arXiv subjects

Arthur P. Ramirez

Publications and source records attributed to Arthur P. Ramirez.

At least 19 recordsLinked to original sources

arXiv2026

Spin correlations, low-energy scales, and anisotropy scaling in kagome frustrated magnets

Neutron scattering is central to identifying quantum states of magnetic materials. In the search for quantum spin liquids, broad spectral features of inelastic spectra have been cited as evidence for spinon excitations, but can also arise from magnon excitations excitations in the presence of quenched disorder and strong magnon interactions. We develop a new approach to this problem, based on the adiabatic continuity in the $XXZ$ Heisenberg model on geometrically frustrating (GF) lattices as a function of the model's anisotropy. Using this approach, we identify universal features and energies of finite-temperature spin correlators. Focusing on the kagome lattice, we show that the low-energy spin spectral function contains robust, momentum-independent peaks with frequencies: $\omega_1 \approx 3.4 T^*$ and $\omega_2 \approx 6.3 T^*$, where the ``hidden energy scale'' $T^*$ is the characteristic scale of a low-temperature peak in the heat capacity, at which many GF magnets also display spin-glass freezing. We show that the spectral features at low energies $\omega\lesssim T^*$ arise from single-magnon scattering and identify the magnetizations of the respective excitations. We explore the evolution of the spectral features with temperature and discuss extensions to other GF lattices. Our results provide a sharp spectroscopic criterion for interpreting neutron scattering in kagome and other GF quantum magnets.

cond-mat.str-el↗

arXiv2026

A footprint of zero-point entropy in higher-temperature magnetic thermodynamics

Identifying extensively degenerate zero-temperature states is key in characterizing spin-liquid-candidate materials and spin ices. In experiments, finding zero-point entropy (ZPE) is often attempted by measuring the entropy released by a material when cooled down from very high to very low temperatures. Such investigations are often unreliable and lead to controversial results because accessible temperatures may be insufficient to accurately capture essential low- and high-temperature features of magnetic materials. The purpose of this paper is to point out a simple, easily accessible signature of nonzero ZPE: the Maxwell's relation $\left(\partial S/\partial H\right)_T = \left(\partial M/\partial T\right)_H$ can appear violated if a vanishing ZPE is assumed incorrectly. This relation can further be used for estimating the ZPE. In many materials below characteristic temperatures, the criterion of non-vanishing ZPE has a particularly simple form: $\left(\frac{\partial C}{\partial H}\right)_T\left(\frac{\partial M}{\partial T}\right)_H<0$. We discuss these effects and the ZPE signature in the benchmark test case of the well-studied spin ice $Dy_2Ti_2O_7$.

cond-mat.mes-hall↗

arXiv2025

Low-temperature entropies and possible states in geometrically frustrated magnets

The entropy that an insulating magnetic material releases upon cooling can reveal important information about the properties of spin states in that material. In many geometrically frustrated (GF) magnetic compounds, the heat capacity exhibits a low-temperature peak that comes from the spin states continuously connected to the ground states of classical models, such as the Ising model, on the same GF lattice, which manifests in the amount of entropy associated with this heat-capacity peak. In this work, we simulate numerically the values of entropy released by higher-spin triangular-lattice layered systems and materials on SCGO lattices. We also compare the experimentally measured values of entropy in several strongly GF compounds, $NiGa_2S_4$, $FeAl_2Se_4$ and SCGO/BSZCGO, with possible theoretical values inferred from the classical models to which the quantum states of those materials may be connected. This comparison suggests that the lowest-energy states of higher-spin layered triangular-lattice compounds can be described in terms of doublet states on individual magnetic sites. Our analyses demonstrate how the values of entropy can reveal the structure of low-energy magnetic states in GF compounds and call for more accurate thermodynamic measurement in GF magnetic materials.

