Giant thermal Hall conductivity in the pseudogap phase of cuprate superconductors

Large oscillatory thermal hall effect in kagome metals

The thermal Hall effect recently provided intriguing probes to the ground state of exotic quantum matters. These observations of transverse thermal Hall signals lead to the debate on the fermionic versus bosonic origins of these phenomena. The recent report of quantum oscillations (QOs) in Kitaev spin liquid points to a possible resolution. The Landau level quantization would most likely capture only the fermionic thermal transport effect. However, the QOs in the thermal Hall effect are generally hard to detect. In this work, we report the observation of a large oscillatory thermal Hall effect of correlated Kagome metals. We detect a 180-degree phase change of the oscillation and demonstrate the phase flip as an essential feature for QOs in the thermal transport properties. More importantly, the QOs in the thermal Hall channel are more profound than those in the electrical Hall channel, which strongly violates the Wiedemann-Franz (WF) law for QOs. This result presents the oscillatory thermal Hall effect as a powerful probe to the correlated quantum materials.

Planar thermal Hall effect from phonons in a Kitaev candidate material

The thermal Hall effect has emerged as a potential probe of exotic excitations in spin liquids. In the Kitaev magnet α -RuCl3, the thermal Hall conductivityκ x y has been attributed to Majorana fermions, chiral magnons, or phonons. Theoretically, the former two types of heat carriers can generate a "planar"κ x y , whereby the magnetic field is parallel to the heat current, but it is unknown whether phonons also could. Here we show that a planarκ x y is present in another Kitaev candidate material, Na2Co2TeO6. Based on the striking similarity betweenκ x y and the phonon-dominated thermal conductivityκ x x , we attribute the effect to phonons. We observe a large difference inκ x y between different configurations of heat current and magnetic field, which reveals that the direction of heat current matters in determining the planarκ x y . Our observation calls for a re-evaluation of the planarκ x y observed inα -RuCl3.

Thermal Hall effects due to topological spin fluctuations in YMnO3

The thermal Hall effect in magnetic insulators has been considered a powerful method for examining the topological nature of charge-neutral quasiparticles such as magnons. Yet, unlike the kagome system, the triangular lattice has received less attention for studying the thermal Hall effect because the scalar spin chirality cancels out between adjacent triangles. However, such cancellation cannot be perfect if the triangular lattice is distorted. Here, we report that the trimerized triangular lattice of multiferroic hexagonal manganite YMnO3 produces a highly unusual thermal Hall effect under an applied magnetic field. Our theoretical calculations demonstrate that the thermal Hall conductivity is related to the splitting of the otherwise degenerate two chiralities of its 120˚ magnetic structure. Our result is one of the most unusual cases of topological physics due to this broken Z2 symmetry of the chirality in the supposedly paramagnetic state of YMnO3, due to strong topological spin fluctuations with the additional intricacy of a Dzyaloshinskii-Moriya interaction.

Phonon Hall Viscosity of Ionic Crystals

When time-reversal symmetry is broken, the low-energy description of acoustic lattice dynamics allows for a dissipationless component of the viscosity tensor, the phonon Hall viscosity, which captures how phonon chirality grows with the wave vector. In this work, we show that, in ionic crystals, a phonon Hall viscosity contribution is produced by the Lorentz forces on moving ions. We calculate typical values of the Lorentz force contribution to the Hall viscosity using a simple square lattice toy model, and we compare it with literature estimates of the strengths of other Hall-viscosity mechanisms.

The Wiedemann-Franz law in doped Mott insulators without quasiparticles

Many metallic quantum materials display anomalous transport phenomena that defy a Fermi liquid description. Here, we use numerical methods to calculate thermal and charge transport in the doped Hubbard model and observe a crossover separating high- and low-temperature behaviors. Distinct from the behavior at high temperatures, the Lorenz number [Formula: see text] becomes weakly doping dependent and less sensitive to parameters at low temperatures. At the lowest numerically accessible temperatures, [Formula: see text] roughly approaches the Wiedemann-Franz constant [Formula: see text], even in a doped Mott insulator that lacks well-defined quasiparticles. Decomposing the energy current operator indicates a compensation between kinetic and potential contributions, which may help to clarify the interpretation of transport experiments beyond Boltzmann theory in strongly correlated metals.

