arXiv subjects

Shovon Pal

Publications and source records attributed to Shovon Pal.

At least 19 recordsLinked to original sources

arXiv2026

The interplay of crystal-field transitions and exchange spin dynamics in a ferrimagnet

Rare-earth iron garnets offer an ideal platform for exploring the interplay of low-energy excitations and the complex temperature-dependent magnetization dynamics. In these systems, exchange coupling between rare-earth and iron sublattices generates high-frequency collective spin excitations. In addition, the robust spin-orbit coupling of localized 4$f$ electrons triggers the crystal-electric-field (CEF) transitions at THz frequencies. Despite extensive research into the garnet spin dynamics, the interplay between CEF excitations and exchange modes has remained largely unmapped. Using temperature-dependent THz time-domain spectroscopy, we demonstrate a hybridization between the Yb-ion CEF excitation and the Yb-Fe exchange mode in Gd$_{3/2}$Yb$_{1/2}$BiFe$_{5}$O$_{12}$. This coupling is characterized by a significant redistribution of spectral and temporal weights as the material approaches its magnetization compensation temperature. Notably, the Yb-Fe exchange mode exhibits an anomalous redshift upon cooling -- a reversal of the conventional blue shift typically driven by increased exchange coupling. We trace this phenomenon to a modification of Yb-Fe exchange anisotropy, driven by the interplay of the Fe exchange field and Yb CEF excitations. These findings highlight the critical role of CEF-mediated exchange coupling in shaping low-energy spin dynamics, positioning rare-earth garnets as a cornerstone for future THz spintronic technologies.

cond-mat.mtrl-sci↗

arXiv2026

Soft-mode nonlinearities away from ferroelectric phase transition

The interplay between ionic and electronic subsystems dictates the behavior of structural phase transitions in polar dielectrics, a coupling mediated by soft optical phonon modes. In incipient ferroelectrics such as SrTiO$_3$ (STO), strong local-field effects can drive the lattice into a non-perturbative regime near the phase boundary. However, disentangling the distinct contributions of local fields from those of spontaneous macroscopic polarization remains an experimental challenge. Here, we isolate these mechanisms by probing paraelectric STO deep within its symmetric phase, where macroscopic spontaneous polarization is suppressed. Linear terahertz (THz) spectroscopy reveals that the soft mode exhibits a hybrid character, predominantly driven by electronic polarizability. Utilizing two-dimensional THz spectroscopy, we map the underlying nonlinear signals, demonstrating that the system persists in a perturbative regime characterized by robust local-field coherence. By implementing a microscopic model of coupled electronic and lattice degrees of freedom mediated by local fields, we qualitatively reproduce these multidimensional coherent signatures. Our findings highlight that while local fields are necessary to initiate non-perturbative lattice dynamics, they are insufficient on their own. This reveals that spontaneous polarization plays a deterministic role in dictating soft-mode nonlinearities in strongly correlated polar dielectrics.

cond-mat.mtrl-sci↗

arXiv2026

Kohn anomaly in a topological phase transition

Topological crystalline insulators extend the concept of topological insulators by hosting surface states protected by crystallographic symmetry. Their topological phase transitions arise from spin-orbit-driven band inversion in the bulk electronic structure, reshaping the low-energy electronic environment and its coupling to lattice excitations. While the electronic aspects of band topology are well established, the corresponding dynamics of lattice and electron-phonon interactions remain largely unexplored. Here, we report a pronounced softening of a low-energy surface phonon mode across the topological phase transition in Pb0.77Sn0.23Se, revealed by temperature-dependent time-domain terahertz spectroscopy. Unlike the well-known phonon softening in ferroelectrics, this effect does not signal a structural instability but instead reflects electronic reconstruction. We attribute the softening to the Kohn anomaly, indicating a strong coupling between lattice vibrations and Dirac-like surface electrons in the topological phase. Consistently, the phonon linewidth deviates from the standard anharmonic temperature dependence, further evidencing enhanced electron-phonon coupling. Our results establish phonon softening as a spectroscopic signature of topological phase transitions and provide a route to distinguish topological and trivial phases.

cond-mat.str-el↗

arXiv2025

Unveiling nonlinearities of electromagnetically induced transparency in a THz metamaterial