cond-mat.str-el↗

arXiv2024

Short-range order and hidden energy scale in geometrically frustrated magnets

In geometrically frustrated (GF) magnets, conventional long-range order is suppressed due to the presence of primitive triangular structural units, and the nature of the ensuing ground state remains elusive. One class of candidate states, extensively sought in experiments and vigorously studied theoretically, is the quantum spin liquid (QSL), a magnetically-disordered state in which all spins participate in a quantum-coherent many-body state. Randomly located impurities, present in all materials, may prevent QSL formation and instead lead to the formation of a spin-glass state. In this article, we review available data on the specific heat, magnetic susceptibility, and neutron scattering in GF materials. Such data show that a pure GF magnet possesses a characteristic ``hidden energy scale'' significantly exceeded by the other microscopic energy scales in the material. When cooled down to a temperature below the hidden energy scale, a GF material develops significant short-range order that dominates its properties and, in particular, dictates the spin-glass transition temperature for experimentally accessible impurity densities. We review the manifestations of short-range order in the commonly observed thermodynamics quantities in GF materials, possible scenarios for the hidden energy scale, and related open questions.

cond-mat.str-el↗

arXiv2024

Origin of the hidden energy scale and the $f$-ratio in geometrically frustrated magnets

Sufficiently clean geometrically frustrated (GF) magnets are the largest class of candidate materials that may host quantum spin liquids (QSLs). Some of them have been shown to exhibit spin-glass freezing, potentially precluding QSLs, at the "hidden energy scale", which is significantly lower than the microscopic energy scale of spin interactions. Here, we investigate the origin of the hidden energy scale and its relationship to the $f$-ratio, the figure of merit for the degree of frustration in GF magnetic materials. The available experimental and numerical data provide evidence that GF magnets display, universally, two distinct temperature scales in the specific heat, the lowest of which is of the order of the hidden energy scale $T^*$. We argue that this scale is determined by non-magnetic excitations, similar to spin exchanges in chains of spins. The collective entropy of such excitations matches the entropy of the ground states of the Ising model on the same lattice, which provides a way to verify the proposed scenario in experiment. We demonstrate that in the presence of quenched disorder, a broad class of materials exhibits spin-glass freezing at temperatures of order $T^*$, in accordance with experimental observations. As $T^*$ is a property of the clean GF medium, it leads to a constraint on the $f$-ratio.

cond-mat.str-el↗

arXiv2024

Quasispins of vacancy defects and their interactions in disordered antiferromagnets

Vacancy defects in disordered magnetic materials are known to act as effective spins, ``quasispins'', in response to an external magnetic field. In the dilute limit, the contributions of such ``quasispins'' to the magnetic susceptibility $\chi_\text{vac}(T)\propto N_\text{vac}/T$ are singular in the limit of low temperatures $T$ and match those of free spins. With increasing the density of vacancies, their interactions may become essential. Motivated by frustrated and quasi-one-dimensional magnetic materials, we study analytically quasispins and their interactions in a generic system that has short-range antiferromagnetic order and lacks long-range order. We predict that if the vacancy defect does not disrupt the short-range antiferromagnetic order around it, the quasispin value matches the value of spins of the magnetic atoms in the material, and the correlators of the quasispins of different vacancies match the spin-spin correlators in the vacancy-free material. We confirm our conclusions by exact calculations for Ising chains with nearest-neighbour and next-to-nearest-neighbour interactions. We also compute the first virial correction to the susceptibility of a magnetic material due to the interactions of vacancy quasispins.

cond-mat.dis-nn↗

arXiv2023

Quasispins of vacancy defects in Ising chains with nearest- and next-to-nearest-neighbour interactions

