arXiv subjects

Alex Buchel

Publications and source records attributed to Alex Buchel.

At least 19 recordsLinked to original sources

arXiv2026

Striped phases of N=4 SYM at finite density

We construct spatially modulated phases of strongly coupled $\mathcal N=4$ supersymmetric Yang--Mills theory at finite $R$-charge density. Compactification on a three-torus and unequal background charges isolate an unstable charge-density mode. Its static threshold seeds a nonlinear branch of striped black branes in the STU model. The striped states have higher entropy than homogeneous states with the same energy and conserved charges. They connect continuously to the homogeneous branch, with an entropy excess that begins at fourth order in the modulation amplitude. These solutions provide a candidate endpoint of the charge-transport instability.

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arXiv2026

Entropically favored scalar hair on AdS black branes

We construct neutral AdS$_5$ black branes with source-free scalar hair that is entropically favored over the symmetric phase. A quadratic scalar--Gauss--Bonnet coupling destabilizes the thermal state while leaving the ultraviolet operator dimension fixed at $Δ=3$. The fully backreacted solutions require only a quadratic scalar potential. We describe how a neural-network search led to this simple model.

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arXiv2026

Causality violation in the holographic approach to N=4 SYM theory at finite 't Hooft coupling

In the AdS/CFT correspondence, correlation functions of the energy-momentum tensor in N=4 supersymmetric Yang-Mills theory at large but finite 't Hooft coupling can be studied using higher-derivative corrections to supergravity. We show that the commonly used holographic prescription for correlation functions which is obtained by truncating the equations of motion at order $(α')^3$ implies that the thermal equilibrium state of N=4 SYM theory in the planar limit is dynamically unstable at arbitrarily large finite 't Hooft coupling. The instability is a reflection of the unphysical causality violation brought about by treating perturbatively-derived equations as if they were non-perturbative, analogous to the instability of AdS black branes in five-dimensional Gauss-Bonnet gravity. While other holographic prescriptions for treating the $O(α'^3)$ corrections to supergravity may alleviate the pathology introduced by the truncation, the validity regime of the results obtained by any prescription at $O(α'^3)$ must be limited to the strict perturbative treatment of the $O(α'^3)$ effects on N=4 SYM observables.

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arXiv2026

PINNs for N=2* holography

Can a holographic theory be reconstructed from its equation of state? We investigate this question using physics-informed neural networks in the bosonic-mass sector of the Pilch--Warner model. We first validate the PINN approach against an analytic solution encoding the leading high-temperature corrections to the conformal equation of state. We then reconstruct nonlinear black-brane geometries and the scalar potential using thermodynamics, ultraviolet asymptotics, Ward identities, and horizon regularity. Independent solutions of the bulk Einstein-scalar equations demonstrate that accurately reproducing thermodynamics with a trained PINN does not guarantee an accurate reconstruction of the scalar potential. High-precision knowledge of the high-temperature equation of state constrains nonlinear interactions in the bulk scalar potential. Fixing the cubic coupling to its exact PW value, motivated by this high-temperature extraction, reduces the maximum sampled error in the reconstructed scalar self-interactions by a factor of $\sim 15$ compared with constraining only the bulk scalar mass. Adding knowledge of the bulk viscosity of the gauge theory plasma further reduces this error by an additional factor of $\sim 13$, with independent forward solutions reproducing the bulk viscosity within $0.09\%$.

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arXiv2026

Instability in ${\cal N}=4$ supersymmetric Yang-Mills theory on $S^3$ at finite density

Homogeneous and isotropic equilibrium states of strongly coupled ${\cal N}=4$ supersymmetric Yang-Mills charged plasma in ${\mathbb R}^3$ with equal chemical potentials for the maximal Abelian subgroup of the $R$-symmetry group become dynamically unstable below some critical temperature. The instabilities arise in the $R$-symmetry charge transport, precisely when the equilibrium state becomes thermodynamically unstable. We study the fate these correlated instabilities when the theory is placed on $S^3$. The curvature of the three-sphere affects the onset of the dynamical and the thermodynamic instabilities differently: increasing the curvature at low temperatures can stabilize its transport, but leave the plasma thermodynamically unstable. Thermodynamic stability is never recovered provided the $S^3$ volume is allowed to fluctuate.

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arXiv2026

Instability of 5D Gauss-Bonnet black branes

We show that Gauss-Bonnet black branes in five-dimensional anti-de Sitter gravity are unstable when the Gauss-Bonnet coupling falls outside the range allowed by the conformal collider bounds. The unstable modes and the boundary causality violating modes are connected by a phase rotation of complex boundary momentum.

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arXiv2026

On stability of baryonic black membranes

Near-extremal black membranes with topological (baryonic) $U(1)_B$ charge of M-theory compactified on the coset space $M^{1,1,0}$ are stable. $M^{1,1,0}$ coset is a ${\mathbb Z}_2$-invariant truncation of a larger $Q^{1,1,1}$ coset, with diagonal $U(1)_B\equiv U(1)_{B,+}\subset U(1)_B^2$ symmetry of the latter. We show that the baryonic black membranes of M-theory $M^{1,1,0}$ compactifications are unstable to ${\mathbb Z}_2$-odd gravitational bulk gauge and scalar fluctuations, but only if this bulk scalar is identified with the holographically dual $2+1$ dimensional superconformal gauge theory operator of conformal dimension $Δ=1$. The instability is associated with the unstable charge transport of the off-diagonal $U(1)_{B,-}\subset U(1)_B^2$ symmetry.

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arXiv2026

Holographic Gauss-Bonnet transport

We extend the computational framework of \cite{Buchel:2023fst} to analysis of shear and bulk viscosities in generic strongly coupled holographic Gauss-Bonnet gauge theories. The finite Gauss-Bonnet coupling constant encodes holographic plasma with non-equal central charges $c\ne a$ at the ultraviolet fixed point. In a simple model we discuss transport coefficients within the causality window $-\frac12\le \frac{c-a}{c}\le \frac 12$ of the theory.

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arXiv2026

Holographic Charged Transport with Higher Derivatives

We compute the first-order hydrodynamic transport coefficients (shear viscosity $η$, bulk viscosity $ζ$, and charge conductivity $σ$) for a broad class of strongly coupled, four-dimensional charged relativistic gauge theory plasma with holographic gravitational duals containing higher-derivative corrections. The landscape of our holographic models captures non-conformal gauge theories with an arbitrary number of relevant coupling constants and a general scalar potential in the gravitational dual, allowing for a systematic exploration of charged transport along generic holographic RG flows. The leading-order higher-derivative corrections probe gauge theories with non-equal central charges $c\ne a$ at the ultraviolet fixed point, and enable the engineering of diverse temperature and charge density profiles for the viscosities and the conductivity. Our results establish the membrane paradigm in higher-derivative holographic models: all the transport coefficients are extracted from the black brane horizon values of the gravitational scalars, and various functions defining the gravitational holographic dual.

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arXiv2025

Near-extremal membranes in M-theory

We consider near-extremal membranes embedded in M-theory, consistently truncated to gauged $\mathcal{N} = 2$ supergravity in four dimensions on the coset space $M^{1,1,0}$. These are holographically dual to $2 + 1$ dimensional superconformal gauge theory with $U(1)_R \times U(1)_B$ global symmetry. Turning on the chemical potential to either the $R$-symmetry or the baryonic symmetry gives access to the quantum critical regime of the boundary gauge theory. We study perturbative stability of the extremal limit, and demonstrate that membranes with topological (baryonic) charge are free from all known instabilities. $R$-charged membranes are free from the superconducting instabilities, but have unstable charge transport and instabilities associated with the condensation of the axions.

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arXiv2025

Instability of Baryonic Black Branes

Baryonic black branes describe the quantum critical phase of the conformal conifold gauge theory at strong coupling. This phase extends to zero temperature at a finite baryonic chemical potential, represented by extremal black branes with $AdS_2\times R^3\times T^{1,1}$ throat in asymptotic $AdS_5\times T^{1,1}$ geometry. We demonstrate here that this phase is dynamically unstable below some critical value of $T_c/μ$: the instability is represented by a diffusive mode in the hydrodynamic sound channel with a negative diffusion coefficient. We also identify a new (exotic) ordered phase of the conifold gauge theory: this phase originates at the same critical value of $T_c/μ$, but extends to arbitrary high temperatures, and is characterized by an expectation value of a dimension-2 operator, ${\cal O}_2\propto T^2$, in the limit $\frac μT\to 0$.

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arXiv2025

On the relevance of GIKS instability of charged N=4 SYM plasma

Recently Gladden, Ivo, Kovtun, and Starinets (GIKS) identified a novel instability of ${\cal N}=4$ supersymmetric Yang-Mills plasma with a diagonal $U(1)$ $R$-symmetry chemical potential. Since ${\cal N}=4$ SYM plasma has $R$-charged matter fields, it might suffer from the standard superconducting instability. We show that while the superconducting instability is indeed present, it happens at a lower critical temperature compare to that of the GIKS instability.

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arXiv2025

Fluxification and scalarization of the conifold black holes

We study thermal states of the strongly coupled Klebanov-Witten ${\cal N}=1$ superconformal gauge theory with R-symmetry chemical potential. The theory has two distinct dimension $Δ=3$ chiral primary operators that develop a condensate at the same critical value of the chemical potential. One instability is associated with the spontaneous fluxification (the appearance of the 3-form flux) of the holographic dual black hole, while the other one is associated with the spontaneous scalarization (the activation of the deformation mode of the conifold). Different phases dominate in grand canonical and microcanonical ensembles.

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arXiv2025

The ordered phase of charged N=4 SYM plasma from STU

In \cite{Buchel:2025cve} we numerically identified the ordered phase of the charged ${\cal N}=4$ supersymmetric Yang-Mills plasma. We explain here how this phase can be obtained analytically within the STU model of Behrnd, Cvetič, and Sabra \cite{Behrndt:1998jd}.

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arXiv2025

The ordered phase of charged N=4 SYM plasma

Recently is has been shown \cite{Gladden:2024ssb} that strongly coupled ${\cal N}=4$ supersymmetric Yang-Mills plasma with diagonal $U(1)$ $R$-charge chemical potential is unstable at $\fracμ{2πT}=\sqrt{2}$. We construct a new homogeneous and isotropic phase of this plasma that dominates in the microcanonical ensemble (but not in the grand canonical one). The new phase extends to arbitrary high temperatures, \ie as $T\gg μ$, and is characterized by an expectation value of a dimension-2 operator, with ${\cal O}_2\propto T^2$ -- it is the {\it ordered conformal phase}. In the limit $\fracμT\to 0$, this ordered phase has vanishing energy density $\frac{\cal E}{T^4}\propto \frac{μ^2}{T^2}$ and is a low entropy density state $\frac{\cal S}{T^3}\propto \frac{μ^2}{T^2}$.

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arXiv2024

Plumbing the wormholes of string theory flux compactifications

Thus far, the known wormholes in string theory connecting disjoint boundaries represented by finite volume quotients of hyperbolic spaces leak: they are non-perturbatively unstable towards brane-anti-brane nucleation in the flux backgrounds that support these wormholes. Turning on additional fluxes suppressed this instability, but would not completely eliminate it. We present the first example of a non-perturbatively stable wormhole in type IIB supergravity on a warped squashed conifold with fluxes.

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arXiv2024

Holographic transport beyond the supergravity approximation

We set up a unified framework to efficiently compute the shear and bulk viscosities of strongly coupled gauge theories with gravitational holographic duals involving higher derivative corrections. We consider both Weyl$^4$ corrections, encoding the finite 't Hooft coupling corrections of the boundary theory, and Riemann$^2$ corrections, responsible for non-equal central charges $c\ne a$ of the theory at the ultraviolet fixed point. Our expressions for the viscosities in higher derivative holographic models are extracted from a radially conserved current and depend only on the horizon data.

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arXiv2023

Holographic conformal order with higher derivatives

Conformal order are isotropic and translationary invariant thermal states of a conformal theory with nonzero expectation value of certain operators. While ubiquitous in bottom-up models of holographic CFTs, conformal order states are unstable in theories dual to bulk two-derivative gravity. We explore conformal order in strongly coupled theories with gravitational holographic duals involving higher derivative corrections.

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