Significant progress toward a theory of high-temperature superconductivity in cuprates has been achieved via the study of effective one- and three-band Hubbard models. Nevertheless, material-specific predictions, while essential for constructing a comprehensive theory, remain challenging due to the complex relationship between real materials and the parameters of the effective models. By combining cluster dynamical mean-field theory and density functional theory in a charge-self-consistent manner, here we show that the goal of material-specific predictions for high-temperature superconductors from first principles is within reach. To demonstrate the capabilities of our approach, we take on the challenge of explaining the remarkable physics of multilayer cuprates by focusing on the two representative ${\mathrm{Ca}}_{(1+n)}{\mathrm{Cu}}_n{\mathrm{O}}_{2n}{\mathrm{Cl}}_2$ and ${\mathrm{HgBa}}_2{\mathrm{Ca}}_{(n-1)}{\mathrm{Cu}}_n{\mathrm{O}}_{(2n+2)}$ families. We shed light on the microscopic origin of many salient features of multilayer cuprates, in particular, the $n$ dependence of their superconducting properties. The growth of $T_c$ from the single-layer to the trilayer compounds is here explained by the reduction of the charge transfer gap and, consequently, the growth of superexchange $J$ as $n$ increases. The origin of both is traced to the appearance of low-energy conduction bands reminiscent of standing wave modes confined within the stack of ${\mathrm{CuO}}_2$ planes. We interpret the ultimate drop of $T_c$ for $n\ge 4$ as a consequence of the inhomogeneous doping between the ${\mathrm{CuO}}_2$ planes, which prevents the emergence of superconductivity in the inner planes due to their insufficient effective hole doping, as we also highlight the existence of a minimal doping (4%) required for superconductivity to appear in one of the planes. We explain material-specific properties such as the larger propensity of ${\mathrm{HgBa}}_2{\mathrm{Ca}}_{(n-1)}{\mathrm{Cu}}_n{\mathrm{O}}_{(2n+2)}$ to superconduct compared with ${\mathrm{Ca}}_{(1+n)}{\mathrm{Cu}}_n{\mathrm{O}}_{2n}{\mathrm{Cl}}_2$. We also find the coexistence of arcs and pockets observed with photoemission, the charge redistribution between copper and oxygen, and the link to the pseudogap. Our work establishes a framework for comprehensive studies of high-temperature superconducting cuprates, enables detailed comparisons with experiment, and, through its ab initio settings, unlocks opportunities for theoretical material design of high-temperature superconductors.

Predicting the emergent properties of impurities immersed in a quantum bath is a fundamental challenge that can defy quasiparticle treatments. Here, we measure the spectral properties and real-time dynamics of mobile impurities injected into a weakly interacting homogeneous Bose-Einstein condensate, using two broad Feshbach resonances to tune both the impurity-bath and intrabath interactions. For attractive impurity-bath interactions, the impurity spectrum features a single branch, which away from the resonance corresponds to a well-defined attractive polaron; near the resonance, we observe dramatic broadening of this branch, suggesting a breakdown of the quasiparticle picture. For repulsive impurity-bath interactions, the spectrum features two branches: the attractive branch that is dominated by excitations with energy close to that of the Feshbach dimer, but has a many-body character, and the repulsive polaron branch. Our measurements show that the behavior of impurities in weakly interacting baths is remarkably universal, controlled only by the bath density and a single dimensionless interaction parameter.

One of the main interests of 2D materials is their ability to be assembled with many degrees of freedom for tuning and manipulating excitonic properties. There is a need to understand how the structure of the interfaces between atomic layers influences exciton properties. Here we use cathodoluminescence and time-resolved cathodoluminescence experiments to study how excitons interact with the interface between two twisted hexagonal boron nitride ($h$-BN) crystals with various angles. An efficient capture of free excitons by the interface is demonstrated, which leads to a population of long-lived and interface-localized (2D) excitons. Temperature-dependent experiments indicate that for high twist angles, these excitons localized at the interface further undergo a self-trapping. It consists in a distortion of the lattice around the exciton on which the exciton traps itself. Our results suggest that this exciton-interface interaction causes the broad 4-eV optical emission of highly twisted $h$-BN–$h$-BN structures. Exciton self-trapping is finally discussed as a common feature of $s{p}^2$ hybridized boron nitride polytypes and nanostructures due to the ionic nature of the B—N bond and the small size of their excitons.