Circularly Polarized Light-Dependent Pyro-Phototronic Effect from 2D Chiral-Polar Double Perovskites

Chiral hybrid perovskites combine the advantages of chiral materials and halide perovskites, offering an ideal platform for the design of circularly polarized light (CPL) detectors. The pyro-phototronic effect, as a special mechanism of the photoexcited pyroelectric signal, can significantly improve the performance of photodetectors, whereas it remains a great challenge to achieve pyroelectricity-based CPL detection. In this work, the chiroptical phenomena and the pyro-phototronic effect are combined in chiral-polar perovskites to achieve unprecedented pyroelectric-based CPL detection. Two novel two-dimensional (2D) lead-free chiral-polar double perovskites, S/R-[(4-aminophenyl)ethylamine]2AgBiI8·0.5H2O, are successfully designed and synthesized by introducing chiral organic ligands into metal halide frameworks. Strikingly, the photoresponse is substantially boosted with the support of the pyro-phototronic effect, showing an increased pyro-phototronic current that is 40 times greater than the photovoltaic current. Furthermore, the pyroelectric-based detector possesses excellent CPL detection capacity to distinguish different polarization states of CPL photons, which achieve an impressive glph of up to 0.27 at zero bias. This study provides a brand new process for CPL detection by utilizing the pyro-phototronic effect in chiral-polar perovskites, which opens a new avenue for chiral materials in optoelectronic applications.

Giant, magnet-free, and room-temperature Hall-like heat transfer

Thermal chirality, generically referring to the handedness of heat flux, provides a significant possibility for modern heat control. It may be realized with the thermal Hall effect yet at the high cost of strong magnetic fields and extremely low temperatures. Here, we reveal magnet-free and room-temperature Hall-like heat transfer in an active thermal lattice composed of a stationary solid matrix and rotating solid particles. Rotation breaks the Onsager reciprocity relation and generates giant thermal chirality about two orders of magnitude larger than ever reported at the optimal rotation velocity. We further achieve anisotropic thermal chirality by breaking the rotation invariance of the active lattice, bringing effective thermal conductivity to a region unreachable by the thermal Hall effect. These results could enlighten topological and non-Hermitian heat transfer and efficient heat utilization in ways distinct from phonons.

Gate-Tunable Phonon Magnetic Moment in Bilayer Graphene

We develop a first-principles quantum scheme to calculate the phonon magnetic moment in solids. As a showcase example, we apply our method to study gated bilayer graphene, a material with strong covalent bonds. According to the classical theory based on the Born effective charge, the phonon magnetic moment in this system should vanish, yet our quantum mechanical calculations find significant phonon magnetic moments. Furthermore, the magnetic moment is highly tunable by changing the gate voltage. Our results firmly establish the necessity of the quantum mechanical treatment, and identify small-gap covalent materials as a promising platform for studying tunable phonon magnetic moment.

Frequency Splitting of Chiral Phonons from Broken Time-Reversal Symmetry in CrI_{3}

Conventional approaches for lattice dynamics based on static interatomic forces do not fully account for the effects of time-reversal-symmetry breaking in magnetic systems. Recent approaches to rectify this involve incorporating the first-order change in forces with atomic velocities under the assumption of adiabatic separation of electronic and nuclear degrees of freedom. In this Letter, we develop a first-principles method to calculate this velocity-force coupling in extended solids and show via the example of ferromagnetic CrI_{3} that, due to the slow dynamics of the spins in the system, the assumption of adiabatic separation can result in large errors for splittings of zone-center chiral modes. We demonstrate that an accurate description of the lattice dynamics requires treating magnons and phonons on the same footing.

The phonon thermal Hall angle in black phosphorus

The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal (κii), and the transverse, κij, thermal conductivities peak at the same temperature and at this peak temperature, the κij/κjj/B is ≈ 10-4-10-3 T-1. Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field.

