Dr Dharmalingam Prabhakaran

Abstract:

Magnets with isotropic easy-plane symmetry host Goldstone modes that can be leveraged for efficient spin transport. Here, we present a time-resolved optical polarimetry technique that allows us to detect and characterize such low-frequency modes, and use it to observe the Goldstone mode in the multi-Q broken helix phase of EuIn2As2. The strength of our technique comes from the ability to distinguish between nematic and magnetization dynamics in order to yield information about the mode structure, in addition to its frequency. We find that the nearly uniform spin precession characteristic of a Goldstone mode is realized only when a small magnetic field is used to unpin the broken helix from local strain generated during crystal growth. In this regime, the mode frequency scales linearly with the applied field due to the ground state C2z symmetry of the broken helix. Our work shows how optical polarimetry can be used to study the Goldstone modes of complex magnets.

Abstract:

Understanding how light modifies long-range order in quantum materials is key to improving our ability to control functionality. However, this is challenging if the response is heterogeneous. Here we address the most common form of light-induced heterogeneity—surface melting—and measure the dynamics of orbital order in the layered manganite La_0.5Sr_1.5MnO₄. We isolate the surface dynamics from the bulk by measuring the orbital truncation rod and orbital Bragg peak. After photoexcitation, the orbital Bragg peak shows an unusual narrowing, which suggests an increase in correlation length of the probed volume. By contrast, the correlation length at the surface decreases. These differences can be reconciled if the material is heterogeneous, and light melts a less ordered surface. By isolating the surface response, we determine that the loss of long-range order is an incoherent process, which is probably accompanied by the formation of local polarons.

Abstract:

Ultrafast switching of ferroic phases is an active research area with technological potential. Yet, some key challenges remain, ranging from limited speeds in ferromagnets to intrinsic volatility of switched domains owing to depolarizing fields in ferroelectrics. Unlike these ferroic systems, ferroaxial materials host bistable states that preserve spatial-inversion and time-reversal symmetry and are therefore immune to depolarizing fields but also difficult to manipulate with conventional methods. We demonstrate photo-induced switching of ferroaxial order by engineering an effective axial field composed of circularly driven terahertz phonon modes. A switched ferroaxial domain remains stable for many hours and can be reversed back with a second terahertz pulse of opposite helicity. The effects demonstrated in this work may lead to the development of a robust platform for ultrafast information storage.

Abstract:

<jats:p>Iridium-containing conducting materials are widely investigated for their strong spin-orbit coupling and potential topological properties. Recently the commonly used electrode material iridium dioxide was found to host a large spin-Hall conductivity and was shown to support Dirac nodal lines. Here we present quantum-oscillation experiments on high-quality <a:math xmlns:a="http://www.w3.org/1998/Math/MathML"><a:msub><a:mi>IrO</a:mi><a:mn>2</a:mn></a:msub></a:math> single crystals using the de Haas-van Alphen effect measured using torque magnetometry with a piezoresistive microcantilever as well as density functional theory-based band-structure calculations. The angle, temperature, and field dependencies of the oscillations and the calculated band dispersion provide valuable information on the properties of the charge carriers, including the Fermi-surface geometry and electronic correlations. Comparison of experimental results to calculations allows us to assigns the observed de Haas-van Alphen frequencies to the calculated Fermi surface topology. We find that the effective masses of <b:math xmlns:b="http://www.w3.org/1998/Math/MathML"><b:msub><b:mi>IrO</b:mi><b:mn>2</b:mn></b:msub></b:math> are enhanced compared to the rest electron mass <c:math xmlns:c="http://www.w3.org/1998/Math/MathML"><c:msub><c:mi>m</c:mi><c:mi>e</c:mi></c:msub></c:math>, ranging from 1.9 to 3.0 <d:math xmlns:d="http://www.w3.org/1998/Math/MathML"><d:msub><d:mi>m</d:mi><d:mi>e</d:mi></d:msub></d:math>, whereas the scattering times indicate excellent sample quality. We discuss our results in context with recent ARPES and band-structure calculation results that found Dirac nodal lines in <e:math xmlns:e="http://www.w3.org/1998/Math/MathML"><e:msub><e:mi>IrO</e:mi><e:mn>2</e:mn></e:msub></e:math> and compare the effective masses and other electronic properties to those of similar materials like the nodal chain metal <f:math xmlns:f="http://www.w3.org/1998/Math/MathML"><f:msub><f:mi>ReO</f:mi><f:mn>2</f:mn></f:msub></f:math> in which Dirac electrons with very light effective masses have been observed.</jats:p>

Abstract:

Photocatalytic ammonia decomposition offers a sustainable route for hydrogen production, but its development is limited by low catalytic efficiency and poorly understood mechanisms. Here, a protonated layered perovskite, HPrNb2O7 (HPNO), is reported as an efficient catalyst for ammonia decomposition under mild photo‐thermal conditions. Upon exposure to NH3 at elevated temperatures, HPNO promotes the in situ formation and intercalation of hydrazine intermediates within its interlayer galleries, enabled by thermally generated oxygen vacancies and hydrogen bonding. Advanced characterization techniques have been applied to confirm the formation and stabilization of hydrazine. It is also shown that thermal energy prolongs charge carrier lifetimes and enhances oxygen vacancy formation, contributing to a strong photo‐thermal synergy. The stabilization of hydrazine intermediate promotes the associative mechanism, lowering the activation barrier, thus leading to an enhanced hydrogen evolution rate of 1311.2 µmol·g−1·h−1 at 200 °C under simulated solar irradiation without any noble metal co‐catalyst. This work reveals a distinct, hydrazine‐mediated reaction pathway and positions layered protonated perovskites as promising materials for efficient, solar‐driven ammonia decomposition and sustainable hydrogen generation.