Electromagnetically induced transparency (EIT) in terahertz (THz) metamaterials relies on the coherent coupling between a radiative (bright) mode and a subradiant (dark) mode. Understanding the dynamic interplay between the bright and dark modes holds the key to manipulate the mutual interference and hence the transparency. Here, we use nonlinear 2D-THz spectroscopy to scrutinize the dynamics through nonlinearities of the EIT-like phenomenon in a metamaterial platform that comprises of two coupled resonators. From the temporal profiles of the nonlinear pump-probe and photon-echo signals, we found that the bright mode relaxation time is almost twice the time for the coherent exchange of energy between the two coupled resonators. The multi-peak nature of photon-echo signal and the corresponding temporal signatures further provides a direct visualization of the interference between the dressed states that drives the transparency window in our THz metamaterial. A time-resolved density matrix model accurately describes the observed features, including the cross-peak behavior and the temporal dynamics, establishing the coherent mode coupling as the origin of the transparency window.

physics.optics↗

arXiv2025

The interplay of magnetic order with the electronic scattering and crystal-field effects in a metallic ferromagnet

The interplay between magnetic order, charge dynamics, and crystal field excitations underpins the emergent ground states of rare-earth intermetallics. Using time-domain terahertz spectroscopy, we probe this coupling in PrSi, a metallic ferromagnet. The optical response exhibits pronounced Drude-Smith behavior over a broad temperature range, indicating persistent carrier scattering. A classical Kondo-lattice model (CKLM) attributes this non-Drude conductivity to scattering of itinerant electrons by localized magnetic moments, persisting down to temperatures well below the magnetic ordering scale. At lower temperatures, beyond the scope of CKLM, our experiment reveals that the response is dominated by crystal-field excitations, with sharp transitions at 0.6 THz and 1.54 THz. The mode at 1.54 THz shows a dynamic correlation with the onset of ferromagnetic order, marking the onset of a crystal-field-governed low temperature regime.

cond-mat.str-el↗

arXiv2025

Phonon-polariton mediated dual electromagnetically induced transparency-like response in a THz metamaterial

Light and matter can intertwine to create entirely new quantum states in the so-called strong-coupling regime, allowing unprecedented control over electromagnetic waves. In this work, strong-coupling mediated polaritonic states are exploited to demonstrate tunable dual electromagnetically induced transparency (EIT) like response in the terahertz (THz) frequency range using a micron-sized metamaterial system coupled with the phonon mode of a lead halide perovskite film. This architecture allows us to reversibly switch between the single and the dual EIT-like behavior without modifying the metamaterial structures. The dual EIT-like nature is further confirmed through the in-plane electric field distributions and the slow-light effects. The specific structural symmetry further allowed us to effectively switch between the dual EIT-like response and the conventional strong-coupling responses. Such tunability bears potential implications for developing metamaterial-phonon-based tunable THz devices such as switches, filters, and slow-light devices.

physics.optics↗

arXiv2025

Field-derivative torque induced magnetization reversal in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$

Understanding the mechanism of spin switching in ferrimagnets via the excitation of THz pulses holds promise for future-generation magnetic memory devices. Such spin switching can be accomplished by the Zeeman torque exerted by the THz pulses on the magnetic spins. Theoretical and experimental works have established that the field-derivative of a terahertz pulse also exerts a torque, field derivative torque (FDT). Here, we investigate the role of the FDT in the spin switching in ferrimagnetic Gd$_{3/2}$Yb$_{1/2}$BiFe$_5$O$_{12}$ using a computational approach. Our results foresee that the spin switching in the presence of the FDT requires less THz magnetic fields than the spin switching without the FDT. Without the FDT terms, the spin switching in the considered system requires an extremely high magnetic field. Furthermore, we compute the switching and non-switching contour diagrams to show that the FDT tremendously enhances the possibility of spin switching. These results not only shed light on the significance of the FDT in magnetization switching but also suggest materials where the switching effect is pronounced.

