🔬 mesoscale physics

Van Hove singularity-driven giant Nernst signal in twisted double bilayer graphene

This study demonstrates that van Hove singularities in twisted double bilayer graphene generate a giant, electrically tunable Nernst signal driven by Lifshitz transitions, establishing the effect as a sensitive probe of Fermi-surface topology and a universal thermoelectric signature of these singularities.

Original authors: Ujjal Roy, Monosij Roy, Unmesh Ghorai, Arkaprava Mukherjee, Ravi Kumar, Kenji Watanabe, Takashi Taniguchi, Nandini Trivedi, Rajdeep Sensarma, Subroto Mukerjee, Anindya Das

Published 2026-07-29

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Ujjal Roy, Monosij Roy, Unmesh Ghorai, Arkaprava Mukherjee, Ravi Kumar, Kenji Watanabe, Takashi Taniguchi, Nandini Trivedi, Rajdeep Sensarma, Subroto Mukerjee, Anindya Das

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 the world of electrons not as a chaotic swarm of tiny, invisible bugs, but as a bustling crowd of people trying to move through a city. In most materials, this city is flat and boring; the people (electrons) can walk anywhere with roughly the same ease. But in a special class of materials called "moiré materials," scientists have discovered a way to twist the city streets into a complex, wavy pattern. This twisting creates a unique landscape where the rules of movement change dramatically.

Two key ideas help us understand this landscape. First, think of a "density of states" as a measure of how crowded a specific area of the city is. Usually, the crowd is spread out evenly. But in these twisted materials, there are specific spots where the crowd suddenly becomes incredibly dense, like a massive concert crowd spilling out of a stadium. Scientists call these crowded spots "van Hove singularities." Second, there is the "Nernst effect." You might know the Hall effect, where a magnetic field pushes moving electric charges to the side, creating a voltage. The Nernst effect is a cousin to this: instead of pushing charges with electricity, we push them with heat. If you heat one side of a material and cool the other, the electrons try to diffuse from hot to cold. If you then add a magnetic field, this heat-driven flow gets pushed sideways, creating a transverse voltage. Usually, this sideways push is tiny because the hot and cold electrons push in opposite directions and cancel each other out. The big question in physics has been: what happens to this heat-driven sideways push when the electrons are packed into those super-crowded "van Hove singularities"?

This paper takes a deep dive into that question using a material called "twisted double bilayer graphene" (tDBLG). Think of this material as a sandwich made of four layers of graphene (a single layer of carbon atoms) that have been stacked and slightly twisted relative to each other. This twisting creates the wavy "moiré" pattern mentioned earlier. The researchers didn't just look at how electricity flows through this sandwich; they measured how heat and electricity interact in a magnetic field. They found that when the electrons are sitting right at those super-crowded "van Hove singularities," the Nernst effect explodes in size. Instead of a tiny, barely noticeable signal, they measured a "giant" Nernst signal.

The team discovered that these giant signals appear as sharp peaks exactly where the "van Hove singularities" are located. These peaks are so large that they rival some of the best-known materials for this effect, reaching values of about 40 µV K⁻¹T⁻¹ at very low temperatures (around 1 Kelvin). What makes this even more fascinating is that the researchers could tune these peaks like a radio dial. By applying an electric field (a "displacement field"), they could move the position of the singularities and watch the giant signal appear and disappear right on cue.

To understand why this happens, the authors used a clever analogy involving the "effective mass" of the electrons. In normal metals, hot electrons and cold electrons are pushed in opposite directions by a magnetic field, canceling each other out. However, at the "van Hove singularities," the landscape changes so drastically (a "Lifshitz transition") that the effective mass of the electrons flips sign. It's as if the rules of the road suddenly reversed for half the crowd. Instead of pushing in opposite directions, both the hot and cold electrons get deflected toward the same side. This lack of cancellation leads to a massive buildup of charge on one side, creating that giant voltage. The researchers confirmed this by comparing their measurements with theoretical calculations, which matched their experimental data almost perfectly.

The paper explicitly argues against the idea that this giant signal comes from exotic, complex interactions or superconducting fluctuations, which are often blamed for large Nernst signals in other materials. Instead, they show that this phenomenon arises purely from the geometry of the electron energy bands—the "topology" of the landscape. They demonstrate that a standard, non-interacting model of electron movement is enough to explain the huge signal, provided you account for the unique shape of the energy bands at the singularities.

In summary, this study establishes the Nernst effect as a powerful new tool for mapping the hidden shapes of electron landscapes in twisted materials. It proves that "van Hove singularities" leave a universal, giant fingerprint in thermal transport. The findings suggest that this isn't just a quirk of one specific material, but a general rule for any flat-band or moiré material, offering a new way for scientists to "see" the topology of electron surfaces without needing complex, high-energy probes. The authors are confident in these results, having measured them directly in the lab and backed them up with solid theoretical models, marking a significant step in understanding how heat and magnetism dance together in the quantum world.

