Thickness-driven crossover from conventional to chiral nonreciprocal superconductivity in kagome metal CsV3Sb5

This study demonstrates that reducing the thickness of the kagome metal CsV3Sb5 induces a dimensional crossover from conventional bulk superconductivity to a chiral, nonreciprocal phase characterized by broken inversion and time-reversal symmetries, thereby resolving controversies over its pairing symmetry and enabling new quantum device applications.

The Gist

Imagine a material called CsV3Sb5 as a bustling city built on a unique, honeycomb-like grid of triangles (a "kagome" lattice). For a long time, scientists thought this city operated like a standard, predictable metropolis where electricity flows smoothly and symmetrically in all directions. This was the "bulk" version of the material—a thick, massive block of the substance.

However, this new study reveals that if you shrink this city down to a very thin, flat sheet (like peeling a single layer off a loaf of bread), the rules of the game change completely. The city transforms from a standard metropolis into a one-way, chiral super-city where electricity has a preferred direction, even without any external help.

Here is the breakdown of what the researchers found, using simple analogies:

1. The "Bulk" vs. The "Thin Sheet"

• The Thick Block (Bulk): When the material is thick (hundreds of nanometers), it behaves like a normal, conventional superconductor. Think of it as a wide, two-way highway where cars (electrons) can drive equally well in both directions. It follows standard physics rules.
• The Thin Sheet (Ultrathin Flakes): When the researchers peeled the material down to be thinner than about 100 nanometers (roughly 1,000 times thinner than a human hair), the behavior flipped. The material suddenly started acting like a one-way street.

2. The "Superconducting Diode" Effect

The most exciting discovery is something called the Superconducting Diode Effect.

• The Analogy: Imagine a turnstile at a subway station. Usually, a turnstile lets you spin through easily in one direction but locks up if you try to spin the other way. In a normal superconductor, electricity flows perfectly in both directions.
• The Discovery: In these thin sheets, the material acts like a perfect, zero-resistance turnstile. Electricity flows effortlessly in one direction but hits a "speed bump" (resistance) if it tries to go the other way.
• Why it matters: This only happens when the sheet is thin enough. The researchers found that once the material gets thicker than ~100 nm, this "one-way" behavior disappears, and it goes back to being a normal two-way highway.

3. Breaking the Rules of Symmetry

In physics, "symmetry" is like a mirror. If you look in a mirror, left and right are swapped, but the laws of physics usually stay the same.

• The Problem: For a material to act like a one-way street (a diode), it has to break two fundamental rules:
  1. Inversion Symmetry: It can't look the same if you flip it inside out.
  2. Time-Reversal Symmetry: It can't look the same if you play the movie of the electrons moving backward.
• The Solution: The study shows that in the thick blocks, these rules are respected. But in the thin sheets, the material spontaneously breaks these rules. It creates an internal "chiral" (handed) state, like a spiral staircase that only goes up one way, forcing the electricity to follow that specific path.

4. The "Height" of the City

The researchers also looked at how "tall" the electrons feel they are in this city.

• In the thick blocks, the electrons feel like they are in a tall, 3D skyscraper where they can move up, down, and sideways freely.
• In the thin sheets, the electrons feel like they are trapped on a flat, 2D table. As the sheet gets thinner, the "height" of their movement shrinks until it is almost as thin as a single atomic layer. This confinement forces the electrons to rearrange themselves into this new, exotic one-way state.

5. Solving a Mystery

For years, scientists were confused. Some experiments on thick blocks said, "It's a normal superconductor!" while other experiments on thin flakes said, "It's a weird, exotic one!"

• The Verdict: This paper solves the argument by showing that both are right. The material isn't one thing or the other; it depends entirely on how thick it is.
  • Thick = Normal.
  • Thin = Exotic, one-way, chiral superconductor.

Summary

The researchers discovered that by simply making a piece of kagome metal thinner, they can switch its personality from a standard, two-way superconductor to a futuristic, one-way superconductor that breaks the laws of symmetry. This doesn't just clear up a scientific debate; it shows that we can "tune" the quantum behavior of materials just by changing their thickness, turning a simple sheet of metal into a versatile platform for future quantum devices.

Technical Summary

Technical Summary: Thickness-Driven Crossover from Conventional to Chiral Nonreciprocal Superconductivity in Kagome Metal CsV3Sb5

Problem Statement
The superconducting pairing symmetry in the kagome metal CsV3Sb5 has been a subject of intense debate. Bulk studies, including nuclear magnetic resonance (NMR), nuclear quadrupole resonance (NQR), and tunneling measurements, consistently indicate a conventional, fully gapped multi-gap $s$-wave state with no time-reversal symmetry breaking (TRSB). Conversely, recent studies on ultrathin flakes have reported unconventional signatures, such as nonreciprocal transport and a zero-field superconducting diode effect (SDE), which necessitate the breaking of both inversion ($P$) and time-reversal ($T$) symmetries. The origin of these conflicting observations—whether they stem from extrinsic factors, sample quality, or a genuine thickness-dependent phase transition—remains unresolved. Specifically, it is unclear if the nonreciprocal phenomena are intrinsic to the two-dimensional (2D) limit of CsV3Sb5 or if they represent an extrinsic anomaly.

