arXiv subjects

S. -J. Lee

Publications and source records attributed to S. -J. Lee.

11 recordsLinked to original sources

arXiv2026

Mitigation of Initial Transients in Total-f Gyrokinetic Turbulence Simulations Using Neoclassically Relaxed Distribution Function

Total-f five-dimensional gyrokinetic simulations are essential for self-consistent studies of multi-scale, multiphysics transport in the edge region of diverted tokamak plasmas. However, conventional initialization with a local Maxwellian distribution often generates large-amplitude transients, particularly geodesic acoustic modes (GAMs). These transients are especially severe in the plasma edge because of steep profile gradients, strong radial electric fields, and high safety factors, and they increase the computational time required to reach a saturated turbulent state. To address this problem, we present a new initialization scheme for the total-f XGC code that uses a relaxed particle distribution obtained from a computationally inexpensive axisymmetric simulation. Before the distribution is transferred to the full turbulence simulation, phase-space smoothing is applied to reduce particle noise while preserving its neoclassical structure. Applications to the Cyclone Base Case and an ASDEX Upgrade I-mode discharge demonstrate substantial suppression of transient GAMs, reduced particle noise, and a significant reduction in time to solution.

physics.plasm-ph↗

arXiv2026

A Shifted $t$-Schur Weight from the Modified Odd Operator

We study the one-time weight on strict partitions obtained from the modified odd Greaves--Jing--Zhu operator. The shifted $t$-Schur functions generated by this operator are obtained from the classical Schur $Q$-functions by the plethystic substitution $X\mapsto X-tX$. Thus the corresponding weight \[ λ\longmapsto \mathcal Q_λ(X;t)P_λ(Y) \] is a shifted Schur weight with a virtual first alphabet. We give its normalization, its Pfaffian correlation kernel, its Fredholm Pfaffian for the largest part, and its size cumulants. For $t=-q$ with $q\geq 0$ the virtual alphabet becomes the positive alphabet $X+qX$, giving a genuine probability measure. This positive specialization is the one-time marginal of the two-color lift considered in a companion note.

math.CO↗

arXiv2026

A Two-Color Lift of the Shifted $t$-Schur Measure

At the specialization $t=-q$, $q\geq0$, the shifted $t$-Schur function associated with the modified odd Greaves--Jing--Zhu operator is $Q_λ[X+qX]$. Instead of merging the two alphabets $X$ and $qX$, we insert an intermediate strict partition between the two corresponding half-vertex operators. This gives a two-color lift of the shifted Schur measure on pairs $μ\subseteqλ$ with weight \[ Q_μ(qX)Q_{λ/μ}(X)P_λ(Y). \] We compute the normalization and both marginals, identify an explicit Markov transition kernel, prove a semigroup property, and show that the two color volumes $|μ|$ and $|λ|-|μ|$ are independent. We also realize the model as a two-time shifted Schur process and write its Pfaffian correlation kernel in Vuletić's convention. Rectangular specializations give closed formulas and Gaussian limits for the color volumes.

math.PR↗

arXiv2026

Transition Matrices between Shifted $t$-Schur Bases and Cyclotomic Schur $Q$-Positivity

For a strict partition $λ$, let $\mathcal Q_λ(X;t)=Q_λ[X-tX]$ be the shifted $t$-Schur function arising from the modified Greaves--Jing--Zhu operator on the odd power-sum ring. We study transition matrices between the shifted bases with parameters $t$ and $s$. The relative scaling operator is diagonal in the odd power-sum basis, leading to explicit spectral data, determinant and trace formulas, weighted symmetry, a spin-character formula, and a transition Cauchy identity. For the cyclotomic specialization $C_{λμ}^{[M]}(t)=C_{λμ}(t^M,t)$, the relative operator becomes plethystic substitution by $1+t+\cdots+t^{M-1}$. We prove Schur $Q$-positivity and reciprocity, derive factorization and root-of-unity rank formulas, and give an exact computation method. For $M=2$, all one-row transitions are computed explicitly, and the nonzero coefficients are unimodal.

math.CO↗

arXiv2026

Mixed Products of Modified Greaves--Jing--Zhu Operators

Let $\mathcal Y(z;t)$ be the modified Greaves--Jing--Zhu operator on the odd power-sum ring. We first point out that this operator can be obtained from the classical neutral operator by a simple diagonal change of variables. We then study products in which the two deformation parameters are not necessarily the same. For two parameters $t$ and $s$, we compute the scalar factor that appears in the mixed product. This factor has an explicit exponential form and, in a completed setting, can also be written as a quotient of infinite $t$-Pochhammer products. We also give a recurrence for its coefficients, a product formula for several mixed operators, and formulas for the coefficients obtained after applying the operators to $\mathbf 1$. A particularly simple case occurs when $s=t^M$. In this case the scalar factor becomes the finite quotient $(u;t)_M/(-u;t)_M$. Its coefficients are signed principal specializations of one-row Schur $Q$-functions. As a result, after removing the signs, these coefficients are nonnegative palindromic polynomials. We also give a Gaussian-binomial formula and a finite-order recurrence.

math.CO↗

arXiv2026

A Modified Greaves--Jing--Zhu Operator and a Shifted $t$-Gessel Formula

The recent work of Greaves, Jing, and Zhu gives an operator construction for the $t$-Schur functions and the $t$-Schur measure. Motivated by their construction, we consider the same type of vertex operator on the odd power-sum ring. Its Fourier modes generate a family of symmetric functions indexed by strict partitions, which we call shifted $t$-Schur functions. These functions specialize to Schur $Q$-functions at $t=0$. We derive a two-row formula, a Pfaffian Giambelli formula, a Cauchy identity, and a finite shifted Gessel-type formula. This note is intended as a first step toward further study of the odd-operator analogue of the Greaves--Jing--Zhu construction.