Technical Summary: Van Hove Singularity-Driven Giant Nernst Signal in Twisted Double Bilayer Graphene

Problem Statement
Twisted double bilayer graphene (tDBLG) hosts prominent van Hove singularities (vHSs) within its moiré minibands, which are associated with Lifshitz transitions and are believed to drive correlated electronic phases such as superconductivity and itinerant magnetism. While the electrical transport signatures of these vHSs are well-documented, their impact on thermal transport remains unexplored. Specifically, the behavior of the Nernst effect—the transverse voltage generated by a longitudinal temperature gradient in a magnetic field—at these singularities is unknown. Understanding this is critical because the Nernst effect is a sensitive probe of Fermi-surface topology, Berry curvature, and superconducting fluctuations, yet no equilibrium transport studies had previously characterized the Nernst signal at vHSs in twisted platforms.

Methodology
The authors fabricated hBN-encapsulated tDBLG devices with twist angles of approximately 1.66° and 1.20° using a standard "cut-and-stack" transfer method. The devices were patterned into Hall bar geometries with integrated metallic heaters to generate longitudinal temperature gradients.

Experimental Setup: Measurements were conducted in a 3He cryostat at base temperatures (~250 mK) with a perpendicular magnetic field (up to 1 T). The study utilized the 2 $\omega$ technique to measure thermoelectric voltages.
Tuning: Independent control of carrier density ( $n$ ) and out-of-plane displacement field ( $D$ ) was achieved via top and bottom gates, allowing the researchers to map the electronic structure across the moiré bands.
Analysis: The authors measured longitudinal resistance ( $R_{xx}$ ), Hall resistance ( $R_{xy}$ ), longitudinal thermopower ( $S_{xx}$ ), and transverse Nernst signal ( $S_{yx}$ ). They correlated these experimental data with theoretical calculations based on a continuum model for the density of states (DOS) and semiclassical Boltzmann transport equations.

Key Results

Mapping vHSs: By analyzing the divergence and sign change of the Hall density ( $n_H$ ) as a function of $n$ and $D$ , the authors successfully mapped the positions of vHSs in both the conduction and valence bands. These positions align with Lifshitz transitions where isolated Fermi pockets merge into a single pocket, consistent with theoretical DOS calculations.
Giant Nernst Signal: The most significant finding is the observation of unusually large Nernst signals ( $S_{yx}$ $S_{y x}$ ) precisely at the vHS locations.
- The signal exhibits pronounced positive peaks for both conduction and valence band vHSs.
- The maximum magnitude reaches $\sim 20$ $\mu$ V K $^{-1}$ T $^{-1}$ for the 1.66° device and $\sim 40$ $\mu$ V K $^{-1}$ T $^{-1}$ for the 1.20° device at $\sim 1$ K.
- These values are comparable to heavy fermion metals and are among the largest reported, surpassed only by bismuth.
- In contrast, the Nernst signal remains negligibly small throughout the rest of the bandwidth, and the longitudinal thermopower ( $S_{xx}$ ) changes sign and vanishes at the vHS due to particle-hole symmetry.
Mechanism: The large enhancement is attributed to the Lifshitz transition around the vHS. Unlike in conventional metals where high- and low-energy carriers deflect in opposite transverse directions (leading to cancellation), the sign reversal of the effective mass across the saddle point in tDBLG causes both carrier types to deflect in the same direction. This constructive interference generates the giant Nernst signal.
Theoretical Validation: Semiclassical Boltzmann transport calculations, utilizing the measured conductivity tensor and theoretical DOS, quantitatively reproduce the experimental $S_{yx}$ data. The analysis confirms that the large signal arises from the energy dependence of the Hall angle and mobility near the vHS, without requiring exotic many-body interactions in the non-interacting limit.

Significance and Claims
The paper establishes the Nernst effect as a highly sensitive probe of Fermi-surface topology in moiré materials. The authors claim that the giant, universally positive Nernst signal is a direct thermoelectric signature of van Hove singularities associated with Lifshitz transitions.

Universality: The positive sign of the Nernst signal around the vHS is argued to be universal, contrasting with the sign variability seen in metals or semimetals.
Generic Feature: Since the mechanism relies purely on band topology (effective mass sign reversal at saddle points), the authors posit that such giant vHS-driven Nernst signals should be a generic feature across various moiré and flat-band materials.
New Probe: This work identifies a new, robust method to detect and characterize vHSs and topological transitions in correlated systems, complementing existing electrical transport measurements.

The study does not claim to have discovered new correlated phases (like superconductivity) via this method, but rather provides a fundamental transport signature that can be used to locate the vHSs that often precede such phases.

More like this