arXiv subjects

B. S. Shivaram

Publications and source records attributed to B. S. Shivaram.

At least 19 recordsLinked to original sources

arXiv2026

Geometric Analysis of Magnetic Labyrinthine Stripe Evolution via Deep Learning Segmentation

Labyrinthine stripe patterns are common in many physical systems, yet their lack of long-range order makes quantitative characterization challenging. We investigate the evolution of such patterns in bismuth-doped yttrium iron garnet (Bi:YIG) films subjected to a magnetic field annealing protocol. A U-Net deep learning model, trained with synthetic degradations including additive white Gaussian and Simplex noise, enables robust segmentation of experimental magneto-optical images despite noise and occlusions. Building on this segmentation, we develop a geometric analysis pipeline based on skeletonization, graph mapping, and spline fitting, which quantifies local stripe propagation through length and curvature measurements. Applying this framework to 444 images from 12 annealing protocol trials, we analyze the transition from the "quenched" state to a more parallel and coherent "annealed" state, and identify two distinct evolution modes (Type A and Type B) linked to field polarity. Our results provide a quantitative analysis of geometric and topological properties in magnetic stripe patterns and offer new insights into their local structural evolution, and establish a general tool for analyzing complex labyrinthine systems.

cond-mat.mtrl-sci↗

arXiv2026

Disorder-induced spin-cluster magnetism in a doped kagome spin liquid candidate

The search for new quantum spin liquid materials relies on systems with strong frustration such as spins on an ideal kagome lattice. However, lattice imperfections can have substantial effects which are as yet not well understood. In recent work, the two-dimensional kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] has emerged as a leading candidate hosting a Dirac spin liquid which appears to survive at least for x<0.4, associated with alternating-bond hexagon (ABH) disorder. Here in magnetic samples with x=0.58, y=0.1 we report unusual in-plane ferromagnetic canting (FM) of the in-plane antiferromagnet (AFM), with an unusually wide regime of short-ranged order, and propose theoretical models to explain this behavior. First, we show that Kitaev type exchanges naturally arise on the kagome lattice to second order in the known Dzyaloshinskii-Moriya exchanges, and that these interactions can produce the unusual in-plane FM canting from antichiral AFM. Second, we propose a phenomenological model of weakly-FM-canted spin clusters to describe the short-ranged regime and analyze quantum fluctuations in an ABH toy model to show how ABH disorder can stabilize this regime. The combination of experimental observation and theory suggests that kagome-Kitaev interactions and ABH disorder are necessary for describing the magnetic fluctuations in this family of materials, with potential implications for the proposed proximate spin liquid phase.

cond-mat.str-el↗

arXiv2026

Coarsening dynamics of fingerprint labyrinthine patterns: Machine learning assisted characterization

Fingerprint labyrinthine patterns exhibit a level of structural complexity beyond simple stripe phases, combining local stripe order with a dense network of point-like defects. Unlike symmetry-breaking phases, where coarsening proceeds via diffusive defect annihilation, or conventional stripe phases, where curvature-driven motion of extended grain boundaries dominates, the coarsening of fingerprint labyrinths is governed primarily by localized junction and terminal defects. Using the Turing-Swift-Hohenberg equation, we study the nonequilibrium relaxation of fingerprint labyrinthine patterns following a quench. To go beyond conventional Fourier-based diagnostics, we employ a template-matching convolutional neural network (TM-CNN) to identify and track junctions and terminals directly in real space, enabling a quantitative characterization of defect statistics and spatial correlations. We show that, although these point-like defects drive coarsening, their motion is strongly constrained by the surrounding stripe geometry, leading to slow, nondiffusive dynamics that are qualitatively distinct from both conventional phase ordering and stripe coarsening. Together, these results establish defect-mediated dynamics as the central organizing principle of fingerprint labyrinthine coarsening and demonstrate the effectiveness of machine-learning-assisted approaches for complex pattern-forming systems.