Motivated by frustrated magnets and quasi-one-dimensional magnetic materials, we study the magnetic properties of 1D Ising chains with nearest-neighbour (NN) and weaker next-to-nearest neighbour (NNN) interactions in the presence of vacancy defects. The effect of a vacancy on the magnetic susceptibility of a spin chain is two-fold: it reduces the length of the chain by an effective ``vacancy size'' and may also act as a free spin, a ``quasispin'', with a Curie-type $\chi_\text{quasi}=\langle S^2\rangle/T$ contribution to the susceptibility. In chains with antiferromagnetic short-range order, the susceptibility of vacancy-free chains is exponentially suppressed at low temperatures, and quasispins dominate the effect of impurities on the chains' magnetic properties. For chains with antiferromagnetic NN interactions, the quasispin matches the value $\langle S^2\rangle=1$ of the Ising spins in the chain for ferromagnetic NNN interactions and vanishes for antiferromagnetic NNN interactions. For chains with ferromagnetic short-range order, quasispin effects are insignificant due to exponentially large low-temperature susceptibilities, and the dominant effect of a vacancy is effectively changing the length of the chain.

cond-mat.mtrl-sci↗

arXiv2020

Extremely Weakly Interacting $ΔS_z = 0$ and $ΔS_z = 1$ Excitations and Evidence for Fractional Quantization in a Magnetization Plateau: CeSb

The plateau at 1/3 of the saturation magnetization, $M_s$, in the metamagnet CeSb is accompanied by a state of ferromagnetic layers of spins in an up-up-down sequence. We measured $M$ and the specific heat, $C$, in the plateau, spin wave analyses of which reveal two distinct branches of excitations. Those with $ΔS_z = 1$ as measured by $M$, coexist with a much larger population of $ΔS_z = 0$ excitations measured by $C$ but invisible to $M$. The large density of $ΔS_z = 0$ excitations, their energy gap, and their seeming lack of interaction with $ΔS_z = 1$ excitations suggest an analogy with astrophysical dark matter. Additionally, in the middle of the plateau three sharp jumps in $M(H)$ are seen, the size of which, $0.15 $%$M_s$, is consistent with fractional quantization of magnetization-per-site in the down-spin layers.

cond-mat.str-el↗

arXiv2020

A Néel-type antiferromagnetic order in the spin 1/2 rare-earth honeycomb YbCl$_3$

Most of the searches for Kitaev materials deal with $4d/5d$ magnets with spin-orbit-coupled ${J=1/2}$ local moments such as iridates and $α$-RuCl$_3$. Here we propose the monoclinic YbCl$_3$ with a Yb$^{3+}$ honeycomb lattice for the exploration of Kiteav physics. We perform thermodynamic, $ac$ susceptibility, angle-dependent magnetic torque and neutron diffraction measurements on YbCl$_3$ single crystal. We find that the Yb$^{3+}$ ion exhibits a Kramers doublet ground state that gives rise to an effective spin ${J_{\text{eff}}=1/2}$ local moment. The compound exhibits short-range magnetic order below 1.20 K, followed by a long-range Néel-type antiferromagnetic order at 0.60 K, below which the ordered Yb$^{3+}$ spins lie in the $ac$ plane with an angle of 16(11)$^{\circ}$ away from the $a$ axis. These orders can be suppressed by in-plane and out-of-plane magnetic fields at around 6 and 10 T, respectively. Moreover, the Néel temperature varies non-monotonically under the out-of-plane magnetic fields. The in-plane magnetic anisotropy and the reduced order moment 0.8(1) $μ_B$ at 0.25 K indicate that YbCl$_3$ could be a two-dimensional spin system to proximate the Kitaev physics.

cond-mat.str-el↗

arXiv2019

A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling

Magnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall (QAH) effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~ 0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.

cond-mat.mtrl-sci↗

arXiv2019

Probing the Electronic Structure of Graphene Near and Far from the Fermi Level via Planar Tunneling Spectroscopy