Unconventional short-range structural fluctuations in cuprate superconductors

The interplay between structural and electronic degrees of freedom in complex materials is the subject of extensive debate in physics and materials science. Particularly interesting questions pertain to the nature and extent of pre-transitional short-range order in diverse systems ranging from shape-memory alloys to unconventional superconductors, and how this microstructure affects macroscopic properties. Here we use neutron and X-ray diffuse scattering to uncover universal structural fluctuations in La2-xSrxCuO4 and Tl2Ba2CuO6+δ, two cuprate superconductors with distinct point disorder effects and with optimal superconducting transition temperatures that differ by more than a factor of two. The fluctuations are present in wide doping and temperature ranges, including compositions that maintain high average structural symmetry, and they exhibit unusual, yet simple scaling behaviour. The scaling regime is robust and universal, similar to the well-known critical fluctuations close to second-order phase transitions, but with a distinctly different physical origin. We relate this behaviour to pre-transitional phenomena in a broad class of systems with structural and magnetic transitions, and propose an explanation based on rare structural fluctuations caused by intrinsic nanoscale inhomogeneity. We also uncover parallels with superconducting fluctuations, which indicates that the underlying inhomogeneity plays an important role in cuprate physics.

Phonon drag thermal Hall effect in metallic strontium titanate

SrTiO₃, a quantum paralectric, displays a detectable phonon thermal Hall effect (THE). Here, we show that the amplitude of the THE is extremely sensitive to stoichiometry. It drastically decreases upon substitution of a tiny fraction of Sr atoms with Ca, which stabilizes the ferroelectric order. It drastically increases by an even lower density of oxygen vacancies, which turn the system to a dilute metal. The enhancement in the metallic state exceeds by far the sum of the electronic and the phononic contributions. We explain this observation as an outcome of three features: 1) Heat is mostly transported by phonons; 2) the electronic Hall angle is extremely large; and 3) there is substantial momentum exchange between electrons and phonons. Starting from Herring's picture of phonon drag, we arrive to a quantitative account of the enhanced THE. Thus, phonon drag, hitherto detected as an amplifier of thermoelectric coefficients, can generate a purely thermal transverse response in a dilute metal with a large Hall angle. Our results reveal a hitherto-unknown consequence of momentum-conserving collisions between electrons and phonons.

Large phonon thermal Hall conductivity in the antiferromagnetic insulator Cu3TeO6

Phonons are known to generate a thermal Hall effect in certain insulators, including oxides with rare-earth impurities, quantum paraelectrics, multiferroic materials, and cuprate Mott insulators. In each case, a special feature of the material is presumed relevant for the underlying mechanism that confers chirality to phonons in a magnetic field. A fundamental question is whether a phonon Hall effect is an unusual occurrence-linked to special characteristics such as skew scattering off rare-earth impurities, structural domains, ferroelectricity, or ferromagnetism-or a much more common property of insulators than hitherto believed. To help answer this question, we have turned to a material with none of the previously encountered special features: the cubic antiferromagnet Cu3TeO6. We find that its thermal Hall conductivity [Formula: see text] is among the largest of any insulator so far. We show that this record-high [Formula: see text] signal is due to phonons, and it does not require the presence of magnetic order, as it persists above the ordering temperature. We conclude that the phonon Hall effect is likely to be a fairly common property of solids.

Phonon thermal Hall effect in a metallic spin ice

It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κxx and transverse κxy thermal conductivity in Pr2Ir2O7, which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κxx and κxy are dominated by phonons. A T/H scaling of κxx unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κxx deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κxx and κxy data clearly shows that κxy correlates with κxx in its response to magnetic field including a success of the T/H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

Anomalous Thermal Hall Effect in an Insulating van der Waals Magnet

Two-dimensional (2D) van der Waals (vdW) magnets have been a fertile playground for the discovery and exploration of physical phenomena and new physics. In this Letter, we report the observation of an anomalous thermal Hall effect (THE) with κ_{xy}∼1×10^{-2}  W K^{-1} m^{-1} in an insulating van der Waals ferromagnet VI_{3}. The thermal Hall signal persists in the absence of an external magnetic field and flips sign upon the switching of the magnetization. In combination with theoretical calculations, we show that VI_{3} exhibits a dual nature of the THE, i.e., dominated by topological magnons hosted by the ferromagnetic honeycomb lattice at higher temperatures and by phonons induced by the magnon-phonon coupling at lower temperatures. Our results not only position VI_{3} as the first ferromagnetic system to investigate both anomalous magnon and phonon THEs, but also render it as a potential platform for spintronics-magnonics applications.