math.CO↗

arXiv2022

Characterization of photoinduced normal state through charge density wave in superconducting YBa$_2$Cu$_3$O$_{6.67}$

The normal state of high-Tc cuprates has been considered one of the essential topics in high-temperature superconductivity research. However, compared to the high magnetic fields study of it, understanding a photoinduced normal state remains elusive. Here, we explore a photoinduced normal state of YBa$_2$Cu$_3$O$_{6.67}$ (YBCO) through a charge density wave (CDW) with time-resolved resonant soft x-ray scattering, as well as a high-magnetic field x-ray scattering. In the non-equilibrium state in which people predict a quenched superconducting state based on the previous optical spectroscopies, we experimentally observed a similar analogy to the competition between superconductivity and CDW shown in the equilibrium state. We further observe that the broken pairing states in the superconducting CuO$_2$ plane via the optical pump lead to nucleation of three-dimensional CDW precursor correlation, revealing that the photoinduced CDW is similar to phenomena shown under magnetic fields. Ultimately, these findings provide a critical clue that the characteristics of the photoinduced normal state show a solid resemblance to those under magnetic fields in equilibrium conditions.

cond-mat.supr-con↗

arXiv2021

Two-dimensional superconducting fluctuations associated with charge density wave stripes in La$_{1.87}$Sr$_{0.13}$Cu$_{0.99}$Fe$_{0.01}$O$_4$

The presence of a small concentration of in-plane Fe dopants in La$_{1.87}$Sr$_{0.13}$Cu$_{0.99}$Fe$_{0.01}$O$_4$ is known to enhance stripe-like spin and charge density wave (SDW and CDW) order, and suppress the superconducting $T_c$. Here, we show that it also induces highly two-dimensional (2D) superconducting correlations that have been argued to be signatures of a new form of superconducting order, so-called pair-density-wave (PDW) order. In addition, using the resonant soft x-ray scattering, we find that the 2D superconducting fluctuation is strongly associated with the CDW stripe. In particular, the PDW signature first appears when the correlation length of the CDW stripe grows over eight times the lattice unit ($\sim$ 8$a$). These results provide critical conditions for the formation of PDW order.

cond-mat.supr-con↗

arXiv2020

Detection of the Chiral Spin Structure in Ferromagnetic SrRuO$_3$ Thin Film

A SrRuO$_3$ (SRO) thin film and its heterostructure have brought much attention because of the recently demonstrated fascinating properties, such as topological Hall effect and skyrmions. Critical to the understanding of those SRO properties is the study of the spin configuration. Here, we conduct resonant soft x-ray scattering (RSXS) at oxygen K-edge to investigate the spin configuration of a 4 unit-cell SRO film that was grown epitaxially on a single crystal SrTiO$_3$. The RSXS signal under a magnetic field (~0.4 Tesla) clearly shows a magnetic dichroism pattern around the specular reflection. Model calculations on the RSXS signal demonstrate that the magnetic dichroism pattern originates from a Néel-type chiral spin structure in this SRO thin film. We believe that the observed spin structure of the SRO system is a critical piece of information for understanding its intriguing magnetic and transport properties.

cond-mat.mtrl-sci↗

arXiv2019

Observation of two types of charge density wave orders in superconducting La$_{2-x}$Sr$_x$CuO$_4$

The discovery of charge- and spin-density-wave (CDW/SDW) orders in superconducting cuprates has altered our perspective on the nature of high-temperature superconductivity (SC). However, it has proven difficult to fully elucidate the relationship between the density wave orders and SC. Here using resonant soft X-ray scattering we study the archetypal cuprate, La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) over a broad doping range. We reveal the existence of two types of CDW orders in LSCO, namely CDW stripe order and CDW short-range order (SRO). While the CDW-SRO is suppressed by SC, it is partially transformed into the CDW stripe order with developing SDW stripe order near the superconducting $T_{\rm c}$. These findings indicate that the stripe orders and SC are inhomogeneously distributed in the superconducting CuO$_2$ planes of LSCO. This further suggests a new perspective on the putative pair-density-wave order that coexists with SC, SDW, and CDW orders.

cond-mat.supr-con↗

arXiv2011

Sondheimer Oscillation as a Fingerprint of Surface Dirac Fermions

Topological states of matter challenge the paradigm of symmetry breaking, characterized by gapless boundary modes and protected by the topological property of the ground state. Recently, angle-resolved photoemission spectroscopy (ARPES) has revealed that semiconductors of Bi$_{2}$Se$_{3}$ and Bi$_{2}$Te$_{3}$ belong to such a class of materials. Here, we present undisputable evidence for the existence of gapless surface Dirac fermions from transport in Bi$_{2}$Te$_{3}$. We observe Sondheimer oscillation in magnetoresistance (MR). This oscillation originates from the quantization of motion due to the confinement of electrons within the surface layer. Based on Sondheimer's transport theory, we determine the thickness of the surface state from the oscillation data. In addition, we uncover the topological nature of the surface state, fitting consistently both the non-oscillatory part of MR and the Hall resistance. The side-jump contribution turns out to dominate around 1 T in Hall resistance while the Berry-curvature effect dominates in 3 T $\sim$ 4 T.

cond-mat.str-el↗