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Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu_1/3Nb_2/3O_3 Revealed by EXAFS and Thermodynamics

This study reveals that local chemical ordering in the disordered perovskite BaCu $_{1/3}$ Nb $_{2/3}$ O $_3$ , which preferentially forms Cu-Nb bonds and suppresses Cu-Cu linkages, creates a specific exchange network that drives the material's low-temperature random-singlet behavior rather than conventional magnetic ordering or spin-glass freezing.

Original authors: Sagar Mahapatra, Francesco De Angelis, Martin Etter, Edmund Welter, M. P. Saravanan, Rajeev Rawat, Carlo Meneghini, Surjeet Singh

Published 2026-08-05

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CC BY 4.0

Original authors: Sagar Mahapatra, Francesco De Angelis, Martin Etter, Edmund Welter, M. P. Saravanan, Rajeev Rawat, Carlo Meneghini, Surjeet Singh

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where the tiniest building blocks of matter, like electrons, are constantly playing a game of "tag" with their neighbors. In a perfect, orderly crystal, these electrons would line up in neat rows, all agreeing on who to tag next. But in the messy, real world, things get chaotic. Sometimes, atoms are swapped out or mixed up randomly, creating a "disordered" system. When this happens, the electrons can't agree on a single pattern. Instead of forming a solid, ordered army, they might get stuck in a weird, fluid state where they pair up randomly with whoever is nearby, no matter how far away they are. Scientists call this a "Random Singlet" state. It's like a giant, invisible dance floor where partners are chosen by chance, and the music changes speed depending on how hot or cold the room is. Understanding these strange, disordered states is a big deal because they might hold the secrets to new types of superconductors or quantum computers, technologies that could change how we live. But to understand the dance, you first have to figure out exactly who is standing next to whom on the crowded floor.

This is exactly what a team of scientists set out to do with a special crystal called BaCu1/3Nb2/3O3 (or BCNO for short). Think of this crystal as a 3D grid of tiny boxes, where some boxes are supposed to hold copper atoms and others niobium atoms. The recipe says there should be one copper for every two niobiums, but in a real crystal, the atoms don't always follow the recipe perfectly; they might jumble up. The scientists wanted to know: Do the copper atoms clump together in big groups, or are they scattered far apart? This question matters because if the copper atoms clump, they should form a solid magnetic order (like a frozen magnet). But if they are scattered and mixed with niobium, they might form that mysterious "Random Singlet" state.

To solve this mystery, the researchers used two different "eyes" to look at the crystal. First, they used a powerful X-ray machine (synchrotron XRD) to see the average shape of the crystal. This was like looking at a city from a satellite; it showed that the city looked perfectly mixed, with no big neighborhoods of just copper. However, the satellite view can't see the tiny details of who is standing next to whom on the street corner. So, the team used a second, more zoomed-in tool called XAFS. This tool acts like a super-magnifying glass that can tell exactly which atom is next to which.

The results were surprising. Even though the satellite view said the copper atoms were randomly mixed, the zoomed-in view revealed a secret rule: the copper atoms were actively avoiding each other! Instead of standing next to other copper atoms, they were almost always standing next to niobium atoms. It's as if the copper atoms were shy and refused to hold hands with their own kind, preferring to hold hands with the niobium neighbors instead. This "shyness" meant that even though there were enough copper atoms to theoretically form a giant connected chain, they were actually broken up into tiny, isolated pairs or single atoms.

Because the copper atoms were so isolated, they couldn't form a solid magnetic order. Instead, they behaved exactly like the "Random Singlet" theory predicted. The scientists measured how the material reacted to heat and magnetic fields and found that it followed a very specific mathematical pattern (a power-law behavior) that only happens in these random, disordered states. They also noticed something cool: when they turned up the magnetic field, the material's behavior changed again, shifting from a messy, random state to a smoother, more fluid state that looks a bit like a "quantum spin liquid," where the electrons are free to move without getting stuck.

In short, the paper proves that in this specific crystal, the atoms arrange themselves in a very specific, local way that prevents them from clumping together. This local arrangement is the hidden reason why the material acts like a random singlet instead of a normal magnet. It's a beautiful example of how the tiny, invisible choices atoms make about who their neighbors are can completely change the behavior of the whole material, turning a potential magnet into a strange, quantum fluid.