Giant phonon anomalies in the proximate Kitaev quantum spin liquid α-RuCl3

The Kitaev quantum spin liquid epitomizes an entangled topological state, for which two flavors of fractionalized low-energy excitations are predicted: the itinerant Majorana fermion and the Z2 gauge flux. It was proposed recently that fingerprints of fractional excitations are encoded in the phonon spectra of Kitaev quantum spin liquids through a novel fractional-excitation-phonon coupling. Here, we detect anomalous phonon effects in α-RuCl3 using inelastic X-ray scattering with meV resolution. At high temperature, we discover interlaced optical phonons intercepting a transverse acoustic phonon between 3 and 7 meV. Upon decreasing temperature, the optical phonons display a large intensity enhancement near the Kitaev energy, JK~8 meV, that coincides with a giant acoustic phonon softening near the Z2 gauge flux energy scale. These phonon anomalies signify the coupling of phonon and Kitaev magnetic excitations in α-RuCl3 and demonstrates a proof-of-principle method to detect anomalous excitations in topological quantum materials.

Sizable Suppression of Thermal Hall Effect upon Isotopic Substitution in SrTiO_{3}

We report measurements of the thermal Hall effect in single crystals of both pristine and isotopically substituted strontium titanate. We discovered a 2 orders of magnitude difference in the thermal Hall conductivity between SrTi^{16}O_{3} and ^{18}O-enriched SrTi^{18}O_{3} samples. In most temperature ranges, the magnitude of thermal Hall conductivity (κ_{xy}) in SrTi^{18}O_{3} is proportional to the magnitude of the longitudinal thermal conductivity (κ_{xx}), which suggests a phonon-mediated thermal Hall effect. However, they deviate in the temperature of their maxima, and the thermal Hall angle ratio (|κ_{xy}/κ_{xx}|) shows anomalously decreasing behavior below the ferroelectric Curie temperature T_{c}∼25  K. This observation suggests a new underlying mechanism, as the conventional scenario cannot explain such differences within the slight change in phonon spectrum. Notably, the difference in magnitude of thermal Hall conductivity and rapidly decreasing thermal Hall angle ratio in SrTi^{18}O_{3} is correlated with the strength of quantum critical fluctuations in this displacive ferroelectric. This relation points to a link between the quantum critical physics of strontium titanate and its thermal Hall effect, a possible clue to explain this example of an exotic phenomenon in nonmagnetic insulating systems.

Proposed Model of the Giant Thermal Hall Effect in Two-Dimensional Superconductors: An Extension to the Superconducting Fluctuation Regime

We extend the thermodynamic approach for the description of the thermal Hall effect in two-dimensional superconductors above the critical temperature, where fluctuation Cooper pairs contribute to the conductivity, as well as in disordered normal metals where the particle-particle channel is important. We express the Hall heat conductivity in terms of the product of temperature derivatives of the chemical potential and of the magnetization of the system. Based on this general expression, we derive the analytical formalism that qualitatively reproduces the superlinear increase of the thermal Hall conductivity with the decrease of temperature observed in a large variety of experimentally studied systems [Grissonnanche et al., Nature (London) 571, 376 (2019)NATUAS0028-083610.1038/s41586-019-1375-0]. We also predict a nonmonotonic behavior of the thermal Hall conductivity in the regime of quantum fluctuations, in the vicinity of the second critical field and at very low temperatures.