Global symmetries greatly enrich the landscape of topological quantum phases, playing an essential role from topological insulators to fractional quantum Hall effect. Topological phases in mixed quantum states, originating from decoherence in open quantum systems or disorders in imperfect crystalline solids, have recently garnered significant interest. Unlike pure states, mixed quantum states can exhibit average symmetries—symmetries that keep the total ensemble invariant but not on each individual state. In this work, we present a systematic classification and characterization of average symmetry-protected topological (ASPT) phases applicable to generic symmetry groups, encompassing both average and exact symmetries, for bosonic and fermionic systems. Moreover, we formulate the theory of average symmetry-enriched topological (ASET) orders in disordered bosonic systems. Our systematic approach helps clarify nuanced issues in previous literature and uncovers compelling new physics. Notably, we discover that (1) the definition and classification of ASPT phases in decohered and disordered systems exhibit subtle differences, (2) despite these differences, ASPT phases in both settings can be classified and characterized under a unified framework of defect decoration and spectral sequence, (3) this systematic classification uncovers a plethora of ASPT phases that are intrinsically mixed, implying they can exclusively manifest in decohered or disordered systems where part of the symmetry is average, and (4) similarly for ASET, we find intrinsically disordered phases exhibiting exotic anyon behaviors—the ground states of such phases necessarily contain localized anyons, with gapless (yet still localized) excitation blue spectra.

We propose a tensor network approach known as the locally purified density operator (LPDO) to investigate the classification and characterization of symmetry-protected topological phases in open quantum systems. We extend the concept of injectivity, originally associated with matrix product states and projected entangled pair states, to LPDOs in $(1+1)\mathrm{D}$ and $(2+1)\mathrm{D}$ systems, unveiling two distinct types of injectivity conditions that are inherent for short-range entangled density matrices. Within the LPDO framework, we outline a classification scheme for decohered average symmetry-protected topological (ASPT) phases, consistent with earlier results obtained through spectrum sequence techniques. However, our approach offers an intuitive and explicit construction of ASPT states with the decorated domain-wall picture emerging naturally. We illustrate our framework with ASPT phases protected by a weak global symmetry and strong fermion parity symmetry and then extend it to a general group structure. Moreover, we derive both the classification data and the explicit forms of the obstruction functions using the LPDO formalism, particularly in the case of nontrivial group extension between strong and weak symmetries, where intrinsic ASPT phases may emerge. We demonstrate constructions of fixed-point LPDOs for ASPT phases in both $(1+1)\mathrm{D}$ and $(2+1)\mathrm{D}$ and discuss their physical realization in decohered or disordered systems. In particular, we construct examples of intrinsic ASPT states in $(1+1)\mathrm{D}$ and $(2+1)\mathrm{D}$ using the LPDO formalism.

Proximity ferroelectricity has recently been reported as a new design paradigm for inducing ferroelectricity, where a nonferroelectric polar material becomes a ferroelectric one by interfacing with a thin ferroelectric layer. Strongly polar materials, such as AlN and ZnO, which were previously unswitchable with an external field below their dielectric breakdown fields, can now be switched with practical coercive fields when they are in intimate proximity to a switchable ferroelectric. Here, we develop a general Landau-Ginzburg theory of proximity ferroelectricity in multilayers of nonferroelectrics and ferroelectrics to analyze their switchability and coercive fields. The theory predicts regimes of both “proximity switching,” where the multilayers collectively switch, and “proximity suppression,” where they collectively do not switch. The mechanism of the proximity ferroelectricity is an internal electric field determined by the polarization of the layers and their relative thickness in a self-consistent manner that renormalizes the double-well ferroelectric potential to lower the steepness of the switching barrier. Further reduction in the coercive field emerges from charged defects in the bulk that act as nucleation centers. The application of the theory to proximity ferroelectricity in ${\mathrm{Al}}_{\mathrm{x}\text{−}1}{\mathrm{Sc}}_{\mathrm{x}}\mathrm{N}/\mathrm{AlN}$ and ${\mathrm{Zn}}_{1\text{−}\mathrm{x}}{\mathrm{Mg}}_{\mathrm{x}}\mathrm{O}/\mathrm{ZnO}$ bilayers is demonstrated. The theory further predicts that dielectric-ferroelectric and paraelectric-ferroelectric multilayers can potentially lead to induced ferroelectricity in the dielectric or paraelectric layers, resulting in the entire stack being switched, an exciting avenue for new discoveries. This thawing of “frozen ferroelectrics,” paraelectrics, and potentially dielectrics with high dielectric constants promises a large class of new ferroelectrics with exciting prospects for previously unrealizable domain-patterned optoelectronic and memory technologies.

