🔬 materials science

Observation of Room-temperature Charge Density Wave Correlations via Coherent Phonon Spectroscopy in Sn-doped Kagome Superconductor CsV $_3$ Sb $_5$

Using ultrafast time-resolved reflectivity and synchrotron X-ray diffraction, this study reveals that Sn doping introduces quenched disorder that pins short-range charge density wave correlations in the Kagome superconductor CsV $_3$ Sb $_5$ , allowing these correlations to persist robustly up to room temperature well beyond the long-range order phase boundary.

Original authors: Qinwen Deng, Andrea Capa Salinas, Suchismita Sarker, Leon Balents, Stephen D. Wilson, Liang Wu

Published 2026-02-23

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CC BY 4.0

Original authors: Qinwen Deng, Andrea Capa Salinas, Suchismita Sarker, Leon Balents, Stephen D. Wilson, Liang Wu

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

The Big Picture: Finding Ghosts in the Machine

Imagine a crystal called CsV₃Sb₅ as a giant, perfectly organized dance floor. Inside this crystal, electrons (the dancers) usually move in a chaotic, free-flowing way. But at a certain temperature, they suddenly decide to hold hands and form a rigid, repeating pattern. In physics, we call this a Charge Density Wave (CDW). It's like the dancers suddenly forming a perfect, synchronized line dance that covers the whole floor.

Usually, if you heat up the dance floor, the dancers get too jittery, the line breaks, and the pattern disappears. In this specific crystal, that "line dance" usually stops existing above about -179°C (94 Kelvin).

The Mystery:
Scientists took this crystal and started sprinkling it with a different ingredient: Tin (Sn). They expected that adding this "impurity" would mess up the dance floor so badly that the line dance would vanish immediately. And indeed, standard measurements showed the big, long-range line dance was gone.

The Surprise:
However, the researchers used a special "super-speed camera" (ultrafast laser spectroscopy) to look at the crystal. They discovered that even though the big line dance was gone, tiny, local groups of dancers were still holding hands and dancing in patterns.

Even more shocking? In the sample with the most Tin, these tiny patterns didn't just survive the heat; they survived all the way up to Room Temperature (25°C / 296 K).

The Analogy: The Traffic Jam vs. The Parking Lot

To understand what's happening, let's use a traffic analogy.

The Perfect Crystal (No Tin): Imagine a highway where all cars are moving in a perfect, synchronized wave. This is the Long-Range CDW. It's a massive, organized traffic jam that stretches for miles. If you heat it up (add energy), the cars speed up, the wave breaks, and traffic flows freely.
Adding Tin (The Disorder): Now, imagine you drop a few broken-down cars (Tin atoms) randomly onto the highway.
- The Old Theory: You'd think the broken cars would just cause chaos, and the synchronized wave would disappear completely.
- The New Discovery: Instead of total chaos, the broken cars act like parking spots or speed bumps. They "pin" the cars nearby. Even though the cars can't form a wave that stretches for miles anymore, they get stuck in little, tight clusters around the broken cars.
- The Result: You don't have a highway-wide traffic jam, but you have hundreds of tiny, localized traffic jams that stay stuck together even when the temperature rises.

How They Found It: The "Snap" Test

How did they see these invisible tiny patterns?

The Method: They hit the crystal with a super-fast laser pulse (like a camera flash). This makes the atoms in the crystal vibrate, like plucking a guitar string.
The Clue: When the atoms vibrate, they make a specific sound (a frequency).
- In a normal crystal, they hear one main note (4.1 THz).
- In a crystal with the "line dance" (CDW), they hear a second, special note (1.3 THz).
The Discovery: Even in the samples where the big line dance was supposed to be dead, they still heard that special 1.3 THz note.
- In the pure crystal, this note fades away when it gets hot.
- In the Tin-doped crystal, this note kept ringing all the way up to room temperature. It was like hearing a ghostly echo of the dance that shouldn't exist.

Why Does This Matter?

This paper changes how we understand these materials in three big ways:

Disorder isn't always bad: We usually think "impurities" (like Tin) ruin perfect materials. Here, the impurities actually helped preserve the electronic pattern by "pinning" it in place, preventing it from melting away when heated.
The "Room Temperature" Breakthrough: Finding these patterns at room temperature is huge. It means these quantum effects aren't just for freezing labs; they are robust enough to exist in our everyday environment.
Superconductivity Connection: These crystals are also superconductors (conduct electricity with zero resistance). The fact that these "tiny dances" (short-range CDW) survive alongside superconductivity suggests they might be working together, rather than fighting each other. This gives scientists a new clue on how to build better superconductors.

The Takeaway

Think of the Tin atoms as anchors. They stopped the electrons from forming a giant, perfect wave across the whole crystal, but they also acted as glue, holding small groups of electrons together in a pattern that refuses to break, even when the crystal gets hot.

The researchers proved that even when a "perfect" order is destroyed, hidden, local order can survive and thrive, thanks to the very things we thought were ruining the system.

1. Problem Statement

The Kagome superconductor family $AV_3Sb_5$ ($A = K, Rb, Cs$) exhibits complex intertwined orders, including charge density waves (CDW), superconductivity, and time-reversal symmetry breaking. In pristine $CsV_3Sb_5$ , the long-range CDW order transitions at $T_{CDW} \approx 94$ K.

The Gap: While chemical doping (specifically Sn substitution on Sb sites) is known to suppress long-range CDW order (disappearing in thermodynamic measurements above $x \approx 0.06$ ), the behavior of short-range CDW correlations across extended temperature and doping ranges remains poorly understood.
The Question: Do CDW correlations persist in the absence of long-range order? If so, what is their temperature limit, and what is the role of the disorder introduced by Sn doping? Previous studies suggested a competition between superconductivity and CDW, but the specific mechanism allowing CDW signatures to survive at room temperature in highly doped samples was unclear.