Revival of Charge Density Waves and Charge Density Fluctuations in Cuprate High-Temperature Superconductors

I present here a short memory of my scientific contacts with K.A. Müller starting from the Interlaken Conference (1988), Erice (1992 and 1993), and Cottbus (1994) on the initial studies on phase separation (PS) and charge inhomogeneity in cuprates carried out against the view of the majority of the scientific community at that time. Going over the years and passing through the charge density wave (CDW) instability of the correlated Fermi liquid (FL) and to the consequences of charge density fluctuations (CDFs), I end with a presentation of my current research activity on CDWs and the related two-dimensional charge density fluctuations (2D-CDFs). A scenario follows of the physics of cuprates, which includes the solution of the decades-long problem of the strange metal (SM) state.

Thermal Hall conductivity in the cuprate Mott insulators Nd2CuO4 and Sr2CuO2Cl2

The heat carriers responsible for the unexpectedly large thermal Hall conductivity of the cuprate Mott insulator La₂CuO₄ were recently shown to be phonons. However, the mechanism by which phonons in cuprates acquire chirality in a magnetic field is still unknown. Here, we report a similar thermal Hall conductivity in two cuprate Mott insulators with significantly different crystal structures and magnetic orders – Nd₂CuO₄ and Sr₂CuO₂Cl₂ – and show that two potential mechanisms can be excluded – the scattering of phonons by rare-earth impurities and by structural domains. Our comparative study further reveals that orthorhombicity, apical oxygens, the tilting of oxygen octahedra and the canting of spins out of the CuO₂ planes are not essential to the mechanism of chirality. Our findings point to a chiral mechanism coming from a coupling of acoustic phonons to the intrinsic excitations of the CuO₂ planes.

What makes the phonons in cuprates become chiral, as measured by their thermal Hall effect, is an unresolved question. Here, the authors rule out two extrinsic mechanisms and argue that chirality comes from a coupling of acoustic phonons to the intrinsic excitations of the CuO₂ planes.

Universal signatures of Majorana-like quasiparticles in strongly correlated Landau-Fermi liquids

Motivated by recent experiments in the Kitaev honeycomb lattice, Kondo insulators, and the 'Luttinger's theorem-violating' Fermi liquid phase of the underdoped cuprates, we extend the theoretical machinery of Landau-Fermi liquid theory to a system of itinerant, interacting Majorana-like particles. Building upon a previously introduced model of 'nearly self-conjugate' fermionic polarons, a Landau-Majorana kinetic equation is introduced to describe the collective modes and Fermi surface instabilities in a fluid of particles whose fermionic degrees of freedom obey the Majorana reality condition. At large screening, we show that the Landau-Majorana liquid harbors a Lifshitz transition for specific values of the driving frequency. Moreover, we find the dispersion of the zero sound collective mode in such a system, showing that there exists a specific limit where the Landau-Majorana liquid harbors a stability against Pomeranchuk deformations unseen in the conventional Landau-Fermi liquid. With these results, our work paves the way for possible extensions of the Landau quasiparticle paradigm to nontrivial metallic phases of matter.

Universal Behavior of the Thermal Hall Conductivity

We report theoretical and experimental analyses of the thermal Hall conductivity in correlated systems. For both fermionic and bosonic excitations with nontrivial topology, we show that at "intermediate" temperatures, the thermal Hall conductivity exhibits an unexpected universal scaling with a simple exponential form. At low temperatures, it behaves differently and reflects the spectral properties of underlying excitations. Our predictions are examined as examples in two prototype compounds, the quantum paraelectric SrTiO_{3} and the spin-liquid compound RuCl_{3}. The experimental data can be largely covered by our proposed minimal phenomenological model independent of microscopic details, revealing dominant bosonic contributions in SrTiO_{3} and gapped fermionic excitations in RuCl_{3}. Our work establishes a phenomenological link between microscopic models and experimental data and provides a unified basis for analyzing the thermal Hall conductivity in correlated systems over a wide temperature region.

Enhanced Thermal Hall Effect in Nearly Ferroelectric Insulators

In the context of recent experimental observations of an unexpectedly large thermal Hall conductivity, κ_{H}, in insulating La_{2}CuO_{4} (LCO) and SrTiO_{3} (STO), we theoretically explore conditions under which acoustic phonons can give rise to such a large κ_{H}. Both the intrinsic and extrinsic contributions to κ_{H} are large in proportion to the dielectric constant, ε, and the "flexoelectric" coupling, F. While the intrinsic contribution is still orders of magnitude smaller than the observed effect, an extrinsic contribution proportional to the phonon mean-free path appears likely to account for the observations, at least in STO. We predict a larger intrinsic κ_{H} in certain insulating perovskites.