Achieving quantum speedups in practical tasks remains challenging for current noisy intermediate-scale quantum (NISQ) devices. These devices always encounter significant obstacles such as inevitable physical errors and the limited scalability of current near-term algorithms. Meanwhile, assuming a typical architecture for fault-tolerant quantum computing (FTQC), realistic applications inevitably require a vast number of qubits, typically exceeding ${10}^6$, which seems far beyond near-term realization. In this work, to bridge the gap between the NISQ and FTQC eras, we propose an alternative approach to achieve practical quantum advantages on early-FTQC devices. Our framework is based on partially fault-tolerant logical operations to minimize spatial overhead and avoids the costly distillation techniques typically required for executing non-Clifford gates. To this end, we develop a space-time efficient state preparation protocol to generate an ancillary non-Clifford state consumed for implementing an analog rotation gate with an arbitrary small angle $\theta$ and a remarkably low worst-case error rate below $\mathcal{O}(|\theta |p_{\mathrm{ph}})$, where $p_{\mathrm{ph}}$ is the physical error rate. Furthermore, we propose several error suppression schemes tailored to our preparation protocol, which are essential to minimize the overhead for mitigating errors. Based on this framework, we present several promising applications that leverage the potential of our framework, including the Trotter simulation and quantum phase estimation (QPE). Notably, we demonstrate that our framework allows us to perform the QPE for an $(8\times 8)$-site Hubbard model with fewer than $6.8\times {10}^4$ qubits and an execution time of 10.6 days (or 14 min with full parallelization) under $p_{\mathrm{ph}}={10}^{-4}$, which is significantly faster than recent classical estimation with tensor network techniques (density matrix renormalization group and projected entangled pair states).

The observation of delicate correlated phases in twisted heterostructures of graphene and transition metal dichalcogenides suggests that moiré flat bands are intrinsically resilient against certain types of disorder. Here, we investigate the robustness of moiré flat bands in the chiral limit of the Bistritzer-MacDonald model—applicable to both platforms in certain limits—and demonstrate drastic differences between the first magic angle and higher magic angles in response to chiral symmetric disorder that arise, for instance, from lattice relaxation. We understand these differences using a hidden constant of motion that permits the decomposition of the non-Abelian gauge field induced by interlayer tunnelings into two decoupled Abelian ones. At all magic angles, the resulting effective magnetic field splits into an anomalous contribution and a fluctuating part. The anomalous field maps the moiré flat bands onto a zeroth Dirac Landau level, whose flatness withstands any chiral symmetric perturbation such as nonuniform magnetic fields due to a topological index theorem—thereby underscoring a topological mechanism for band flatness. Only the first magic angle can fully harness this topological protection due to its weak fluctuating magnetic field. In higher magic angles, the amplitude of fluctuations largely exceeds the anomalous contribution, which we find results in a physically meaningless chiral operator and an extremely large sensitivity to microscopic details and an exponential collapse of the single-particle gap. Through numerical simulations, we further study various types of disorder and identify the scattering processes that are enhanced or suppressed in the chiral limit. Interestingly, we find that the topological suppression of disorder broadening persists away from the chiral limit and is further accentuated by isolating a single sublattice polarized flat band in energy. Our analysis suggests the Berry curvature hot spot at the top of the $K$ and $K^{\prime }$ valence band in the transition metal dichalcogenide monolayers is essential for the stability of its moiré flat bands and their correlated states.

Materials where flattened electronic dispersions arise from destructive phase interference, rather than localized orbitals, have emerged as promising platforms for studying emergent quantum phenomena. Crucial next steps involve tuning such flat bands to the Fermi level, where they can be studied at low energy scales, and assessing their potential for practical applications. Here, we show that the interplay of highly dispersive and ultraflat bands inherent to these systems can lead to extreme interband scattering-induced electron-hole asymmetry, which can be harnessed in thermoelectrics. Our comprehensive theoretical and experimental investigation of ${\mathrm{Ni}}_3{\mathrm{In}}_{1-x}{\mathrm{Sn}}_x$ kagome metals supports this concept, showing that it could lead to thermoelectric performance on par with state-of-the-art semiconductors such as ${\mathrm{Bi}}_2{\mathrm{Te}}_3$. In ${\mathrm{Ni}}_3\mathrm{In}$, scattering-induced electron-hole asymmetry is, however, subdued by an exotic conduction mechanism arising from quantum tunneling of charge carriers between Dirac bands, unrelated to the flat band itself. We outline strategies to selectively switch off this tunneling transport through negative chemical pressure or strain. Our study proposes a new direction to explore in topological flat-band systems and vice versa introduces a novel tuning knob for thermoelectric materials.

