arXiv subjects

Seokjin Bae

Publications and source records attributed to Seokjin Bae.

11 recordsLinked to original sources

arXiv2025

Microscopic evidence of dominant excitonic instability in Ta2NiSe5

An excitonic insulator (EI) is a charge-neutral bosonic condensate of spontaneously formed electron-hole pairs. Exotic quantum phenomena such as dissipationless charge neutral transport and huge potential for optoelectronic applications have led to an extensive search for intrinsic bulk materials that are EIs at ambient pressure without sophisticated device fabrication. The narrow gap semimetal Ta2NiSe5 has been proposed as a rare example of an intrinsic EI, but its ground state has remained controversial since the EI phase transition is accompanied by a structural distortion. Here, we use scanning tunneling microscopy and spectroscopy to present microscopic evidence that supports an excitonic origin of the insulating phase in Ta2NiSe5. First, we find that the insulating gap persists at structural domain boundaries where the bulk structural distortion is absent suggesting that the structural distortion is not the primary origin of insulating ground state. Second, the insulating gap is suppressed at localized charge puddles which indicates that charge correlations are important in producing the insulating gap. Finally, the decay length of the in-gap states at the puddles shows similar value to the estimated size of the exciton wavefunction from previous photoemission studies. Taken together, these findings suggest that Ta2NiSe5 is an EI and may be a versatile platform to study the physics of bosonic condensates in solid-state system at ambient conditions.

cond-mat.str-el↗

arXiv2025

Reply to "Limitations of detecting structural changes and time-reversal symmetry breaking in scanning tunneling microscopy experiments"

In their comment, the authors attribute our observations of light- and magnetic-field-induced manipulation of the CDW state in RbV3Sb5 to random experimental artifacts such as tip changes, drift, and scan conditions, concluding that no intrinsic effect exists. In this reply, we clarify three points. First, our reported magnetic-field-induced CDW switching has been independently replicated in multiple STM studies and corroborated by transport measurements. Second, in our data, the switching of CDW intensities and Bragg vector ratios exhibit high fidelity (nearly 100 $\%$), and this cannot be explained by random artifacts. Lastly, we disprove the comment's unsubstantiated claim about magnetic field values by showing a system-dependent correlation between z-piezo shift and magnetic field. Taken together, our Reply reaffirms the reproducibility and intrinsic nature of light and magnetic field induced CDW manipulation in RbV3Sb5.

cond-mat.supr-con↗

arXiv2025

Dynamic competition between phason and amplitudon observed by ultrafast multimodal scanning tunneling microscopy

The intertwining between two ordered states that arise from the same interactions is reflected in the dynamics of their coupled collective excitations. While the equilibrium phase diagram resulting from such intertwined orders has been extensively studied, the dynamic competition between non-equilibrium modes is a largely unexplored territory. Here, we introduce a multimodal STM-based pump-probe technique, that combines ultrafast tunneling microscopy (USTM), ultrafast point-contact spectroscopy (UPC), and optical pump-probe reflectance (OPPR) on femtosecond timescale, all within a single instrument. Using this platform, we investigate the collective excitations of the unconventional charge density wave insulator (TaSe4)2I. Our UPC measurements reveal charge oscillations at 0.22 THz, with a temperature dependence that matches the theoretically predicted behavior of the long-sought massive phason gaining mass through the Anderson-Higgs mechanism. Unexpectedly, the data also reveals a second mode at 0.11 THz exhibiting similar temperature and polarization dependence with comparable mode intensity. These features, along with the robust 1/2 frequency ratio locking suggest that the 0.11 THz phason is a 'daughter mode' that arises from the splitting of the 0.22 THz massive phason into two massless phasons via parametric amplification, analogous to the decay of a neutral pion into two photons. Strikingly, comparison with OPPR data reveals that the daughter phason competes with and suppresses the amplitudon at proximate frequency. Our studies reveal an unexplored mechanism for the generation and extinction of collective excitations in quantum materials and pave the way for a microscopic understanding of ultrafast phenomena.

cond-mat.str-el↗

arXiv2025

Atomic-scale imaging and charge state manipulation of NV centers by scanning tunneling microscopy