cond-mat.mtrl-sci↗

arXiv2024

Discovery of a non-Hermitian phase transition in a bulk condensed-matter system

Phase transitions are fundamental in nature. A small parameter change near a critical point leads to a qualitative change in system properties. Across a regular phase transition, the system remains in thermal equilibrium and, therefore, experiences a change of static properties, like the emergence of a magnetisation upon cooling a ferromagnet below the Curie temperature. When driving a system far from equilibrium, novel, otherwise inaccessible quantum states of matter may arise. Such states are typically non-Hermitian, that is, their dynamics break time-reversal symmetry, a basic law of equilibrium physics. Phase transitions in non-Hermitian systems are of fundamentally new nature in that the dynamical behaviour rather than static properties may undergo a qualitative change at a critical, here called exceptional point. Here we experimentally realize a non-Hermitian phase transition in a bulk condensed-matter system. Optical excitation creates charge carriers in the ferromagnetic semiconductor EuO. In a temperature-dependent interplay with the Hermitian transition to ferromagnetic order, a non-Hermitian change of the relaxation dynamics occurs, manifesting in our time-resolved reflection data as a transition from bi-exponential real to single-exponential complex decay. Our theory models this behavior and predicts non-Hermitian phase transitions for a large class of condensed-matter systems, where they may be exploited to sensitively control bulk-dynamic properties.

cond-mat.str-el↗

arXiv2024

Role of material-dependent properties in THz field-derivative-torque-induced nonlinear magnetization dynamics

The traditional Landau-Lifshitz-Gilbert (LLG) equation has often delineated the linear and nonlinear magnetization dynamics, even at ultrashort timescales e.g., femtoseconds. In contrast, several other non-relativistic and relativistic spin torques have been reported as an extension of the LLG spin dynamics. Here, we explore the contribution of the relativistic field-derivative torque (FDT) in the nonlinear THz magnetization dynamics response applied to ferrimagnets with high Gilbert damping and exchange magnon frequency. Our findings suggest that the FDT plays a significant role in magnetization dynamics in both linear and nonlinear regimes, bridging the gap between the traditional LLG spin dynamics and experimental observations. We find that the coherent THz magnon excitation amplitude is enhanced with the field-derivative torque. Furthermore, a phase shift in the magnon oscillation is induced by the FDT term. This phase shift is almost 90 for the antiferromagnet, while it is almost zero for the ferrimagnet under our investigation. Analyzing the dual THz excitation and their FDT, we find that the nonlinear signals can not be distinctly observed without the FDT terms. However, the inclusion of the FDT terms produces distinct nonlinear signals which matches extremely well with the previously reported experimental results.

cond-mat.mtrl-sci↗

arXiv2024

Missing spectral weight in a heavy-fermion system far above N\'eel temperature

The competition between the Kondo spin-screening effect and the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in heavy-fermion systems drives the quantum phase transition between the magnetically ordered and the heavy-Fermi-liquid ground states. Despite intensive investigations of heavy quasiparticles on the Kondo-screened side of the quantum phase transition and of their breakdown at the quantum critical point, the magnetically ordering side has not systematically been studied. Using terahertz time-domain spectroscopy, we report a suppression of the Kondo quasiparticle weight in CeCu$_{6-x}$Au$_x$ samples on the antiferromagnetic side of the quantum phase transition at temperatures as much as two orders of magnitude above the N\'{e}el temperature $T_\text{N}$. With our systematic investigations into the high-temperature, paramagnetic region on the antiferromagnetic side of the phase diagram of CeCu$_{6-x}$Au$_x$, i.e., with $x =$ 0.2, 0.3, and 0.5, we show that the suppression results from a quantum frustration effect induced by the temperature-independent RKKY interaction. Hence, our results emphasize that besides critical fluctuations, the RKKY interaction may play an important role in the quantum-critical scenario.

cond-mat.str-el↗

arXiv2024

Evidence of relativistic field-derivative torque in nonlinear THz response of magnetization dynamics