Identifying efficient pathways to modulate quantum coherence is a crucial step toward realizing ultrafast switching of macroscopic orders, which requires the microscopical understanding of the interplay between multidegrees of freedom. Here, we demonstrate an all-optical method to control the coherent electron and lattice excitation in a prototypical nodal-line semimetal ZrSiS. We show the displacive excitation of two coherent Raman-active phonon modes, which results in a mode-selective renormalization of its topological band structure comparable with previous experimental observations. We subsequently realize an effective manipulation of the coherent lattice vibration, not only for their amplitude, but also a $\pi$-phase shift by tuning the laser intensity and frequency. We pinpoint that such a phase shift originates from the photoinduced carrier redistribution and can, in turn, determine the quasiparticle renormalization, for example, to induce an ultrafast topological Lifshitz transition, which we anticipate can be detected by pump-probe transport measurements. These results address the requirements for a directional pumping of coherent phonons with laser fields and provide the opportunity to explore exotic nonequilibrium physics.

The Hubbard model is believed to capture the essential physics of cuprate superconductors. However, recent theoretical studies suggest that it fails to reproduce a robust and homogeneous superconducting ground state. Here, using resonant inelastic x-ray scattering and density matrix renormalization group calculations, we show that magnetic excitations in the prototypical cuprate ladder ${\mathrm{Sr}}_{14}{\mathrm{Cu}}_{24}{\mathrm{O}}_{41}$ are inconsistent with those of a simple Hubbard model. The magnetic response of hole carriers, contributing to an emergent branch of spin-flip excitations, is strongly suppressed. This effect is the consequence of strong $d$-wavelike pairing, enhanced by nearly an order of magnitude through a large nearest-neighbor attractive interaction and persisting up to at least 260 K. The close connection between the physics of cuprate ladders and that of the two-dimensional compounds suggests that such an enhanced hole pairing may be a universal feature of superconducting cuprates.

We report evidence for superconductivity with onset temperatures up to 11 K in thin films of the infinite-layer nickelate parent compound ${\mathrm{NdNiO}}_2$. A combination of oxide molecular beam epitaxy and atomic hydrogen reduction yields samples with high crystallinity and low residual resistivities, a substantial fraction of which exhibit superconducting transitions. We survey a large series of samples with a variety of techniques, including electrical transport, scanning transmission electron microscopy, x-ray absorption spectroscopy, and resonant inelastic x-ray scattering, to investigate the possible origins of superconductivity. We propose that superconductivity could be intrinsic to the undoped infinite-layer nickelates but suppressed by disorder due to a possibly sign-changing order parameter, a finding which would necessitate a reconsideration of the nickelate phase diagram. Another possible hypothesis is that the parent materials can be hole doped from randomly dispersed apical oxygen atoms, which would suggest an alternative pathway for achieving superconductivity.

In this report, we apply a suite of ultrafast spectroscopic techniques and advanced calculations to reveal the interplay between electronic and lattice degrees of freedom in ferroelectric ${\mathrm{BiFeO}}_3$. Using transient sum frequency generation spectroscopy, which is sensitive to electronic polarizations, we observe a transient electronic dipole reduction upon optical excitation which recovers at 0.5 and 10 ps timescale. The time-dependent density functional theory calculation reveals that both ligand-metal charge transfer and local excitation transition occurred upon photo excitation. To reveal the nature of electronic dipole restoration, we employ transient extreme ultraviolet (EUV) spectroscopy—an element-specific ultrafast technique that follows charge dynamics of Bi, Fe, and O altogether. The transient EUV dynamics observed both ultrafast free charge carrier relaxation to excitons, as well as polaron formation. However, a timescale comparison suggests that only the polaron formation is responsible for the 0.5 ps electronic dipole restoration, whereas the faster electronic relaxation does not contribute to the ferroelectric property changes. Multireference configuration interaction calculation further corroborates this result by showing both Fe and Bi atoms shift from the ground state equilibrium—leading to the polaron formation. Our result disentangles the multidegrees of freedom in ultrafast ferroelectric modulation and identifies the pivotal motion—a local polaron formation—for the fast ferroelectric recovery. It provides crucial insights on the specific lattice distortion that could modulate properties or phase transitions of condensed matter materials.