Nitrogen-vacancy (NV) centers in diamond are among the most promising solid-state qubit candidates, owing to their exceptionally long spin coherence times, efficient spin-photon coupling, room-temperature operation, and steadily advancing fabrication and integration techniques. Despite significant progress in the field, atomic-scale characterization and control of individual NV centers have remained elusive. In this work, we present a novel approach utilizing a conductive graphene capping layer to enable direct imaging and manipulation of $NV^{-}$ defects via scanning tunneling microscopy (STM). By investigating over 40 individual $NV^{-}$ centers, we identify their spectroscopic signatures and spatial configurations. Our dI/dV conductance spectra reveal the ground state approximately 300 meV below the Fermi level. Additionally, density-of-states mapping uncovers a two-lobed wavefunction aligned along the [111] crystallographic direction. Remarkably, we demonstrate the ability to manipulate the charge state of the NV centers from $NV^{-}$ to $NV^{0}$ through STM tip-induced gating. This work represents a significant advancement in the atomic-scale understanding and engineering of NV centers, paving the way for future quantum device development.

quant-ph↗

arXiv2024

Optical Manipulation of the Charge Density Wave state in RbV3Sb5

Broken time-reversal symmetry in the absence of spin order indicates the presence of unusual phases such as orbital magnetism and loop currents. The recently discovered family of kagome superconductors AV$_3$Sb$_5$ (A = K, Rb, or Cs), hosting an exotic charge-density wave (CDW) state, has emerged as a strong candidate for this phase. While initial experiments suggested that the CDW phase breaks time-reversal symmetry, this idea is being intensely debated due to conflicting experimental data. In this work we use laser-coupled scanning tunneling microscopy (STM) to study RbV$_3$Sb$_5$. STM data shows that the Fourier intensities of all three CDW peaks are different, implying that the CDW breaks rotational and mirror symmetries. By applying linearly polarized light along high-symmetry directions, we show that the relative intensities of the CDW peaks can be reversibly switched, implying a substantial electro-striction response, indicative of strong non-linear electron-phonon coupling. A similar CDW intensity switching is observed with perpendicular magnetic fields, which implies an unusual piezo-magnetic response that, in turn, requires time-reversal symmetry-breaking. We show that the simplest CDW that satisfies these constraints and reconciles previous seemingly contradictory experimental data is an out-of-phase combination of bond charge order and loop currents that we dub congruent CDW flux phase. Our laser-STM data opens the door to the possibility of dynamic optical control of complex quantum phenomenon in correlated materials.

cond-mat.str-el↗

arXiv2021

Anomalous normal fluid response in a chiral superconductor UTe2

A chiral superconductor has been proposed as one pathway to realize topological quantum computation utilizing the predicted Majorana normal fluid at its boundary (i.e., a point, edge, or surface). The search for experimental realizations has led to the discovery of 1D chiral superconducting systems. However, the long-sought 2D and 3D chiral superconductors with edge and surface Majorana normal fluid are yet to be conclusively found. Here we report evidence for a chiral spin-triplet pairing state of UTe$_2$ with significant surface normal fluid response. The microwave surface impedance of the UTe$_2$ crystal was measured and converted to complex conductivity, which is sensitive to both normal and superfluid responses. The anomalous residual normal fluid conductivity in the zero temperature limit supports the presence of a significant normal fluid response. The superfluid conductivity follows the low temperature behavior predicted for the axial spin-triplet state, which is further narrowed down to the chiral spin-triplet state with evidence of broken time-reversal symmetry. The temperature dependence of the superfluid conductivity also reveals a low bulk impurity scattering rate and low frequency-to-energy-gap ratio, implying that the observed normal fluid response does not have a trivial origin. Our findings suggest that UTe$_2$ can be a new platform to study exotic topological excitations in higher dimension, and may play the role of a versatile 3D building block in the future era of topological quantum computation.

cond-mat.supr-con↗

arXiv2019

Point Node Gap Structure of Spin-Triplet Superconductor UTe2

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe2 to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic a- and b-axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasi-linear increase consistent with the presence of nodal quasiparticles, rising toward the a-axis upper critical field where the Wiedemann-Franz law is recovered. Together with a quadratic temperature dependence of the magnetic penetration depth up to T/T_c=0.3, these measurements provide evidence for an unconventional spin-triplet superconducting order parameter with point nodes. Millikelvin specific heat measurements performed on the same crystals used for thermal transport reveal an upturn below 300 mK that is well described by a divergent quantum-critical contribution to the density of states (DOS). Modeling this contribution with a T^{-1/3} power law allows restoration of the full entropy balance in the superconducting state and a resultant cubic power law for the electronic DOS below T_c, consistent with the point-node gap structure determined by thermal conductivity and penetration depth measurements.

