arXiv subjects

Jian-Huang She

Publications and source records attributed to Jian-Huang She.

At least 19 recordsLinked to original sources

arXiv2018

Neural Network for CVA: Learning Future Values

A new challenge to quantitative finance after the recent financial crisis is the study of credit valuation adjustment (CVA), which requires modeling of the future values of a portfolio. In this paper, following recent work in [Weinan E(2017), Han(2017)], we apply deep learning to attack this problem. The future values are parameterized by neural networks, and the parameters are then determined through optimization. Two concrete products are studied: Bermudan swaption and Mark-to-Market cross-currency swap. We obtain their expected positive/negative exposures, and further study the resulting functional form of future values. Such an approach represents a new framework for modeling XVA, and it also sheds new lights on other methods like American Monte Carlo.

q-fin.CP↗

arXiv2017

Mechanism for nematic superconductivity in FeSe

Despite its seemingly simple composition and structure, the pairing mechanism of FeSe remains an open problem due to several striking phenomena. Among them are nematic order without magnetic order, nodeless gap and unusual inelastic neutron spectra with a broad continuum, and gap anisotropy consistent with orbital selection of unknown origin. Here we propose a microscopic description of a nematic quantum spin liquid that reproduces key features of neutron spectra. We then study how the spin fluctuations of the local moments lead to pairing within a spin-fermion model. We find the resulting superconducting order parameter to be nodeless $s\pm d$-wave within each domain. Further we show that orbital dependent Hund's coupling can readily capture observed gap anisotropy. Our prediction calls for inelastic neutron scattering in a detwinned sample.

cond-mat.str-el↗

arXiv2016

Strange Metals from Quantum Geometric Fluctuations of Interfaces

Our current understanding of strongly correlated electron systems is based on a homogeneous framework. Here we take a step going beyond this paradigm by incorporating inhomogeneity from the beginning. Specifying to systems near the Mott metal-insulator transition, we propose a real space picture of itinerant electrons functioning in the fluctuating geometries bounded by interfaces between metallic and insulating regions. In 2+1-dimensions, the interfaces are closed bosonic strings, and we have a system of strings coupled to itinerant electrons. When the interface tension vanishes, the geometric fluctuations become critical, which gives rise to non-Fermi liquid behavior for the itinerant electrons. In particular, the poles of the fermion Green's function can be converted to zeros, indicating the absence of propagating quasiparticles. Furthermore, the quantum geometric fluctuations mediate Cooper pairing among the itinerant electrons, indicating the intrinsic instability of electronic systems near the Mott transition.

cond-mat.str-el↗

arXiv2016

Topological Superconductivity in Metal/Quantum-Spin-Ice Heterostructures

The original proposal to achieve superconductivity by starting from a quantum spin-liquid (QSL) and doping it with charge carriers, as proposed by Anderson in 1987, has yet to be realized. Here we propose an alternative strategy: use a QSL as a substrate for heterostructure growth of metallic films to design exotic superconductors. By spatially separating the two key ingredients of superconductivity, i.e., charge carriers (metal) and pairing interaction (QSL), the proposed setup naturally lands on the parameter regime conducive to a controlled theoretical prediction. Moreover, the proposed setup allows us to "customize" electron-electron interaction imprinted on the metallic layer. The QSL material of our choice is quantum spin ice well-known for its emergent gauge-field description of spin frustration. Assuming the metallic layer forms an isotropic single Fermi pocket, we predict that the coupling between the emergent gauge-field and the electrons of the metallic layer will drive topological odd-parity pairing. We further present guiding principles for materializing the suitable heterostructure using ab initio calculations and describe the band structure we predict for the case of Y$_2$Sn$_{2-x}$Sb$_x$O$_7$ grown on the (111) surface of Pr$_2$Zr$_2$O$_7$. Using this microscopic information, we predict topological odd-parity superconductivity at a few Kelvin in this heterostructure, which is comparable to the $T_c$ of the only other confirmed odd-parity superconductor Sr$_2$RuO$_4$.

cond-mat.str-el↗

arXiv2015

Anomalous Scaling of the Penetration Depth in Nodal Superconductors

Recent findings of anomalous super-linear scaling of low temperature ($T$) penetration depth (PD) in several nodal superconductors near putative quantum critical points suggest that the low temperature PD can be a useful probe of quantum critical fluctuations in a superconductor. On the other hand, cuprates which are poster child nodal superconductors have not shown any such anomalous scaling of PD, despite growing evidence of quantum critical points. Then it is natural to ask when and how can quantum critical fluctuations cause anomalous scaling of PD? Carrying out the renormalization group calculation for the problem of two dimensional superconductors with point nodes, we show that quantum critical fluctuations associated with point group symmetry reduction result in non-universal logarithmic corrections to the $T$-dependence of the PD. The resulting apparent power law depends on the bare velocity anisotropy ratio. We then compare our results to data sets from two distinct nodal superconductors: YBa$_2$Cu$_3$O$_{6.95}$ and CeCoIn$_5$. Considering all symmetry-lowering possibilities of the point group of interest, $C_{4v}$, we find our results to be remarkably consistent with YBa$_2$Cu$_3$O$_{6.95}$ being near vertical nematic QCP, and CeCoIn$_5$ being near diagonal nematic QCP. Our results motivate search for diagonal nematic fluctuations in CeCoIn$_5$.