Scanning tunneling spectroscopy (STS) has yielded significant insight on the electronic structure of graphene and other two-dimensional (2D) materials. STS directly measures a fundamental and directly calculable quantity: the single particle density of states (SPDOS). Due to experimental setup limitations, however, STS has been unable to explore 2D materials in ultra-high magnetic fields where electron-electron interactions can drastically change the SPDOS. Recent developments in the assembly of heterostructures composed of graphene and hexagonal boron nitride have enabled a device-based alternative to potentially overcome these roadblocks. Thus far, however, these nascent efforts are incomplete in analyzing and understanding tunneling spectra and have yet to explore graphene at high magnetic fields. Here we report an experiment at magnetic fields up to 18 T that uses graphene tunneling field effect transistors (TFETs) to establish a clear benchmark for measurement and analysis of graphene planar tunneling spectroscopy. We acquire gate tunable tunneling spectra of graphene and then use these data and electrostatic arguments to develop a systematic analysis scheme. This analysis reveals that TFET devices directly probe electronic structure features near and far from the Fermi level. In particular, our study yields identification of the Dirac point and numerous Landau levels as they fill and empty with charge via application of a gate voltage. Our work demonstrates that TFET devices are a viable platform for directly probing the electronic structure of graphene and other 2D materials in high magnetic fields, where novel electronic states emerge.

cond-mat.mes-hall↗

arXiv2019

Mini Volume Collapse as Evidence for a 3-Body Magnetic Polaron in $Sm_{1-x}Eu_xS$

Samarium sulfide (SmS) is a non-magnetic narrow-gap (0.06 eV) semiconductor which undergoes a transition to a metallic intermediate valence state at 6.5 kbar. Europium sulfide (EuS) is a ferromagnetic semiconductor with a Curie temperature of 16K and a gap of 1.6 eV. Here we present a study of the lattice constant, magnetic susceptibility, and resistivity of the substitution series $Sm_{1-x}Eu_xS$ for 0 < x < 1. We observe a smooth interpolation of magnetic and transport behavior across the series, consistent with a virtual crystal scenario and Vegard's law. Surprisingly, however, the lattice constant deviates below Vegard's law in a manner that suggests parametric control of the Sm-Sm distance by the Eu moment in the manner of a magnetic polaron.

cond-mat.mes-hall↗

arXiv2018

Degeneracy of the 1/8 plateau and antiferromagnetic phases in the Shastry-Sutherland magnet TmB4

The 1/8 fractional plateau phase (1/8-FPP) in Shastry-Sutherland Lattice (SSL) spin systems has been viewed an exemplar of emergence on an Archimedean lattice. Here we explore this phase in the Ising magnet TmB4 using high-resolution specific heat (C) and magnetization (M) in the field-temperature plane. We show that the 1/8-FPP is smoothly connected to the antiferromagnetic (AF) phase on ramping the field from H= 0. Thus, the 1/8-FPP is not a distinct ground state of TmB4. The implication of these results for Heisenberg spins on the SSL is discussed.

cond-mat.str-el↗

arXiv2017

Implications of the Measured Angular Anisotropy at the Hidden Order Transition of URu2Si2

The heavy fermion compound URu2Si2 continues to attract great interest due to the long- unidentified nature of the hidden order that develops below 17.5K. Here we discuss the implications of an angular survey of the linear and nonlinear susceptibility of URu2Si2 in the vicinity of the hidden order transition. While the anisotropic nature of spin fluctuations and low-temperature quasiparticles was previously established, our recent results suggest that the order parameter itself has intrinsic Ising anisotropy, and that moreover this anisotropy extends far above the hidden order transition. Consistency checks and subsequent questions for future experimental and theoretical studies of hidden order are discussed.

cond-mat.str-el↗

arXiv2016

Thermodynamic Measurement of Angular Anisotropy at the Hidden Order Transition of URu$_2$Si$_2$

The heavy fermion compound URu$_2$Si$_2$ continues to attract great interest due to the unidentified hidden order it develops below 17.5K. The unique Ising character of the spin fluctuations and low temperature quasiparticles is well established. We present detailed measurements of the angular anisotropy of the nonlinear magnetization that reveal a $\cos^4 θ$ Ising anisotropy both at and above the ordering transition. With Landau theory, we show this implies a strongly Ising character of the itinerant hidden order parameter.