cond-mat.soft↗

arXiv2024

Magnetic Nonlinear Response of UPt$_3$: An augmented Landau approach

Several heavy fermion materials, including UPt$_3$, exhibit a rapid but gradual rise in the magnetization at a critical field, without an apparent phase transition at any temperature $T>0$, with the possibility of a first order transition at $T \equiv0$. To model such a quantum phase transition it is most appropriate to develop approaches considering the quantum nature of the spins. Within a fully classical framework, we show that it is sufficient to start from a Landau-type free energy with an added Bragg-Williams entropy term to arrive at a number of key experimental features as seen in UPt$_3$. In particular, we show that correctly arriving at the measured (low-field) higher order susceptibilities necessarily invokes an isobestic (crossing) point at a high field in the magnetization isotherms. We also present a full analysis of the angular dependence of the (low-field) linear and nonlinear susceptibilities which when extended also capture the anisotropic high field response of the magnetization. Key to this success is the proper conversion of the evaluated magnetization from constant volume to a constant pressure situation relevant at high fields in heavy fermion materials.

cond-mat.str-el↗

arXiv2024

Machine Learning Assisted Characterization of Labyrinthine Pattern Transitions

We present a comprehensive approach to characterizing labyrinthine structures that often emerge as a final steady state in pattern forming systems. We employ advanced machine learning based pattern recognition techniques to identify the types and locations of topological defects of the local stripe ordering. Applying this method to single-crystal Bi-substituted Yttrium Iron Garnet films, we uncover a distinct morphological transition between two zero-field labyrinthine structures. Crucially, the pair distribution functions of the topological defects reveal subtle differences between labyrinthine structures which are beyond conventional characterization methods. By systematically analyzing the spatial correlations and geometric properties of these defects, we provide new insights into the athermal dynamics governing the observed morphological transitions. Our work demonstrates that machine learning based recognition techniques enable novel studies of rich and complex labyrinthine type structures universal to many pattern formation systems.

cond-mat.soft↗

arXiv2024

Characterization of Magnetic Labyrinthine Structures Through Junctions and Terminals Detection Using Template Matching and CNN

Defects influence diverse properties of materials, shaping their structural, mechanical, and electronic characteristics. Among a variety of materials exhibiting unique defects, magnets exhibit diverse nano- to micro-scale defects and have been intensively studied in materials science. Specifically, defects in magnetic labyrinthine patterns, called junctions and terminals are ubiquitous and serve as points of interest. While detecting and characterizing such defects is crucial for understanding magnets, systematically investigating large-scale images containing over a thousand closely packed junctions and terminals remains a formidable challenge. This study introduces a new technique called TM-CNN (Template Matching - Convolutional Neural Network) designed to detect a multitude of small objects in images, such as the defects in magnetic labyrinthine patterns. TM-CNN was used to identify 641,649 such structures in 444 experimental images, and the results were explored to deepen understanding of magnetic materials. It employs a two-stage detection approach combining template matching, used in initial detection, with a convolutional neural network, used to eliminate incorrect identifications. To train a CNN classifier, it is necessary to annotate a large number of training images. This difficulty prevents the use of CNN in many practical applications. TM-CNN significantly reduces the manual workload for creating training images by automatically making most of the annotations and leaving only a small number of corrections to human reviewers. In testing, TM-CNN achieved an impressive F1 score of 0.991, far outperforming traditional template matching and CNN-based object detection algorithms.

cs.CV↗

arXiv2024

Non-Analytic Magnetic Response and Intrinsic Ferromagnetic Clusters in a Dirac Spin Liquid Candidate

