Stacking and Twisting of Freestanding Complex Oxide Thin Films.

Thermodynamic metric usually works only for those black holes with more than one conserved charge, thereby excluding the Schwarzschild black hole. In this paper, however, different versions of thermodynamic metric are computed and compared for the Schwarzschild-like black hole by introducing new degrees of freedom. These new degrees of freedom have two purposes. First, the deformed metric may be treated offshell to the ordinary Schwarzschild black hole, and onshell physics corresponds to the submanifold by gauge fixing of this additional degree of freedom. In particular, the thermal Ricci scalar for the Schwarzschild black hole, though different for various deformations, can be obtained by switching off the deformation. Second, while deformed metric is treated onshell, a divergent Ricci scalar may signal an exotic phase in which new physical degrees of freedom manifest. This paper considers the new degree of freedom as the running Newton constant, a cutoff scale for regular black holes, a noncommutative deformation or the deformed parameter in the nonextensive Tsallis–Rènyi entropy.

The standard theory of metals, Fermi liquid theory, hinges on the key assumption that although the electrons interact, the low-energy excitation spectrum stands in a one-to-one correspondence with that of a non-interacting system. In the normal state of the copper-oxide high-temperature superconductors, drastic deviations from the Fermi liquid picture are obtained, highlighted by a pseudogap, broad spectral features and T-linear resistivity. A successful theory in this context must confront the highly constraining scaling argument which establishes that all 4-Fermi interactions are irrelevant (except for pairing) at a Fermi surface. This argument lays plain that new low-energy degrees of freedom are necessary. This paper focuses on the series of experiments on copper-oxide superconductors which reveal that the number of low-energy addition states per electron per spin exceeds unity, in direct violation of the key Fermi liquid tenet. These experiments point to new degrees of freedom, not made out of the elemental excitations, as the key mechanism by which Fermi liquid theory breaks down in the cuprates. A recent theoretical advance which permits an explicit integration of the high-energy scale in the standard model for the cuprates reveals the source of the new dynamical degrees of freedom at low energies, a charge 2e bosonic field which has nothing to do with pairing but rather represents the mixing with the high-energy scales. We demonstrate explicitly that at half-filling, this new degree of freedom provides a dynamical mechanism for the generation of the charge gap and antiferromagnetism in the insulating phase. At finite doping, many of the anomalies of the normal state of the cuprates including the pseudogap, T-linear resistivity and the mid-infrared band are reproduced. A possible route to superconductivity is explored.

The twist angle between the monolayers in van der Waals heterostructures provides a new degree of freedom in tuning material properties. We compare the optical properties of WSe2 homobilayers with 2H and 3R stacking using photoluminescence, Raman spectroscopy, and reflectance contrast measurements under ambient and cryogenic temperatures. Clear stacking-dependent differences are evident for all temperatures, with both photoluminescence and reflectance contrast spectra exhibiting a blue shift in spectral features in 2H compared to 3R bilayers. Density functional theory (DFT) calculations elucidate the source of the variations and the fundamental differences between 2H and 3R stackings. DFT finds larger energies for both A and B excitonic features in 2H than in 3R, consistent with experimental results. In both stacking geometries, the intensity of the dominant A1g Raman mode exhibits significant changes as a function of laser excitation wavelength. These variations in intensity are intimately linked to the stacking- and temperature-dependent optical absorption through resonant enhancement effects. The strongest enhancement is achieved when the laser excitation coincides with the C excitonic feature, leading to the largest Raman intensity under 514 nm excitation in 2H stacking and at 520 nm in 3R stacked WSe2 bilayers.