Phonon Thermal Hall Effect in Strontium Titanate

It has been known for more than a decade that phonons can produce an off-diagonal thermal conductivity in the presence of a magnetic field. Recent studies of thermal Hall conductivity, κ_{xy}, in a variety of contexts, however, have assumed a negligibly small phonon contribution. We present a study of κ_{xy} in quantum paraelectric SrTiO_{3}, which is a nonmagnetic insulator and find that its peak value exceeds what has been reported in any other insulator, including those in which the signal has been qualified as "giant." Remarkably, κ_{xy}(T) and κ(T) peak at the same temperature and the former decreases faster than the latter at both sides of the peak. Interestingly, in the case of La_{2}CuO_{4} and α-RuCl_{3}, κ_{xy}(T) and κ(T) peak also at the same temperature. We also studied KTaO_{3} and found a small signal, indicating that a sizable κ_{xy}(T) is not a generic feature of quantum paraelectrics. Combined to other observations, this points to a crucial role played by antiferrodistortive domains in generating κ_{xy} of this solid.

Thermal-transport studies of kagomé antiferromagnets

Searching for the ground state of a kagomé Heisenberg antiferromagnet (KHA) has been one of the central issues of condensed-matter physics, because the KHA is expected to host spin-liquid phases with exotic elementary excitations. Here, we show our longitudinal ([Formula: see text]) and transverse ([Formula: see text]) thermal conductivities measurements of the two kagomé materials, volborthite and Ca kapellasite. Although magnetic orders appear at temperatures much lower than the antiferromagnetic energy scale in both materials, the nature of spin liquids can be captured above the transition temperatures. The temperature and field dependence of [Formula: see text] is analyzed by spin and phonon contributions, and large sample variations of the spin contribution are found in volborthite. Clear changes in [Formula: see text] are observed at the multiple magnetic transitions in volborthite, showing different magnetic thermal conduction in different magnetic structures. These magnetic contributions are not clearly observed in low-[Formula: see text] crystals of volborthite, and are almost absent in Ca kapellasite, showing the high sensitivity of the magnetic excitation in [Formula: see text] to the defects in crystals. On the other hand, a clear thermal Hall signal has been observed in the lowest-[Formula: see text] crystal of volborthite and in Ca kapellasite. Remarkably, both the temperature dependence and the magnitude of [Formula: see text] of volborthite are found to be very similar to those of Ca kapellasite, despite of about an order of magnitude difference in [Formula: see text] We find that [Formula: see text] of both compounds is well reproduced, both qualitatively and quantitatively, by spin excitations described by the Schwinger-boson mean-field theory applied to KHA with the Dzyaloshinskii-Moriya interaction. This excellent agreement demonstrates not only that the thermal Hall effect in these kagomé antiferromagnets is caused by spins in the spin liquid phase, but also that the elementary excitations of this spin liquid phase are well described by the bosonic spin excitations.

Vortex phase diagram and the normal state of cuprates with charge and spin orders

The phase diagram of underdoped cuprates in a magnetic field (H) is key to understanding the anomalous normal state of these high-temperature superconductors. However, the upper critical field (H _c2), the extent of superconducting (SC) phase with vortices, and the role of charge orders at high H remain controversial. Here we study stripe-ordered La-214, i.e., cuprates in which charge orders are most pronounced and zero-field SC transition temperatures T c 0 are lowest. This enables us to explore the vortex phases in a previously inaccessible energy scale window. By combining linear and nonlinear transport techniques sensitive to vortex matter, we determine the T - H phase diagram, directly detect H _c2, and reveal novel properties of the high-field ground state. Our results demonstrate that quantum fluctuations and disorder play a key role as T → 0, while the high-field ground state is likely a metal, not an insulator, due to the presence of stripes.