Finding distinct signatures of a quantum spin liquid (QSL) is an ongoing quest in condensed matter physics, invariably complicated by the presence of disorder in real materials. In this regard the 2D Kagome system YCu$_3$(OH)$_6$[(Cl$_x$Br$_{(1-x)}$)$_{3-y}$(OH)$_y$] (YCOB-Cl), where the vast mismatch in size of Y and Cu avoids subsitutional disorder, otherwise present in kagome materials, has emerged as a favorable candidate. In crystals of this system, with $x<$ 0.4 and no long range order, we report an unusual field dependent magnetization $M(B)$, where $M/B$ changes linearly with $|B|$, the absolute value of the field, in contrast to the expected quadratic behavior. Model calculations with a distribution of ferromagnetic (FM) clusters faithfully capture observed features suggesting such clusters to be intrinsic to real QSL materials. YCOB-Cl has a field enhanced $T^2$ heat capacity as expected for a Dirac QSL but lacks a linear $T$ behavior in the spin susceptibility. By demonstrating that FM clusters dominate the contribution to the susceptibility but not the heat capacity, our work paves the way towards reconciling the apparent inconsistency with a Dirac QSL.

cond-mat.str-el↗

arXiv2021

Anomalous and Anisotropic Nonlinear Susceptibility in the Proximate Kitaev Magnet $α$-RuCl$_3$

The leading order nonlinear (NL) susceptibility, $χ_3$, in a paramagnet is negative and diverges as $T \rightarrow 0$. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet $α$-RuCl$_3$ which differs from the expected antiferromagnetic behavior. For $T< T_c$ = 7.5 K and field $B$ in the ab-plane, we obtain contrasting NL responses in low ($<$ 2 ${T}$) and high field regions. For low fields the NL behavior is dominated by a quadratic response (positive $χ_2$), which shows a rapid rise below $T_c$. This large $χ_2 >0$ implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard ${K-H-Γ}$ model secure such a quadratic ${B}$ dependence of ${M}$, only for ${T}$ $\approx$ ${T}_c$ with $χ_2$ being zero as ${T}$ $\rightarrow$ 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term ($χ_3$) describes the high field NL behavior well. $χ_3$ is large and positive both below and above ${T}_c$ crossing zero only for ${T}$ $>$ 50 K. In contrast, for $B$~$\parallel$~c-axis, no separate low/high field behaviors is measured and only a much smaller $χ_3$ is apparent.

cond-mat.str-el↗

arXiv2019

Magneto Acoustic Quantum Oscillations in High Fields and the Fermi Surface of UPt$_3$

We report magneto-acoustic quantum oscillations (MAQO) in the heavy fermion system UPt$_3$ in magnetic fields B, upto 33 T. For B in the ab-plane of the hexagonal crystal MAQO in the sound velocity commence at $\approx$ 12 T and grow with field. However, in contrast to typical Lifshitz-Kosevich behaviour the frequency corresponding to the dominant oscillation increases continuously as the metamagnetic transition (MMT) at 20 T is reached. This dominant MAQO arises from the $δ$ orbit of band 1 with a large effective mass of 33 m$_e$, for B $<$ 20 T and disappears after the MMT. Thus, the MMT involves a significant change of the Fermi surface, primarily in band 1. For B $\parallel$ ab-plane and $<$ 20 T we reproduce successfully orbits established through previous de Haas-van Alphen and Shubnikov de Haas measurements. We also observe several new orbits, some that can be identified with existing band theory and others not seen previously with completely new frequencies. For B $\parallel$ c-axis we observe MAQOs which also commence at $\approx$ 12 T and grow gradually but break suddenly into a large amplitude and change in frequency at 24.8 T. These enhanced oscillations get weaker again at 30 T. These abrupt changes at 24.8 T and 30 T are signatures of Lifshitz transitions and coincide with the newly discovered spin density wave states for this orientation reported by us recently.