Understanding the complete light-spin interactions in magnetic systems is the key to manipulating the magnetization using optical means at ultrafast timescales. The selective addressing of spins by terahertz (THz) electromagnetic fields via Zeeman torque is one of the most successful ultrafast means of controlling magnetic excitations. Here we show that this traditional Zeeman torque on the spins is not sufficient, rather an additional relativistic field-derivative torque is essential to realize the observed magnetization dynamics. We accomplish this by exploring the ultrafast nonlinear magnetization dynamics of rare-earth, Bi-doped iron garnet when excited by two co-propagating THz pulses. First, by exciting the sample with an intense THz pulse and probing the magnetization dynamics using magneto-optical Faraday effect, we find the collective exchange resonance mode between rare-earth and transition metal sublattices at 0.48 THz. We further explore the magnetization dynamics via the THz time-domain spectroscopic means. We find that the observed nonlinear trace of the magnetic response cannot be mapped to the magnetization precession induced by the Zeeman torque, while the Zeeman torque supplemented by an additional field-derivative torque follows the experimental evidences. This breakthrough enhances our comprehension of ultra-relativistic effects and paves the way towards novel technologies harnessing light-induced control over magnetic systems.

cond-mat.mtrl-sci↗

arXiv2024

Magnetoelectric phase control at domain-wall-like epitaxial oxide multilayers

Ferroelectric domain walls are nanoscale objects that can be created, positioned, and erased on demand. They often embody functional properties that are distinct from the surrounding bulk material. Enhanced conductivity, for instance, is observed at charged ferroelectric domain walls. Regrettably, domain walls of this type are scarce because of the energetically unfavorable electrostatics. This hinders the current technological development of domain-wall nanoelectronics. Here we overcome this constraint by creating robust domain-wall-like objects in epitaxial oxide heterostructures. We design charged head-to-head (HH) and tail-to-tail (TT) junctions with two ferroelectric layers (BaTiO$_{3}$ and BiFeO$_{3}$) that have opposing out-of-plane polarization. To test domain-wall-like functionalities, we insert an ultrathin ferromagnetic La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ layer into the junctions. The interfacial electron or hole accumulation at the interfaces, set by the HH and TT polarization configurations, respectively, controls the LSMO conductivity and magnetization. We thus propose that trilayers reminiscent of artificial domain walls provide magnetoelectric functionality and may constitute an important building block in the design of oxide-based electronic devices.

cond-mat.mtrl-sci↗

arXiv2024

Terahertz crystal electric field transitions in a Kondo-lattice antiferromagnet

Hybridization between the localized f-electrons and the delocalized conduction electrons together with the crystal electric field (CEF) play a determinant role in governing the many-body ground state of a correlated-electron system. Here, we investigate the low-energy CEF states in CeAg_2Ge_2, a prototype Kondo-lattice antiferromagnet where Kondo correlation is found to exist within the antiferromagnetic phase. Using time-domain THz reflection spectroscopy, we show the first direct evidence of two low-energy CEF transitions at 0.6 THz (2.5 meV) and 2.1 THz (8.7 meV). The presence of low-frequency infrared-active phonon modes further manifests as a Fano-modified lineshape of the 2.1 THz CEF conductivity peak. The temporal spectral weights obtained directly from the THz time traces, in addition, corroborate the corresponding CEF temperature scales of the compound.

cond-mat.str-el↗

arXiv2023

Kondo coherence versus superradiance in THz radiation-driven heavy-fermion systems

In strongly correlated systems such as heavy-fermion materials, the coherent superposition of localized and mobile spin states leads to the formation of Kondo resonant states, which on a dense, periodic array of Kondo ions develop lattice coherence. Characteristically, these quantum-coherent superposition states respond to a terahertz (THz) excitation by a delayed THz pulse on the scale of the material's Kondo energy scale and, hence, independent of the pump-light intensity. However, delayed response is also typical for superradiance in an ensemble of excited atoms. In this case, quantum coherence is established by the coupling to an external, electromagnetic mode and, hence, dependent on the pump-light intensity. In the present work, we investigate the physical origin of the delayed pulse, i.e., inherent, correlation-induced versus light-induced coherence, in the prototypical heavy-fermion compound CeCu_5.9Au_0.1. We study the delay, duration and amplitude of the THz pulse at various temperatures in dependence on the electric-field strength of the incident THz excitation, ranging from 0.3 to 15.2 kV/cm. We observe a robust delayed response at approximately 6 ps with an amplitude proportional to the amplitude of the incident THz wave. This is consistent with theoretical expectation for the Kondo-like coherence and thus provides compelling evidence for the dominance of condensed-matter versus optical coherence in the heavy-fermion compound.