Technical Summary: Microscopic Origin of Random Singlet Behavior in B-site Disordered Spin-1/2 Perovskite BaCu1/3Nb2/3O3

Problem Statement
The paper investigates the microscopic origin of the random-singlet (RS) ground state in the disordered perovskite BaCu $_{1/3}$ Nb $_{2/3}$ O $_3$ (BCNO). While theoretical models predict that quenched disorder in spin-1/2 systems can lead to an RS phase characterized by power-law thermodynamic behaviors, the realization of this phase in three-dimensional (3D) systems is counter-intuitive. In BCNO, the magnetic Cu $^{2+}$ concentration (33%) exceeds the site-percolation threshold for a simple cubic lattice (31%). Standard percolation theory suggests this should lead to the formation of large, connected magnetic networks resulting in long-range magnetic order or spin-glass freezing. However, experimental observations in related compounds (SrCu $_{1/3}$ Nb $_{2/3}$ O $_3$ and SrCu $_{1/3}$ Ta $_{2/3}$ O $_3$ ) indicate an RS ground state instead. The central question addressed is how the local chemical arrangement of Cu and Nb ions suppresses direct Cu-Cu connectivity despite the high global concentration, thereby stabilizing the RS phase.

Methodology
The study employs a combined structural and thermodynamic approach:

Synchrotron X-ray Diffraction (XRD): High-resolution powder XRD was used to determine the average crystal structure and long-range ordering. Rietveld refinement was performed to establish the space group and average site occupancies.
X-ray Absorption Fine Structure (XAFS): Cu-K and Nb-K edge XAFS (including XANES and EXAFS) were utilized to probe the local atomic environment, bond distances, and chemical ordering. This technique provides element-specific sensitivity to short-range correlations that are averaged out in XRD.
Thermodynamic Measurements: Magnetic susceptibility ( $\chi$ ) and specific heat ( $c_p$ ) were measured over a wide temperature range (0.1 K to 300 K) and under magnetic fields up to 9 T using a Physical Property Measurement System (PPMS).

Key Results

Structural Analysis:
- Long-range Order: XRD confirms a tetragonal perovskite structure (space group $P4mm$) with no long-range Cu/Nb ordering; the average B-site occupancy matches the nominal 1:2 stoichiometry.
- Local Chemical Order: EXAFS analysis reveals a distinct local chemical order characterized by strong heteroatomic correlations. Around Cu sites, more than 90% of nearest neighbors are Nb ( $N_{Cu-Nb} > 5.4$ , $N_{Cu-Cu} < 0.6$ ). Conversely, Nb sites have an equal probability of being surrounded by Cu or Nb.
- Suppression of Clustering: This local arrangement strongly suppresses direct Cu-Cu linkages. Despite the global concentration exceeding the percolation threshold, the probability of finding adjacent Cu ions is low (<10%), resulting in a structure dominated by Cu monomers rather than large clusters or infinite networks.
Thermodynamic Behavior:
- Absence of Ordering: No signatures of long-range magnetic order or spin-glass freezing were observed down to 0.1 K, despite a Curie-Weiss temperature $\Theta_{CW} \approx -50$ K indicating substantial antiferromagnetic interactions.
- Random Singlet Signatures:
  - Magnetic susceptibility $\chi(T)$ exhibits a power-law divergence ( $\chi \propto T^{-\gamma}$ ) with $\gamma \approx 0.6$ below 10 K.
  - Specific heat $c_p(T)$ in zero field follows a power law $c_p \propto T^{1-\gamma}$ (specifically $T^{0.42}$ ) over more than three decades in temperature.
  - Both $\chi(T)$ and $c_p(T)$ exhibit single-parameter $T/H$ scaling, where data for different fields collapse onto single curves, consistent with RS phenomenology.
- High-Field Crossover: Under high magnetic fields, the specific heat behavior transitions. The exponent $\alpha$ (where $c_p \propto T^\alpha$ ) evolves from 0.42 (zero field) to approximately 1.0 at 9 T, indicating a crossover to a $T$ -linear dependence. The magnetic entropy saturates at roughly 25% of $R \ln 2$ , independent of the field.

Significance and Claims
The paper claims to reveal the microscopic origin of exchange randomness in 3D disordered perovskites that stabilizes the random-singlet phase. The key finding is that local chemical ordering (preferential Cu-Nb correlations) overrides global stoichiometry, effectively breaking the percolation network of magnetic ions into isolated monomers and small clusters. This local suppression of Cu-Cu connectivity prevents the formation of long-range magnetic order or spin-glass states, allowing the system to enter the RS regime.

Furthermore, the study suggests that the application of high magnetic fields reveals a hidden quantum paramagnetic state. The transition to a linear-in-temperature specific heat ( $c_p \propto T$ ) at high fields is interpreted as the emergence of gapless magnetic excitations, potentially indicative of a quantum spin liquid state, with approximately 25% of the spins forming the random singlet phase while the remainder reside in a correlated quantum paramagnetic phase. The work clarifies how structural disorder in $AM_{1/3}M'_{2/3}O_3$ systems can be engineered to stabilize exotic quantum ground states.

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