cond-mat.str-el↗

arXiv2018

Field Angle Tuned Metamagnetism and Lifschitz Transitions in UPt3

Strongly correlated electronic systems can harbor a rich variety of quantum spin states. Understanding and controlling such spin states in quantum materials is of great current interest. Focusing on the simple binary system UPt3 with ultrasound (US) as a probe we identify clear signatures in field sweeps demarkating new high field spin states. Magnetostriction (MS) measurements performed up to 65 T also show signatures at the same fields confirming these state transitions. At the very lowest temperatures (<200 mK) we also observe magneto-acoustic quantum oscillations which for theta = 90° and vicinity abruptly become very strong in the 24.8-30 T range. High resolution magnetization measurements for this same angle reveal a continuous variation of the magnetization implying the subtle nature of the implied transitions. With B rotated away from the c-axis, the US signatures occur at nearly the same field. These state transitions merge with the separate sequence of the well known metamagnetic transition which commences at 20 T for theta = 0° but moves to higher fields as 1/cos(θ). This merge, suggesting a tricritical behavior, occurs at θ~ 51° from the ab-plane. This is an unique off-symmetry angle where the length change is precisely zero due to the anisotropic nature of MS in UPt3 for all magnetic field values.

cond-mat.str-el↗

arXiv2018

An Effective Spin Hamiltonian Approach to Metamagnetism - I

We describe a minimal model, based on a spin only Hamiltonian with a single energy scale for itinerant electron metamagnetism. Within this model the metamagnetic critical field is directly proportional to the temperature where a peak in the linear susceptibility occurs which in turn is related in a simple manner to the temperature where the nonlinear susceptibilities also peak. The spin dependent thermodynamic properties are derived in a straightforward manner and bear a striking resemblance to observations in such strongly correlated systems as heavy fermion materials. We also consider extensions of the model by including effects such as a mean field to encompass observed deviations from a minimal metamagnetic behavior.

cond-mat.str-el↗

arXiv2017

Nonlinear Pauli Susceptibilities in Sr$_3$Ru$_2$O$_7$ and Universal Features of Itinerant Metamagnetism

We report, for the first time, measurements of the third order, $χ_3$ and fifth order, $χ_5$, susceptibilities in an itinerant oxide metamagnet, Sr$_3$Ru$_2$O$_7$ for magnetic fields both parallel and perpendicular to the c-axis. These susceptibilities exhibit maxima in their temperature dependence such that $T_1 \approx 2T_3 \approx 4T_5$ where the $T_i$ are the position in temperature where a peak in the $i$-th order susceptibility occurs. These features taken together with the scaling of the critical field with the temperature $T_1$ observed in a diverse variety of itinerant metamagnets find a natural explanation in a single band model with one Van Hove singularity (VHS) and onsite repulsion $U$. The separation of the VHS from the Fermi energy $Δ$, sets a single energy scale, which is the primary driver for the observed features of itinerant metamagnetism at low temperatures.

cond-mat.str-el↗

arXiv2017

The Linear and Non-linear Magnetic Response of a Tri-Uranium Single Molecule Magnet

We report here low temperature magnetization isotherms for the single molecule magnet, $(UO_2-L)_3$. By analyzing the low temperature magnetization in terms of $M= X_1*B + X_3*B^3$ we extract the linear susceptibility $X_1$ and the leading order nonlinear susceptibility $X_3$. We find that $X_1$ exhibits a peak at a temperature of $T_1=10.4 K$ with $Chi_3$ also exhibiting a peak but at a reduced temperature $T3 = 5 K$. At the lowest temperatures the isotherms exhibit a critical field $B_c = 11.5 T$ marked by a clear point of inflection. A minimal Hamiltonian employing S=1 (pseudo) spins with only a single energy scale (successfully used to model the behavior of bulk f-electron metamagnets) is shown to provide a good description of the observed linear scaling between $T_1, T_3$ and $B_c$. We further show that a Heisenberg Hamiltonian previously employed by Carretta et al. (2013 J. Phys.Cond. Matt. 25 486001) to model this single molecule magnet gives formulas for the angle averaged susceptibilities (in the Ising limit) very similar to those of the minimal model.

cond-mat.other↗

arXiv2015

High Field Ultrasound Measurements in UPt3 and the Single Energy Scale Model of Metamagnetism

