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Sukjin Yoon

Publications and source records attributed to Sukjin Yoon.

12 recordsLinked to original sources

Scaling Bidirectional Spans and Span Violations in Attention Mechanism

The canonical $O(N^2)$ Transformer remains the empirical performance frontier in sequence modeling, and its training can be further optimized by addressing geometric inefficiency. We propose an optimization framework that leverages an asymmetric projection to decompose the backward-pass gradients into parallel spans and orthogonal violations, while keeping the canonical forward-pass $QKV$ structure intact. Through consistent experimental validation across various decomposition and projection setups, we provide strong theoretical evidence: the standard attention gradient is suboptimal. We demonstrated that selectively scaling these components, focusing primarily on $0^{th}$ order bidirectional parallel spans, yields the most effective learning signal. On the limited WikiText-2 dataset, and using a crude configuration, this method achieved a $0.56\%$ reduction in validation loss, confirming the framework's fundamental validity and suggesting significant potential gains on larger datasets and deeper training regimes

cs.LG

Exciting the long-lived Higgs mode in superfluid Fermi gases with particle removal

Experimental evidence of the Higgs mode in strongly interacting superfluid Fermi gases had not been observed until recently [Behrle et al., Nat. Phys. 14, 781 (2018)]. Due to the coupling with other collective modes and quasiparticle excitations, generating stable Higgs-mode oscillations is challenging. We study how to excite long-lived Higgs-mode oscillations in a homogeneous superfluid Fermi gas in the BCS-BEC crossover. We find that the Higgs mode can be excited by time-periodically modulating the scattering length at an appropriate amplitude and frequency. However, even for a modulation frequency below twice the pairing-gap energy, quasiparticles are still excited through the generation of higher harmonics due to nonlinearity in the superfluid. More importantly, we find that persistent Higgs-mode oscillations with almost constant amplitude can be produced by removing particles at an appropriate momentum, and the oscillation amplitude can be controlled by the number of removed particles. Finally, we propose two ways to experimentally realize particle removal.

cond-mat.quant-gas

Pairing Dynamics of Polar States in a Quenched p-wave Superfluid Fermi Gas

We study the pairing dynamics of polar states in a single species p-wave superfluid Fermi gas following a sudden change of the interaction strength. The anisotropy of pair interaction together with the presence of the centrifugal barrier results in profoundly different pairing dynamics compared to the s-wave case. Depending on the direction of quenches, quench to the BCS regime results in a large oscillatory depletion of momentum occupation inside the Fermi sea or a large filling of momentum occupation. We elucidate a crucial role of the resonant state supported by the centrifugal barrier in the pairing dynamics.

cond-mat.quant-gas

Multiple period states of the superfluid Fermi gas in an optical lattice

We study multiple period states of a two-component unpolarized superfluid Fermi gas in an optical lattice along the Bardeen-Cooper-Schrieffer (BCS) to Bose-Einstein condensate (BEC) crossover. The existence of states whose period is a multiple of the lattice spacing is a direct consequence of the non-linear behavior of the gas, which is due to the presence of the order parameter associated with superfluidity. By solving Bogoliubov-de Gennes equations for a superfluid flow with finite quasimomentum, we find that, in the BCS side of the crossover, the multiple period states can be energetically favorable compared to the normal Bloch states and their survival time against dynamical instability drastically increases, suggesting that these states can be accessible in current experiments, in sharp contrast to the situation in BECs.

cond-mat.quant-gas

Aspects of Superfluid Cold Atomic Gases in Optical Lattices

We review our studies on Bose and Fermi superfluids of cold atomic gases in optical lattices at zero temperature. Especially, we focus on superfluid Fermi gases along the crossover between the Bardeen-Cooper-Schrieffer (BCS) and the Bose-Einstein condensate (BEC) states, which enable us to study the Bose and the Fermi superfluids in a unified point of view. We discuss basic static and long-wavelength properties (such as the equation of state, incompressibility, and effective mass), energetic stability, and energy band structures of the superfluid Fermi gases in an optical lattice periodic along one spatial direction. The periodic potential causes pairs of atoms to be strongly bound, and this can affect the static and long-wavelength properties and the stability of the superflow. Regarding the band structure, a peculiar loop structure called "swallowtail" can appear in superfluid Fermi gases and in the Bose case, but the mechanism of emergence in the Fermi case is very different from that in bosonic case. Other quantum phases that the cold atomic gases in optical lattices can show are also briefly discussed based on their roles as quantum simulators of Hubbard models.

cond-mat.quant-gas

Swallowtail Band Structure of the Superfluid Fermi Gas in an Optical Lattice

We investigate the energy band structure of the superfluid flow of ultracold dilute Fermi gases in a one-dimensional optical lattice along the BCS to BEC crossover within a mean-field approach. In each side of the crossover region, a loop structure (swallowtail) appears in the Bloch energy band of the superfluid above a critical value of the interaction strength. The width of the swallowtail is largest near unitarity. Across the critical value of the interaction strength, the profiles of density and pairing field change more drastically in the BCS side than in the BEC side. It is found that along with the appearance of the swallowtail, there exists a narrow band in the quasiparticle energy spectrum close to the chemical potential and the incompressibility of the Fermi gas consequently experiences a profound dip in the BCS side, unlike in the BEC side.