Methodology
The authors conducted a systematic investigation of superconductivity in CsV3Sb5 across a wide range of sample thicknesses, from bulk crystals down to ultrathin flakes (~30 nm). The study employed the following experimental techniques:

• Sample Preparation: High-quality single crystals were grown via the self-flux method. Flakes were exfoliated and encapsulated with hexagonal boron nitride (h-BN) on Si/SiO2 substrates. Thicknesses were verified using atomic force microscopy (AFM).
• Second-Harmonic Magnetotransport: Using a lock-in amplifier, the authors measured the second-harmonic voltage ($V_{2\omega}$) under magnetic fields applied both parallel to the kagome plane ($H // ab$) and perpendicular to it ($H // c$). This technique probes nonreciprocal transport, which arises from the interplay of broken inversion and time-reversal symmetries.
• Zero-Field Superconducting Diode Effect (SDE): Current-voltage ($V-I$) characteristics were measured in zero applied magnetic field using pulsed currents to minimize Joule heating. The authors compared the critical currents for positive and negative current directions to detect spontaneous TRSB.
• Upper Critical Field ($H_{c2}$) Measurements: Angular-dependent and temperature-dependent $H_{c2}$ measurements were performed to determine the dimensionality of the superconducting state and extract coherence lengths ($\xi_{ab}$ and $\xi_c$).

Key Results

1. Thickness-Dependent Nonreciprocity: Second-harmonic measurements revealed that nonreciprocal signals (characterized by peak-valley asymmetry in $V_{2\omega}$ vs. $H$) emerge only in flakes thinner than a critical thickness of approximately 100 nm. Bulk crystals (435 nm) and thicker flakes (240 nm) showed no such signals, even at high current densities.
2. Emergence of Zero-Field SDE: $V-I$ measurements confirmed the absence of an SDE in the 155 nm flake. However, a reproducible mismatch between positive and negative critical currents appeared in the 100 nm flake and became pronounced in thinner samples (65 nm and 30 nm). This indicates spontaneous TRSB in the zero-field superconducting state for thin flakes.
3. Dimensional Crossover: Angular dependence of $H_{c2}$ showed that bulk CsV3Sb5 follows the 3D anisotropic-mass Ginzburg-Landau model. In contrast, thin flakes (e.g., 435 nm and below) exhibit a cusp at $H // ab$, fitting the 2D Tinkham model. The anisotropy factor $\Gamma = H_{c2}(0^\circ)/H_{c2}(90^\circ)$ increased systematically from ~6.9 in the bulk to 17.5 in the thinnest (37 nm) sample.
4. Coherence Length Evolution: While the in-plane coherence length ($\xi_{ab}$) remained relatively stable, the out-of-plane coherence length ($\xi_c$) decreased significantly with thickness. In the 37 nm flake, $\xi_c$ (~1.8 nm) became comparable to the out-of-plane lattice constant ($c \approx 0.93$ nm), suggesting a transition to a quasi-2D regime where interlayer coupling is weak.

Key Contributions

• Resolution of Pairing Symmetry Controversy: The study clarifies that the conventional $s$-wave state observed in bulk CsV3Sb5 and the unconventional chiral state observed in thin flakes are not contradictory but represent a thickness-driven phase transition.
• Identification of a Chiral Phase: The authors provide evidence for a thickness-induced chiral superconducting phase in the 2D limit that breaks both $P$ and $T$ symmetries. This phase is distinct from the bulk state.
• Mechanism Elucidation: The paper proposes that the reduction in interlayer coupling in thin flakes allows for nontrivial relative phases between superconducting order parameters on different layers. Phenomenological free-energy analysis suggests this can stabilize a chiral state (e.g., an $s+is$ state) that spontaneously breaks time-reversal symmetry, accounting for the zero-field SDE and nonreciprocal transport.

Significance
The paper establishes that thin-flake kagome metals serve as a versatile platform for engineering nonreciprocal quantum devices. By demonstrating that the pairing symmetry and dimensionality of CsV3Sb5 can be tuned via thickness, the work resolves long-standing ambiguities regarding its superconducting nature. It suggests that the 2D limit of kagome superconductors hosts emergent topological phases, such as chiral superconductivity, which are inaccessible in the bulk. This finding opens new avenues for exploring emergent topological phases and developing on-chip nonreciprocal devices based on kagome materials.