Defects in crystals can have a transformative effect on the properties and functionalities of solid-state systems. Dopants in semiconductors are core components in electronic and optoelectronic devices. The control of single color centers is at the basis of advanced applications for quantum technologies. Unintentional defects can also be detrimental to the crystalline structure and hinder the development of novel materials. Whatever the research perspective, the identification of defects is a key, but complicated, and often long-standing issue. Here, we present a general methodology to identify point defects by combining isotope substitution and polytype control, with a systematic comparison between experiments and first-principles calculations. We apply this methodology to hexagonal boron nitride ($h$-BN) and its ubiquitous color center emitting in the ultraviolet spectral range. From isotopic purification of the host $h$-BN matrix, a local vibrational mode of the defect is uncovered, and isotope-selective carbon doping proves that this mode belongs to a carbon-based center. Then, by varying the stacking sequence of the host $h$-BN matrix, we unveil different optical responses to hydrostatic pressure for the nonequivalent configurations of this ultraviolet color center. We conclude that this defect is a carbon dimer in the honeycomb lattice of $h$-BN. Our results show that tuning the stacking sequence in different polytypes of a given crystal provides unique fingerprints contributing to the identification of defects in 2D materials.

Quantized lattice vibrations (i.e., phonons) in solids are robust and unambiguous fingerprints of crystal structures and of their symmetry properties. In metals and semimetals, strong electron-phonon coupling may lead to so-called Kohn anomalies in the phonon dispersion, providing an image of the Fermi surface in a nonelectronic observable. Kohn anomalies become prominent in low-dimensional systems, in particular, in graphene, where they appear as sharp kinks in the in-plane optical phonon branches. However, in spite of intense research efforts on electron-phonon coupling in graphene and related van der Waals heterostructures, little is known regarding the links between the symmetry properties of optical phonons at and near Kohn anomalies and their sensitivity towards the local environment. Here, using inelastic light scattering (Raman) spectroscopy, we investigate a set of custom-designed graphene-based van der Waals heterostructures, wherein dielectric screening is finely controlled at the atomic-layer level. We demonstrate experimentally and explain theoretically that, depending exclusively on their symmetry properties, the two main Raman lines of graphene react differently to the surrounding environment. While the 2D line, which is due to near-zone-edge optical phonons, undergoes changes due to the neighboring dielectric environment, the in-plane, zone-center optical phonons are symmetry protected from the influence of the latter. These results shed new light on the unique electron-phonon coupling properties in graphene and related systems and provide invaluable guidelines to characterize dielectric screening in van der Waals heterostructures and moiré superlattices.

The high coercive field (${\mathcal{E}}_c$) of hafnia-based ferroelectrics presents a major obstacle to their applications. The ferroelectric switching mechanisms in hafnia that dictate ${\mathcal{E}}_c$, especially those related to domain nucleation in the nucleation-limited-switching (NLS) model and domain-wall motion in the Kolmogorov-Avrami-Ishibashi (KAI) model, have remained elusive. We develop a deep-learning-assisted multiscale approach, incorporating atomistic insights into the critical nucleus, to predict both NLS- and KAI-type coercive fields. The theoretical NLS-type ${\mathcal{E}}_c$ values agree with previous experimental results as well as our own measurements and also exhibit the correct thickness scaling for films between 3 and 20 nm. Combined theoretical and experimental investigations reveal that the giant ${\mathcal{E}}_c$ in hafnia-based ferroelectrics arises from the ultrathin geometry, which confines switching to the NLS mechanism. We predict that the theoretical lower limit for KAI-type ${\mathcal{E}}_c$ is $0.1\mathrm{MV}/\mathrm{cm}$ arising from mobile domain walls. The activation of KAI-type switching to achieve lower ${\mathcal{E}}_c$ is supported by our experimental demonstration of a low coercive field of $1\mathrm{MV}/\mathrm{cm}$ in 60 nm ferroelectric $({\mathrm{HfO}}_2{)}_n/({\mathrm{ZrO}}_2{)}_n$ ($n=3$ unit cells) superlattices. These findings establish a comprehensive framework for understanding ferroelectric switching in hafnia and highlight the potential of geometry and domain-wall engineering to achieve low-${\mathcal{E}}_c$ devices.