2. Methodology

The authors employed a multi-modal experimental approach to probe electronic and structural correlations:

Sample Synthesis: Single crystals of $CsV_3Sb_{5-x}Sn_x$ were synthesized with varying doping levels ( $x = 0.03\text{--}0.68$ ) using the self-flux method.
Ultrafast Time-Resolved Reflectivity (TR-Reflectivity):
- Technique: Pump-probe spectroscopy using 1560 nm pump and 780 nm probe pulses (100 fs duration, 80 MHz repetition).
- Goal: To track the transient reflectivity change ( $\Delta R/R$ ) and extract coherent phonon spectra via Fourier transform.
- Rationale: CDW formation modifies the electronic structure and phonon spectrum. Specifically, the breaking of $C_6$ symmetry in the CDW state activates specific phonon modes (e.g., the 1.3 THz mode) that are forbidden in the pristine phase.
Synchrotron X-ray Diffraction (XRD):
- Location: Cornell High Energy Synchrotron Source (CHESS).
- Parameters: Transmission geometry with $\lambda = 0.47686$ Å.
- Goal: To directly visualize charge order patterns and measure correlation lengths ( $\xi$ ) in the $x=0.32$ sample at various temperatures.

3. Key Contributions

Discovery of Room-Temperature Short-Range CDW: The study provides the first evidence that short-range CDW correlations in Sn-doped $CsV_3Sb_5$ persist up to room temperature (296 K), far exceeding the bulk $T_{CDW}$ of the undoped material.
Decoupling of Long-Range Order and Correlations: The work demonstrates that while long-range CDW order is suppressed by Sn doping, robust short-range correlations survive, challenging the notion that CDW signatures vanish immediately upon the loss of long-range coherence.
Disorder-Pinning Mechanism: The authors propose that quenched disorder introduced by Sn atoms acts to "pin" CDW fluctuations, converting dynamic fluctuations into static short-range CDW domains. This explains the persistence of CDW signatures well above the thermodynamic transition temperature.

4. Key Results

A. Coherent Phonon Spectroscopy in Undoped vs. Doped Samples

Undoped $CsV_3Sb_5$ : Below $T_{CDW} \approx 94$ K, two additional intense phonon modes appear at 1.3 THz and 3.1 THz (alongside the main 4.1 THz lattice mode). The 1.3 THz mode is a signature of CDW-induced symmetry breaking (Star-of-David vs. inverse Star-of-David distortions).
Doped $x=0.32$ Sample:
- Thermodynamic measurements show no long-range CDW order.
- However, TR-reflectivity reveals a clear 1.3 THz mode persisting from 3 K up to 296 K.
- Linewidth & Lifetime: The 1.3 THz mode in the doped sample has a significantly broader linewidth and a shorter decay time ( $\sim 7$ ps) compared to the undoped sample ( $>60$ ps), indicating reduced correlation lengths and the presence of short-range, rather than long-range, order.
- Frequency Stability: The mode shows minimal frequency softening from 3 K to 296 K, consistent with a zone-folded phonon mode induced by static correlations.

B. Synchrotron X-ray Diffraction Confirmation

XRD measurements on the $x=0.32$ sample at 200 K and 300 K confirmed the presence of quasi-1D incommensurate charge correlations.
Correlation Length ( $\xi$ ):
- At 200 K: $\xi \approx 46$ Å.
- At 300 K: $\xi \approx 34$ Å.
Symmetry Breaking: The data reveals three-state short-range domains rotated by 120°, breaking $C_6$ rotational symmetry down to $C_2$ , consistent with the phonon observations.

C. Doping Evolution

Low Doping ( $x \approx 0.03\text{--}0.04$ ): Long-range CDW exists, but the 1.3 THz mode persists slightly above the bulk $T_{CDW}$ (up to 110 K).
Intermediate Doping ( $x = 0.09, 0.2$ ): Long-range order is absent, but the 1.3 THz mode persists up to 220 K ( $x=0.09$ ) and 250 K ( $x=0.2$ ).
High Doping ( $x = 0.68$ ): The 1.3 THz mode disappears at lower temperatures ( $\sim 140$ K), suggesting that excessive disorder eventually disrupts the correlations.
Lattice Mode Shift: The main 4.1 THz lattice mode frequency increases with Sn doping due to the reduced atomic mass of Sn compared to Sb.

5. Significance and Discussion

Role of Quenched Disorder: The paper argues that Sn doping introduces quenched disorder (random-field and random-mass effects). According to the Fukuyama-Lee-Rice (FLR) model, this disorder pins the phase of the CDW, preventing the melting of short-range order even at high temperatures. This creates a "Bragg glass" or static short-range ordered state.
Implications for Superconductivity: The persistence of CDW correlations at room temperature in the $x=0.32$ sample (where $T_c$ is low) suggests that these static short-range correlations do not necessarily compete directly with superconductivity in the same way long-range order does. This helps explain the "double-dome" superconductivity feature in this system.
Broader Impact: These findings redefine the phase diagram of Kagome superconductors, highlighting that "disordered" phases may still harbor robust electronic correlations. It underscores the necessity of using local probes (like TR-reflectivity and STM) over bulk thermodynamic measurements to fully understand the electronic landscape of doped quantum materials.

In summary, this work establishes that Sn doping in $CsV_3Sb_5$ does not simply destroy CDW order but transforms it into a robust, room-temperature, static short-range state pinned by disorder, providing a new paradigm for understanding electronic correlations in disordered Kagome metals.