cond-mat.quant-gas

Induced P-wave superfluidity within full energy-momentum dependent Eliashberg approximation in asymmetric dilute Fermi gases

We consider a very asymmetric system of Fermions with an interaction characterized by a positive scattering length only. The minority atoms pair and form a BEC of dimers, while the surplus fermions interact only indirectly through the exchange of Bogoliubov sound modes. This interaction has a finite range, the retardation effects are significant and the surplus fermions will form a P-wave superfluid. We compute the P-wave pairing gap in the BCS and Eliashberg with only energy dependence approximations, and demonstrate their inadequacy in comparison with a full treatment of the momentum and energy dependence of the induced interaction. The pairing gap computed with a full momentum and energy dependence is significantly larger in magnitude and that makes it more likely that this new exotic paired phase could be put in evidence in atomic trap experiments.

cond-mat.stat-mech

Large Amplitude Dynamics of the Pairing Correlations in a Unitary Fermi Gas

A unitary Fermi gas has a surprisingly rich spectrum of large amplitude modes of the pairing field alone, which defies a description within a formalism involving only a reduced set of degrees of freedom, such as quantum hydrodynamics or a Landau-Ginzburg-like description. These modes are very slow, with oscillation frequencies well below the pairing gap, which makes their damping through quasiparticle excitations quite ineffective. In atomic traps these modes couple naturally with the density oscillations, and the corresponding oscillations of the atomic cloud are an example of a new type of collective mode in superfluid Fermi systems. They have lower frequencies than the compressional collective hydrodynamic oscillations, have a non-spherical momentum distribution, and could be excited by a quick time variation of the scattering length.

cond-mat.stat-mech

Adjoint Trapping: A New Phenomenon at Strong 't Hooft Coupling

Adding matter of mass m, in the fundamental representation of SU(N), to N=4 supersymmetric Yang-Mills theory, we study ``generalized quarkonium'' containing a (s)quark, an anti(s)quark, and J massless (or very light) adjoint particles. At large 't Hooft coupling $λ$ >> 1, the states of spin <= 1 are surprisingly light (Kruczenski et al., hep-th/0304032) and small (hep-th/0312071) with a J-independent size of order $\sqrtλ/m$. This ``trapping'' of adjoint matter in a region small compared with its Compton wavelength and compared to any confinement scale in the theory is an unfamiliar phenomenon, as it does not occur at small $λ$. We explore adjoint trapping further by considering the limit of large J. In particular, for J >> $\sqrtλ$ >> 1, we expect the trapping phenomenon to become unstable. Using Wilson loop methods, we show that a sharp transition, in which the generalized quarkonium states become unbound (for massless adjoints) occurs at $J \simeq 0.22 \sqrtλ$. If the adjoint scalars of N=4 are massive and the theory is confining (as, for instance, in N=1* theories) then the transition becomes a cross-over, across which the size of the states changes rapidly from ~$\sqrtλ/m$ to something of order the confinement scale ~ $Λ^{-1}$.

hep-th

On the Couplings of Vector Mesons in AdS/QCD

We address, in the AdS/CFT context, the issue of the universality of the couplings of the rho meson to other hadrons. Exploring some models, we find that generically the rho-dominance prediction f_ρg_{ρH H}=m_ρ^2 does not hold, and that g_{ρH H} is not independent of the hadron H. However, we prove that, in any model within the AdS/QCD context, there are two limiting regimes where the g_{ρH H}, along with the couplings of all excited vector mesons as well, become H-independent: (1) when H is created by an operator of large dimension, and (2) when H is a highly-excited hadron. We also find a sector of a particular model where universality for the rho coupling is exact. Still, in none of these cases need it be true that f_ρg_ρ=m_ρ^2, although we find empirically that the relation does hold approximately (up to a factor of order two) within the models we have studied.

hep-th

On the Couplings of the Rho Meson in AdS/QCD

We argue that in generic AdS/QCD models (confining gauge theories dual to string theory on a weakly-curved background), the couplings g_{rho HH} of any rho meson to any hadron H are quasi-universal, lying within a narrow band near m(rho)^2/f(rho). The argument relies upon the fact that the rho is the lowest-lying state created by a conserved current, and the detailed form of the integrals which determine the couplings in AdS/QCD. Quasi-universality holds even when rho-dominance is violated. The argument fails for all other hadrons, except the lowest-lying spin-two hadron created by the energy-momentum tensor. Explicit examples are discussed.

hep-ph

Quarkonium from the Fifth Dimension

Adding fundamental matter of mass m_Q to N=4 Yang Mills theory, we study quarkonium, and "generalized quarkonium" containing light adjoint particles. At large 't Hooft coupling the states of spin<=1 are anomalously light (Kruczenski et al., hep-th/0304032). We examine their form factors, and show these hadrons are unlike any known in QCD. By a traditional yardstick they appear infinite in size (as with strings in flat space) but we show that this is a failure of the yardstick. All of the hadrons are actually of finite size ~ \sqrt{g^2N}/m_Q, regardless of their radial excitation level and of how many valence adjoint particles they contain. Certain form factors for spin-1 quarkonia vanish in the large-g^2N limit; thus these hadrons resemble neither the observed J/Psi quarkonium states nor rho mesons.

hep-th