[論文レビュー] Efficient photo-Nernst terahertz emission in single heavy-metal films

Lei Wang, Linxuan Song+6 more|arXiv (Cornell University)|Mar 23, 2026

Thermal Radiation and Cooling Technologies被引用数 0

ひとこと要約

The paper demonstrates efficient THz emission from standalone Pt (and other heavy-metal) nanofilms at cryogenic temperature and under magnetic field, identifying ultrafast photo-Nernst effect as the mechanism and showing thickness/alloying strategies to boost emission to levels comparable with spintronic bilayers.

ABSTRACT

State-of-the-art metallic terahertz (THz) emitters rely predominantly on spintronic heterostructures, where heavy metals serve as passive spin-to-charge converters. Here, we demonstrate efficient THz radiation from standalone Pt nanofilms at cryogenic temperatures and under external magnetic fields. The governing mechanism is identified as the ultrafast photo-Nernst effect, wherein a transient thermal gradient drives a transverse charge current. The THz emission polarity is directly dictated by the sign of the Nernst coefficient, as verified by the phase reversal observed between Pt and W or Ta. Remarkably, both thickness scaling and alloying-induced suppression of thermal conductivity independently amplify the single-layer emission to levels comparable with benchmark spintronic bilayers. These findings redefine the established role of heavy metals from passive spin-sinks to active THz emitters, uncovering a universal emission paradigm applicable across diverse spintronic and quantum materials.

研究の動機と目的

• Demonstrate THz emission from a standalone heavy-metal film under cryogenic temperatures and magnetic field.
• Identify the ultrafast photo-Nernst effect as the emission mechanism in non-magnetic metals.
• Explore material-dependent sign and strength of emission to connect to Nernst coefficients.
• Investigate thickness and alloying as routes to enhance single-layer THz emission.
• Contrast emission behavior with conventional spintronic bilayers to redefine heavy metals’ role.

提案手法

• Prepare 5 nm Pt films on MgO substrates and cap with MgO/W to prevent oxidation.
• Excite with fs near-IR pulses and detect THz via electro-optic sampling in a cryostat with an in-plane magnetic field.
• Analyze THz polarity under varying B-field, pump incidence, and polarization to establish symmetry.
• Vary Pt thickness from 1–20 nm to optimize THz amplitude and compare with transmittance.
• Introduce Pt-Ti alloying (Pt0.8Ti0.2) to suppress lattice thermal conductivity and enhance ∇Tz-driven emission.
• Correlate THz sign with Nernst coefficients from steady-state magneto-transport measurements.

実験結果

リサーチクエスチョン

• RQ1 Can a standalone heavy-metal film beyond interfacial spin sources emit THz radiation under ultrafast thermal excitation and magnetic field?
• RQ2 Is the emission governed by an ultrafast Nernst mechanism, and how does it depend on material Nernst coefficients?
• RQ3 How do thickness and alloying affect the efficiency of single-layer heavy-metal THz emitters?
• RQ4 Does the emission polarity track the sign of the Nernst coefficient across Pt, W, and Ta?

主な発見

• Single-layer Pt films emit THz radiation at 10 K and 7 T with a bandwidth up to 3 THz.
• THz amplitude scales linearly with magnetic field at low fields and shows slight sublinear behavior above 3 T; effective mobility ~0.087 m^2/Vs.
• THz polarity follows the sign of the Nernst coefficient, as Pt and W/Ta emitters show opposite phase.
• Emission symmetry is E_y perpendicular to B, with E_x negligible, consistent with a transverse thermoelectric (photo-Nernst) mechanism.
• Alloying Pt with Ti (Pt0.8Ti0.2) boosts THz emission by ~3x due to reduced lattice thermal conductivity and steeper ∇T_z.
• Optimal Pt thickness is 2 nm, with strong attenuation beyond ~5 nm due to THz screening; 1–20 nm scan shows a peak at 2 nm.
• Temperature dependence shows strong cryogenic enhancement of THz emission, unlike conventional ISHE-based or ANE mechanisms, due to ultrafast hot-carrier dynamics.
• The 2.6 nm effective THz screening length explains the thickness dependence, aligning with pump absorption and screening model E_THz ∝ (1 − e^{−α d}) e^{−β d}.