the observation of a two-dimensional (2D) checkerboard charge density wave (CDW) in the low-dimensional superconductor Ta4Pd3Te16. By determining its CDW properties across the temperature-pressure (T-P) phase diagram and comparing with prototypical CDW materials, we conclude that Ta4Pd3Te16 features: a) an incommensurate CDW with a mixed character of dimensions (Q1D considering its needle-like shape along the b-axis, Q2D as the CDW has checkerboard wavevectors, and 3D because of CDW projections along all three axes); and b) one of the weakest CDWs compared to its superconductivity (SC), i.e. enhanced SC with respect to CDW, suggesting an interesting interplay of the two orders.

cond-mat.str-el↗

arXiv2020

Constraining the Parameter Space of a Quantum Spin Liquid Candidate in Applied Field with Iterative Optimization

The quantum spin liquid (QSL) state is an exotic state of matter featuring a high degree of entanglement and lack of long-range magnetic order in the zero-temperature limit. The triangular antiferromagnet YbMgGaO4 is a candidate QSL host, and precise determination of the Hamiltonian parameters is critical to understanding the nature of the possible ground states. However, the presence of chemical disorder has made directly measuring these parameters challenging. Here we report neutron scattering and magnetic susceptibility measurements covering a broad range of applied magnetic field at low temperature. Our data shows a field-induced crossover in YbMgGaO4, which we reproduce with complementary classical Monte Carlo and Density Matrix Renormalization Group simulations. Neutron scattering data above and below the crossover reveal a shift in scattering intensity from M to K points and, collectively, our measurements provide essential characteristics of the phase crossover that we employ to strictly constrain proposed magnetic Hamiltonian parameters despite the chemical disorder. Constrained exchange parameters further suggest the material's proximity to the QSL state in the clean limit. More broadly, our approach demonstrates a means of pursuing QSL candidates where Hamiltonian parameters might otherwise be obscured by disorder.

cond-mat.str-el↗

arXiv2018

Emergent Bound States and Impurity Pairs in Chemically Doped Shastry-Sutherland System

The search for novel unconventional superconductors is a central topic of modern condensed matter physics. Similar to other Mott insulators, Shastry-Sutherland (SSL) systems are predicted to become superconducting when chemically doped. This makes SrCu2(BO3)2, an experimental realization of SSL model, a suitable candidate and understanding of the doping effects in it very important. Here we report doping-induced emergent states in Mg-doped SrCu2(BO3)2, which remain stable up to high magnetic fields. Using four complementary magnetometry techniques and theoretical simulations, a rich impurity-induced phenomenology at high fields is discovered. The results demonstrate a rare example in which even a small doping concentration interacts strongly with both triplets and bound states of triplets, and thus plays a significant role in the magnetization process even at high magnetic fields. Moreover, our findings of the emergence of the very stable impurity pairs provide insights into the anticipated unconventional superconductivity in SrCu2(BO3)2 and related materials.

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