cond-mat.supr-con↗

arXiv2019

Microwave Meissner Screening of Proximity coupled Topological Insulator / Superconductor Bilayers

The proximity coupled topological insulator / superconductor (TI/SC) bilayer system is a representative system to realize topological superconductivity. In order to better understand this unique state and design devices from the TI/SC bilayer, a comprehensive understanding of the microscopic properties of the bilayer is required. In this work, a microwave Meissner screening study, which exploits a high-precision microwave resonator technique, is conducted on the SmB6/YB6 thin film bilayers as an example TI/SC system. The study reveals spatially dependent electrodynamic screening response of the TI/SC system that is not accessible to other techniques, from which the corresponding microscopic properties of a TI/SC bilayer can be obtained. The TI thickness dependence of the effective penetration depth suggests the existence of a bulk insulating region in the TI layer. The spatially dependent electrodynamic screening model analysis provides an estimate for the characteristic lengths of the TI/SC bilayer: normal penetration depth, normal coherence length, and the thickness of the surface states. We also discuss implications of these characteristic lengths on the design of a vortex Majorana device such as the radius of the vortex core, the energy splitting due to intervortex tunneling, and the minimum thickness required for a device.

cond-mat.supr-con↗

arXiv2019

Dielectric Resonator Method For Determining Gap Symmetry Of Superconductors Through Anisotropic Nonlinear Meissner Effect

We present a new measurement method which can be used to image gap nodal structure of superconductors whose pairing symmetry is under debate. This technique utilizes a high quality factor microwave resonance involving the sample of interest. While supporting a circularly symmetric standing wave current pattern, the sample is perturbed by a scanned laser beam, creating a photoresponse that was previously shown to reveal the superconducting gap anisotropy. Simulation and the measurement of the photoresponse of an unpatterned Nb film show less than 8\% anisotropy, as expected for a superconductor with nearly isotropic energy gap along with expected systematic uncertainty. On the other hand, measurement of a YBa$_2$Cu$_3$O$_{7-δ}$ thin film shows a clear 4-fold symmetric image with $\mathtt{\sim}12.5$\% anisotropy, indicating the well-known 4-fold symmetric $d_{x^2-y^2}$ gap nodal structure in the $ab$-plane. The deduced gap nodal structure can be further cross-checked by low temperature surface impedance data, which is simultaneously measured. The important advantage of the presented method over the previous spiral resonator method is that it does not require a complicated lithographic patterning process which limits one from testing various kinds of materials due to photoresponse arising from patterning defects. This advantage of the presented technique, and the ability to measure unpatterned samples such as planar thin films and single crystals, enables one to survey the pairing symmetry of a wide variety of unconventional superconductors.

cond-mat.supr-con↗

arXiv2019

Imaging collective behavior in an rf-SQUID metamaterial tuned by DC and RF magnetic fields

We examine the collective behavior of two-dimensional nonlinear superconducting metamaterials using a non-contact spatially resolved imaging technique. The metamaterial is made up of sub-wavelength nonlinear oscillators in a strongly coupled 27x27 planar array of radio-frequency Superconducting QUantum Interference Devices (rf SQUIDs). By using low-temperature laser scanning microscopy we image microwave currents in the driven SQUIDs while in non-radiating dark modes and identify the clustering and uniformity of like-oscillating meta-atoms. We follow the rearrangement of coherent patterns due to meta-atom resonant frequency tuning as a function of external dc and rf magnetic flux bias. We find that the rf current distribution across the SQUID array at zero dc flux and small rf flux reveals a low degree of coherence. By contrast, the spatial coherence improves dramatically upon increasing of rf flux amplitude, in agreement with simulation.

cond-mat.supr-con↗

arXiv2017

Imaging the paramagnetic nonlinear Meissner effect in nodal gap superconductor

Boundary surfaces of nodal gap superconductors can host Andreev bound states (ABS) which develop a paramagnetic response under external RF field in contrast to the bulk diamagnetic response of the bulk superconductor. At low temperature this surface paramagnetic response dominates and enhances the nonlinear RF response of the sample. With a recently developed photoresponse imaging technique, the anisotropy of this "paramagnetic" nonlinear Meissner response, and its current direction (angular) and RF power dependence has been systematically studied. A theoretical model describing the current flow in the surface paramagnetic Andreev bound state, the bulk diamagnetic Meissner state, and their response to optical illumination is proposed and it shows good agreement with the experimental results.

cond-mat.supr-con↗