cond-mat.str-el↗

arXiv2016

Removing the spin ice cap: magnetic ground states of rare earth tripod kagome lattice Mg$_2$RE$_3$Sb$_3$O$_{14}$ (RE = Gd, Dy, Er)

We present the structural and magnetic properties of a new compound family, Mg$_2$RE$_3$Sb$_3$O$_{14}$ (RE = Gd, Dy, Er), with a hitherto unstudied frustrating lattice, the "tripod kagome" structure. Susceptibility (ac, dc) and specific heat exhibit features that are understood within a simple Luttinger-Tisza type theory. For RE = Gd, we found long ranged order (LRO) at 1.65 K, which is consistent with a 120 $^{\circ}$ structure, demonstrating the importance of diople interactions for this 2D Heisenberg system. For RE = Dy, LRO at 0.37 K is related to the "kagome spin ice (KSI)" physics for a 2D system. This result shows that the tripod kagome structure accelerates the transition to LRO predicted for the related pyrochlore systems. For RE = Er, two transitions, at 80 mK and 2.1 K are observed, suggesting the importance of quantum fluctuations for this putative XY system.

cond-mat.str-el↗

arXiv2009

Magnetic phase evolution in the spinel compounds Zn$_{1-x}$Co$_x$Cr$_2$O$_4$

We present the magnetic properties of complete solid solutions of ZnCr$_2$O$_4$ and CoCr$_2$O$_4$: two well-studied oxide spinels with very different magnetic ground states. ZnCr$_2$O$_4$, with non-magnetic $d^{10}$ cations occupying the A site and magnetic $d^3$ cations on the B site, is a highly frustrated antiferromagnet. CoCr$_2$O$_4$, with magnetic $d^7$ cations (three unpaired electrons) on the A site as well, exhibits both Néel ferrimagnetism as well as commensurate and incommensurate non-collinear magnetic order. More recently, CoCr$_2$O$_4$ has been studied extensively for its polar behavior which arises from conical magnetic ordering. Gradually introducing magnetism on the A site of ZnCr$_2$O$_4$ results in a transition from frustrated antiferromagnetism to glassy magnetism at low concentrations of Co, and eventually to ferrimagnetic and conical ground states at higher concentrations. Real-space Monte-Carlo simulations of the magnetic susceptibility suggest that the first magnetic ordering transition and features of the susceptibility across $x$ are captured by near-neighbor self- and cross-couplings between the magnetic A and B atoms. We present as a part of this study, a method for displaying the temperature dependence of magnetic susceptibility in a manner which helps distinguish between compounds possessing purely antiferromagnetic interactions from compounds where other kinds of ordering are present.

cond-mat.mtrl-sci↗

arXiv2009

Large Low Temperature Specific Heat in Pyrochlore Bi$_2$Ti$_2$O$_7$

Both amorphous and crystalline materials frequently exhibit low temperature specific heats in excess of what is predicted using the Debye model. The signature of this excess specific heat is a peak observed in $C/T^3$ \textit{versus} $T$. To understand the curious absence of long-range ordering of local distortions in the crystal structure of pyrochlore Bi$_2$Ti$_2$O$_7$, we have measured the specific heat of crystalline Bi$_2$Ti$_2$O$_7$ and related compounds. We find that the peak in $C/T^3$ versus $T$ in Bi$_2$Ti$_2$O$_7$ falls at a substantially lower temperature than other similar compounds, consistent with the presence of disorder. This thermodynamic evidence for disorder in crystalline Bi$_2$Ti$_2$O$_7$ is consistent with quenched configurational disorder among Bi lone pairs produced by geometrical frustration, which could represent a possible realization of "charge ice".

cond-mat.mtrl-sci↗