cond-mat.str-el↗

arXiv2014

Negative-U Superconductivity on the Surface of Topological Insulators

We study the effects of a finite density of negative-$U$ centers (NUCs) on the surface of a three-dimensional topological insulator. The surface Dirac fermions mediate a power-law interaction among the local Cooper pairs at the NUCs, and the interaction remains long-ranged for weak disorder. Superconductivity can be generated in the presence of a random distribution of NUCs. The NUCs play dual roles as both pair creator and pair breaker, and the competition of the two effects results in non-monotonic dependence of the mean field superconducting transition temperature on the density of NUCs. Global phase coherence is established through coupling the locally superconducting puddles via Josephson coupling. Rare fluctuations play important roles, and a globally superconducting phase can only be achieved at large enough concentration of NUCs. The p-wave component of the superconducting order parameter gives rise to frustration among the superconducting grains, which is captured by a Potts-XY type model. New phases with chiral order, glass order, and possibly topological order can then emerge in the system of superconducting grains.

cond-mat.str-el↗

arXiv2013

RKKY Interaction and Intrinsic Frustration in Non-Fermi Liquid Metals

We study RKKY interaction in non-Fermi liquid metals. We find that the RKKY interaction mediated by non-Fermi liquid metals can be of much longer range than for a Fermi liquid. The oscillatory nature of RKKY interaction thus becomes more important in non-Fermi liquids, and gives rise to enhanced frustration when the spins form a lattice. Frustration suppresses the magnetic ordering temperature of the lattice spin system. Furthermore, we find that the spin system with longer range RKKY interaction can be described by the Brazovskii model, where the ordering wavevector lies on a shell with constant radius. Strong fluctuations in such model lead to a first-order phase transition and/or glassy phase. This may explain some recent experiments where glassy behavior was observed in stoichiometric heavy fermion material close to a ferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2012

Inelastic Electron Tunneling Spectroscopy for Topological Insulators

Inelastic Electron Tuneling Spectroscopy (IETS) is a powerful spectroscopy that allows one to investigate the nature of local excitations and energy transfer in the system of interest. We study IETS for Topological Insulators (TI) and investigate the role of inelastic scattering on the Dirac node states on the surface of TIs. Local inelastic scattering is shown to significantly modify the Dirac node spectrum. In the weak coupling limit, peaks and steps are induced in second derivative $d^2I/dV^2$. In the strong coupling limit, the local negative U centers are formed at impurity sites, and the Dirac cone structure is fully destroyed locally. At intermediate coupling resonance peaks emerge. We map out the evolution of the resonance peaks from weak to strong coupling, which interpolate nicely between the two limits. There is a sudden qualitative change of behavior at intermediate coupling, indicating the possible existence of a local quantum phase transition. We also find that even for a simple local phonon mode the inherent coupling of spin and orbital degrees in TI leads to the spin polarized texture in inelastic Friedel oscillations induced by local mode.

cond-mat.mes-hall↗

arXiv2012

Berezinskii-Kosterlitz-Thouless Transition in Heavy Fermion Superlattices

We propose an explanation of the superconducting transitions discovered in the heavy fermion superlattices by Mizukami et al. (Nature Physics 7, 849 (2011)) in terms of Berezinskii-Kosterlitz-Thouless transition. We observe that the effective mass mismatch between the heavy fermion superconductor and the normal metal regions provides an effective barrier that enables quasi 2D superconductivity in such systems. We show that the resistivity data, both with and without magnetic field, are consistent with BKT transition. Furthermore, we study the influence of a nearby magnetic quantum critical point on the vortex system, and find that the vortex core energy can be significantly reduced due to magnetic fluctuations. Further reduction of the gap with decreasing number of layers is understood as a result of pair breaking effect of Yb ions at the interface.

cond-mat.str-el↗

arXiv2012

Non-contact Friction and Relaxational Dynamics of Surface Defects

Motion of cantilever near sample surfaces exhibits additional friction even before two bodies come into mechanical contact. Called non-contact friction (NCF), this friction is of great practical importance to the ultrasensitive force detection measurements. Observed large NCF of a micron-scale cantilever found anomalously large damping that exceeds theoretical predictions by 8-11 orders of magnitude. This finding points to contribution beyond fluctuating electromagnetic fields within van der Waals approach. Recent experiments reported by Saitoh et al. (Phys. Rev. Lett. 105, 236103 (2010)) also found nontrivial distance dependence of NCF. Motivated by these observations, we propose a mechanism based on the coupling of cantilever to the relaxation dynamics of surface defects. We assume that the surface defects couple to the cantilever tip via spin-spin coupling and their spin relaxation dynamics gives rise to the backaction terms and modifies both the friction coefficient and the spring constant. We explain the magnitude, as well as the distance dependence of the friction due to these backaction terms. Reasonable agreement is found with the experiments.