The generation of topologically nontrivial magnetic configurations has been a pivotal topic in both basic and applied nanomagnetism research. Localized noncoplanar magnetic defects such as skyrmions or merons were found to interact strongly with currents, making them interesting candidates for future spintronics applications. So far, mostly systems with a high rotational symmetry have been investigated where skyrmions were axially symmetric. Here, we study a low-symmetry system by spin-polarized scanning tunneling microscopy and an atomistic spin model using parameters based on first-principles calculations. We demonstrate how a delicate balance between energy terms generates both topologically trivial and nontrivial domain walls, depending on their nonequivalent crystallographic direction. The topological walls consist of alternating merons and antimerons, and the topological charge is 1 for every 6-nm length of the wall. The incorporation of holes in the films facilitates the transition from an in-plane ferromagnetic ground state to a spin-spiral state. Both domain walls and spirals transition into isolated elongated magnetic skyrmions in applied magnetic fields. These findings establish low-symmetry systems as a versatile platform for spin-texture engineering.

Coherent phonons, light-induced coherent lattice vibrations in solids, provide a powerful route to engineer structural and electronic degrees of freedom using light. In this manuscript, we formulate an ab initio theory of the displacive excitation of coherent phonons (DECP), the primary mechanism for light-induced structural control in semimetals. Our study—based on the ab initio simulations of the ultrafast electron and coherent-phonon dynamics in the presence of electron-phonon interactions—establishes a predictive computational framework for describing the emergence of light-induced structural changes and the ensuing transient band-structure renormalization arising from the DECP mechanism. We validate this framework via a combined theoretical and experimental investigation of coherent phonons in the elemental semimetal antimony. Via a Fourier analysis of time- and angle-resolved photoemission spectroscopy measurements, we retrieve information about transient spectral features and quasiparticle renormalization arising from the coherent $A_{1g}$ phonon as a function of momentum, energy, time, and fluence. The qualitative and quantitative agreement between experiment and theory corroborates the first-principles approach formulated in this study. We further apply this formalism to investigate the coherent-phonon dynamics in the topological Weyl semimetal Td-${\mathrm{WTe}}_2$. Besides reproducing the entire spectrum of coherent phonons observed in experiments, our simulations clearly indicate that the shear $A_{1g}$ mode—the mode orchestrating a light-induced phase transition in Td-${\mathrm{WTe}}_2$—is strongly driven by the DECP mechanism and, thus, provide a conclusive explanation for the driving mechanism underpinning the phase transition. Besides advancing the fundamental understanding of electron-phonon interactions mediated by coherent phonons, this study opens new opportunities for predictively engineering structural and electronic degrees of freedom in semimetals via the DECP mechanism.

We study topologically stable defect structures in systems where the defect line classification in three dimensions and associated algebra of interactions (the fundamental group) are governed by the non-Abelian eight-element group, the quaternions $Q_8$. The non-Abelian character of the defect algebra leads to a topological rigidity of bound defect pairs, and trivalent junctions which are the building blocks of multijunction trivalent networks. We realize such structures in laboratory chiral nematics and analyze their behavior analytically, along with numerical modeling.

Systems of spins with strong dipolar interactions and controlled dimensionality enable new explorations in quantum sensing and simulation. In this work, we investigate the creation of strong dipolar interactions in a two-dimensional ensemble of nitrogen-vacancy (NV) centers generated via plasma-enhanced chemical vapor deposition on (111)-oriented diamond substrates. We find that diamond growth on the (111) plane yields high incorporation of spins, both nitrogen and NV centers, where the density of the latter is tunable via the miscut of the diamond substrate. Our process allows us to form dense, preferentially aligned, 2D NV ensembles with volume-normalized ac sensitivity down to ${\eta }_{\mathrm{ac}}=810\mathrm{pT}\mathrm{\mu }{\mathrm{m}}^{3/2}{\mathrm{Hz}}^{-1/2}$. Furthermore, we show that (111) affords maximally positive dipolar interactions among a 2D NV ensemble, which is crucial for leveraging dipolar-driven entanglement schemes and exploring new interacting spin physics.