cond-mat.str-el↗

arXiv2022

Critical slowing down of fermions near a magnetic quantum phase transition

A universal phenomenon in phase transitions is critical slowing down (CSD) - systems, after an initial perturbation, take an exceptionally long time to return to equilibrium. It is universally observed in the dynamics of bosonic excitations, like order-parameter collective modes, but it is not generally expected to occur for fermionic excitations because of the half-integer nature of the fermionic spin. Direct observation of CSD in fermionic excitations or quasiparticles would therefore be of fundamental significance. Here, we observe fermionic CSD in the heavy-fermion (HF) compound YbRh$_2$Si$_2$ by terahertz time-domain spectroscopy. HFs are compound objects with a strongly enhanced effective mass, composed of itinerant and localized electronic states. We see that near the quantum phase transition in YbRh$_2$Si$_2$ the build-up of spectral weight of the HFs towards the Kondo temperature $T_K\approx 25$ K is followed by a logarithmic rise of the quasiparticle excitation rate on the heavy-Fermi-liquid side of the quantum phase transition below $10$ K. A critical two-band HF liquid theory shows that this is indicative of fermionic CSD. This CSD is a clear indication that the HF quasiparticles experience a breakdown near the quantum phase transition, and the critical exponent of this breakdown introduces a classification of fermionic quantum phase transitions analogous to thermodynamic phase transitions - solution to a long-standing problem.

cond-mat.str-el↗

arXiv2020

Terahertz Conductivity of Heavy-fermion Systems from Time-resolved Spectroscopy

The Drude model describes the free-electron conduction in simple metals, governed by the freedom that the mobile electrons have within the material. In strongly correlated systems, however, a significant deviation of the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate the optical conductivity of the heavy-fermion system CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$, using time-resolved, phase-sensitive terahertz spectroscopy. Terahertz electric field creates two types of excitations in heavy-fermion materials: First, the intraband excitations that leave the heavy quasiparticles intact. Second, the resonant interband transitions between the heavy and light parts of the hybridized conduction band that break the Kondo singlet. We find that the Kondo-singlet breaking interband transitions do not create a Drude peak, while the Kondo-retaining intraband excitations yield the expected Drude response; thus, making it possible to separate these two fundamentally different correlated contributions to the optical conductivity.

cond-mat.str-el↗

arXiv2019

Fermi volume evolution and crystal field excitations in heavy-fermion compounds probed by time-domain terahertz spectroscopy

We measure the quasiparticle weight in the heavy-fermion compound CeCu$_{6-x}$Au$_{x}$ ($x=0,\ 0.1$) by time-resolved THz spectroscopy for temperatures from 2 up to 300\,K. This method distinguishes contributions from the heavy Kondo band and from the crystal-electric-field satellite bands by different THz response delay times. We find that the formation of heavy bands is controlled by an exponentially enhanced, high-energy Kondo scale once the crystal-electric-field states become thermally occupied. We corroborate these observations by temperature-dependent dynamical mean-field calculations for the multi-orbital Anderson lattice model and discuss consequences for quantum critical scenarios.

cond-mat.str-el↗

arXiv2018

Unveiling spectral purity and tunability of terahertz quantum cascade laser sources based on intra-cavity difference frequency generation

Terahertz sources based on intra-cavity difference-frequency generation in mid-infrared quantum cascade lasers (THz DFG-QCLs) have recently emerged as the first monolithic electrically-pumped semiconductor sources capable of operating at room-temperature (RT) across the 1-6 THz range. Despite tremendous progress in power output, that now exceeds 1mW in pulsed and 10 μW in continuous-wave regime at room-temperature, knowledge of the major figure of merits of these devices for high precision spectroscopy, such as spectral purity and absolute frequency tunability, is still lacking. Here, by exploiting a metrological grade system comprising a terahertz frequency comb synthesizer, we measure, for the first time, the free-running emission linewidth (LW), the tuning characteristics, and the absolute frequency of individual emission lines of these sources with an uncertainty of 4 x 10-10. The unveiled emission LW (400 kHz at 1ms integration time) indicates that DFG-QCLs are well suited to operate as local oscillators and to be used for a variety of metrological, spectroscopic, communication, and imaging applications requiring narrow-linewidth THz sources.

physics.app-ph↗