cond-mat.mes-hall↗

arXiv2011

Observing the origin of superconductivity in quantum critical metals

Despite intense efforts during the last 25 years, the physics of unconventional superconductors, including the cuprates with a very high transition temperature, is still a controversial subject. It is believed that superconductivity in many of these strongly correlated metallic systems originates in the physics of quantum phase transitions, but quite diverse perspectives have emerged on the fundamentals of the electron-pairing physics, ranging from Hertz style critical spin fluctuation glue to the holographic superconductivity of string theory. Here we demonstrate that the gross energy scaling differences that are behind these various pairing mechanisms are directly encoded in the frequency and temperature dependence of the dynamical pair susceptibility. This quantity can be measured directly via the second order Josephson effect and it should be possible employing modern experimental techniques to build a `pairing telescope' that gives a direct view on the origin of quantum critical superconductivity.

cond-mat.str-el↗

arXiv2011

Proximity of the Superconducting Dome and the Quantum Critical Point in the Two-Dimensional Hubbard Model

We use the dynamical cluster approximation to understand the proximity of the superconducting dome to the quantum critical point in the two-dimensional Hubbard model. In a BCS formalism, $T_c$ may be enhanced through an increase in the d-wave pairing interaction ($V_d$) or the bare pairing susceptibility ($χ_{0d}$). At optimal doping, where $V_d$ is revealed to be featureless, we find a power-law behavior of $χ_{0d}(ω=0)$, replacing the BCS log, and strongly enhanced $T_c$. We suggest experiments to verify our predictions.

cond-mat.supr-con↗

arXiv2010

Stability of Quantum Critical Points in the Presence of Competing Orders

We investigate the stability of Quantum Critical Points (QCPs) in the presence of two competing phases. These phases near QCPs are assumed to be either classical or quantum and assumed to repulsively interact via square-square interactions. We find that for any dynamical exponents and for any dimensionality strong enough interaction renders QCPs unstable, and drives transitions to become first order. We propose that this instability and the onset of first-order transitions lead to spatially inhomogeneous states in practical materials near putative QCPs. Our analysis also leads us to suggest that there is a breakdown of Conformal Field Theory (CFT) scaling in the Anti de Sitter models, and in fact these models contain first-order transitions in the strong coupling limit.

cond-mat.str-el↗

arXiv2009

BCS Superconductivity in Quantum Critical Metals

Departing from the assumption that pairing is induced by an retarded external 'bosonic glue', we reformulate the Bardeen-Cooper-Schrieffer theory of superconductivity as a scaling theory. Conventional superconductors correspond with the asymptotically free case but the theory can be effortlessly generalized to handle quantum critical metals. We find that superconducting Tc's are strongly enhanced in the latter when retardation is strong while the coupling is weak. It appears that this simple scaling theory does account quite well for superconducting properties near quantum critical points.

cond-mat.supr-con↗

arXiv2008

Statistics, Condensation and the Anderson-Higgs Mechanism: The Worldline Path Integral View

We explain, in the first quantized path integral formalism, the mechanism behind the Anderson-Higgs effect for a gas of charged bosons in a background magnetic field, and then use the method to prove the absence of the effect for a gas of fermions. The exchange statistics are encoded via the inclusion of additional Grassmann coordinates in a manner that leads to a manifest worldline supersymmetry. This extra symmetry is key in demonstrating the absence of the effect for charged fermions.

cond-mat.supr-con↗

arXiv2007

Accelerating universe emergent from the landscape

We propose that the existence of the string landscape suggests the universe can be in a quantum glass state, where an extremely large viscosity is generated, and long distance dynamics slows down. At the same time, the short distance dynamics is not altered due to the separation of time scales. This scenario can help to understand some controversies in cosmology, for example the natural existence of slow roll inflation and dark energy in the landscape, the apparent smallness of the cosmological constant. We see also that moduli stabilization is no longer necessary. We further identify the glass transition point, where the viscosity diverges, as the location of the cosmic horizon. We try to reconstruct the geometry of the accelerating universe from the structure of the landscape, and find that the metric should have an infinite jump when crossing the horizon. We predict that the static coordinate metric for dS space breaks down outside the horizon.

hep-th↗

arXiv2006

Weak Gravity Conjecture for Noncommutative Field Theory

We investigate the weak gravity bounds on the U(1) gauge theory and scalar field theories in various dimensional noncommutative space. Many results are obtained, such as the upper bound on the noncommutative scale $g_{YM}M_p$ for four dimensional noncommutative U(1) gauge theory. We also discuss the weak gravity bounds on their commutative counterparts. For example, our result on 4 dimensional noncommutative U(1) gauge theory reduces in certain limit to its commutative counterpart suggested by Arkani-Hamed et.al at least at tree-level.

hep-th↗

arXiv2006

Brane Inflation After WMAP Three Year Results

WMAP three-year data favors a red power spectrum at the level of 2 standard deviations, which provides a stringent constraint on the inflation models. In this note we use this data to constrain brane inflation models and find that KKLMMT model can not fit WMAP+SDSS data at the level of 1 standard deviation and a fine-tuning, eight parts in thousand at least, is needed at the level of 2 standard deviation.

hep-th↗