Following the graphene isolation, strong interest in two dimensional (2D) materials has been driven by their outstanding properties. Their typical intrinsic structure, including strong in-plane covalent bonding and weak out-of-plane Van der Waals interaction, makes them highly promising in diverse areas such as electronics, catalysis, and environment. Growth of 2D materials and staking of such materials, called van der Waals (VdW) heterostructures, require a synthesis approach able to control the deposition onto a support at the atomic scale. Thanks to their simplicity, versatility and ability to control thickness at the angstrom level, Atomic Layer Deposition (ALD) and its variant Atomic Layer Etching (ALET) appear as ones of the most suited techniques to synthesize 2D materials and based-heterostructures. In line with the spectacular improvement and emergence of new physical properties in 2D VdW heterostructures, the curvature and confinement effects provided by the reduced dimensionality of 1D VdW heterostructures make them highly attractive.Herein, ALD of hexagonal Boron Nitride (hBN) is discussed towards fabrication of VdW heterostructures. After introducing the two-step ALD approach for BN, based on polymer derived ceramic chemistry, deposition of hBN onto various carbon nanomaterials is presented. The influence of the degree of graphitization of the carbon substrate on BN growth (nucleation delay) and structure (amorphous, turbostratic, hexagonal phase) is discussed. Indeed, inertness of highly graphitic carbon might inhibit the initiation of ALD growth. The obtained BN-coated carbon materials are characterized by advanced analytic transmission electron microscopy. Particular attention is paid here to the successful fabrication of 1D VdW heterostructures made of few layer hBN coated-single wall carbon nanotubes. These new 1D materials are of particular interest for applications ranging from sensing and quantum optics, to biological labeling.

Van der Waals (vdWs) heterostructures based on 2D materials demonstrate great potential in high‐performance photodetectors. Strategic integration of topological insulators into van der Waals heterostructures provides new degrees of freedom for photocarrier generation, separation, and collection processes, leading to enhanced device metrics. Here, a MoSe 2 /Bi 2 Se 3 /MoTe 2 heterotrilayer photodetector is fabricated, where the Bi 2 Se 3 interlayer serves dual functions: i) as a dirac‐cone‐enabled ultrafast charge transport highway and ii) as a band alignment modulator creating built‐in fields in the MoSe 2 /Bi 2 Se 3 and Bi 2 Se 3 /MoTe 2 interfaces, respectively. Through Kelvin probe force microscopy, the band alignment in MoSe 2 /Bi 2 Se 3 /MoTe 2 is revealed. Under 532 nm illumination with a power density of 439.2 µW mm − 2 , the fabricated device exhibits exceptional performance metrics: a high responsivity (R) of 4.61 A W −1 , an external quantum efficiency (EQE) of 1070%, and a specific detectivity (D * ) of 2.29 × 10 11 Jones. The transient photoresponse demonstrates rapid response/recovery time of 4.0/4.1 ms, respectively. Furthermore, spectral response measurements confirm broadband detection capabilities spanning from the visible to near‐infrared range of 400–1550 nm. These metrics surpass conventional MoSe 2 /MoTe 2 devices by a near four‐fold higher photocurrent along with approximately twelve‐fold enhancements in R, EQE, and D * . This work provides a promising strategy for designing high‐performance photodetectors.

Complex oxide thin films have highly tunable magnetic properties, not only due to the correlated degrees of freedom within these materials, but also due to the constraints imposed by choice of substrate material. For example, the magnetic properties of manganite materials depend heavily on the bond lengths and angles, resulting in a dead layer at the substrate/film interface [1,2,3]. Recently, techniques have been developed to separate oxide thin films from the substrate after growth, decoupling structural and chemical degrees of freedom at the interface [4]. This technique therefore may allow us to reduce dead layers at the interface of manganite thin films.Thin films of (001)-oriented LaMnO3+δ (LMO) were grown using pulsed laser deposition on a water-soluble Sr3Al2O6 (SAO) film on top of a (001) SrTiO3 substrate. After coating the film with a 1μm thick support layer of PMMA polymer, the SAO was dissolved in water, allowing the film to be separated from the substrate; this freestanding film could then be transferred to a different substrate, in this case, oxidized Si or LSAT.X-ray diffraction measurements show that freestanding films remain highly crystalline and oriented, but are no longer strained to the substrate. