arXiv subjects
Publications and source records attributed to Alex Buchel.
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.
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.
We compute the first-order hydrodynamic transport coefficients (shear viscosity $\eta$, bulk viscosity $\zeta$, and charge conductivity $\sigma$) 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.
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 $\Delta=1$. The instability is associated with the unstable charge transport of the off-diagonal $U(1)_{B,-}\subset U(1)_B^2$ symmetry.
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.
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.
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 $T_c/\mu$: 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 $T_c/\mu$, 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 $\frac \mu T\to 0$.
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, Cveti\v{c}, and Sabra \cite{Behrndt:1998jd}.
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 $\frac{\mu}{2\pi T}=\sqrt{2}$. 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 $T\gg \mu$, 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 $\frac\mu T\to 0$, this ordered phase has vanishing energy density $\frac{{\cal E}}{T^4}\propto \frac{\mu^2}{T^2}$ and is a low entropy density state $\frac{{\cal S}}{T^3}\propto \frac{\mu^2}{T^2}$.
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 $\Delta=3$ 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.
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.
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.
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.
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.
We use gauge/gravity correspondence to study the gravitational reheating of strongly coupled gauge theories in the rapid exit from the long inflationary (de Sitter) phase. We estimate the maximal reheating temperature of holographic models in $d$-spatial dimensions, which scale invariance is explicitly broken by a source term for an operator ${\cal O}_\Delta$ of dimension $\frac d2+\frac 12< \Delta< d+1$, as well as the top-down ${\cal N}=2^*$ and the cascading gauge theories. The reheating is most efficient when the inflationary phase Hubble constant is much larger than the mass scale of the conformal symmetry breaking of the theory, and for theories with the conformal symmetry breaking operators that are close to marginal.
According to the standard lore, a prolonged inflation leaves a quantum field theory in a cold, low entropy state. Thus, some mechanism is needed to reheat this post-inflationary state, leaving a hot, thermal, radiation-dominated Universe. Typically, reheating is achieved coupling the inflaton field to the QFT degrees of freedom. We argue that the nonequilibrium dynamics of a non-conformal QFT in (post-)inflationary background space-time can produce hot quark-gluon plasma with the reheating temperature of order the inflationary Hubble scale, without the inflaton coupling.
We use ${\cal N}=2^*$ and cascading gauge theory holographic models to extract the general features of the gravitational susceptibility $\kappa$ of strongly coupled nonconformal quark-gluon plasma. We show that in theories with a relevant coupling constant the gravitational susceptibility is renormalization scheme dependent. We propose to use its temperature derivative, i.e., $\frac{d\kappa}{d\ln T}$, as a scheme-independent characteristic of a QGP. Although $\kappa$ is a thermodynamic quantity, its critical behavior can be drastically distinct in the vicinity of seemingly identical thermal phase transitions.
We study stability of the Dynamical Fixed Points (DFPs) of the cascading gauge theory at strong coupling in de Sitter space-time. We compute the spectra of the perturbative fluctuations and identify stable/unstable DFPs, characterized by the ratio of the strong coupling scale $\Lambda$ of the gauge theory and the Hubble constant $H$ of the background space-time. We discover a new phenomenon in the spectrum of gravitational fluctuations of a non-conformal holographic model: distinct branches of the fluctuations for $H\gg \Lambda$ coalesce for sufficiently low $\frac{H}{\Lambda}$, leading to the removal of some excited modes from the spectrum. We establish that, at least in a dual supergravity approximation, cascading gauge theory does not have a stable DFP for $H\in (H_{crit_1},H_{crit_2})$. Initial states of the theory for $H>H_{crit_2}$ evolve to a stable DFP with unbroken chiral symmetry; while for $H< H_{crit_1}$ the states evolve to a de Sitter vacuum with spontaneously broken chiral symmetry.