We report longitudinal ultrasound velocity measurements for magnetic fields up to 33 T applied parallel to the a-axis of the heavy electron compound UPt$_{3}$. A characteristic dip in the sound velocity at the metamagnetic critical field, $H_{c}=20$ T, reported in earlier work is reproduced and shown to be independent of temperature at very low temperatures. We show that the single energy scale model (B.S. Shivaram et al., Phys. Rev. B89, 241107(R), 2014) captures the observed key features of the field dependence in the sound velocity shift, $δv_{s}$. The shift $δv_{s}$ at $H_{c}$ is found to be inversely dependent on temperature above 3\thinspace K and assumes a fixed value at low T. This saturation in $δv_{s}$ below 3 K is accounted for by level broadening of the Uranium spin states.

cond-mat.str-el↗

arXiv2014

Observation of a Zeeman Induced Lifshitz Transition in URu2Si2

High resolution longitudinal sound velocity measurements in a magnetic field performed at T tending to zero in URu2Si2 reveal a second signature at a field BLT in addition to the step change in velocity expected at the superconducting upper critical field, Bc2. Characteristic Fermi surface related magneto-acoustic quantum oscillations (MAQO) emerge beyond the field BLT at a frequency ~160 T for B||a-axis. Measurements performed with B oriented at various angles between the a and c-axes reveal an anisotropy for BLT nearly identical to that of BC2 suggesting similarity of the electron states involved in both the transitions. Given the observed frequency of ~160 T in the MAQO the transition is most likely related to the emergence of the \ηFermi surface in URu2Si2 (Lifshitz transition).

cond-mat.str-el↗

arXiv2014

Metamagnetism and the Fifth Order Susceptibility in UPt3

An enhanced susceptibility is a natural consequence of the "heavy fermion" (HF) state rendering the possibility of measurably large nonlinear susceptibilities. In recent work a universal behavior of the peaks observed in the linear (\Chi_1) and the third order (\Chi_3) susceptibility in HF metamagnets has been identified. This universality is well accounted for by a single energy scale model considering on-site correlations only. A prediction of this model is a peak in the fifth order susceptibility, \Chi_5, as well. In the first measurements on a HF metamagnet, UPt3 reported herein, we find that Chi_5 rather than attaining a peak, saturates at low temperatures and is positive. The thermodynamic implications of these towards the stability of the metamagnetic HF state are discussed.

cond-mat.str-el↗

arXiv2014

The Superconducting Phases of URu2Si2: Longitudinal Sound Velocity Measurements

High resolution longitudinal sound velocity measurements in a magnetic field performed in the temperature tending to zero limit reveal two distinct signatures attributable to multiple superconducting phases in URu2Si2. A step change in the sound velocity, for propagation in the basal plane, is observed at the critical field, Bc2. This step broadens considerably as T tends to Tc with a concomitant decrease in magnitude. A second step is observed at a field ~0.5 Bc2 and it's magnitude remains constant at all temperatures. Inductive measurements of the transitions in a magnetic field, however, exhibit a single signature which coincides with the lower step with no discernible in-phase signature at the upper transition. Measurements performed with B oriented at various angles between the a and c-axes reveal a weaker angular dependence of the lower step and confirm the rapid fall off of Bc2 close to B||c-axis. An off axis superconducting phase diagram is proposed.

cond-mat.supr-con↗

arXiv2014

Non Linear Susceptibility from High DC Field Torque Magnetometry

Torque magnetometry is a convenient technique to measure the magnetic properties of anisotropic materials. Advances in micromachining and the availability of robust materials with which such magnetometers can be fabricated has made them reliable even in adverse conditions such as very high magnetic fields and both high and very low temperatures. In most applications with such magnetometers the measured torque signals are used to arrive at the linear magnetic susceptibilities only. In this short note we extend torque magnetometry to measure nonlinear susceptibilities and illustrate our methods with representative data on the heavy fermion compound UPt3

cond-mat.str-el↗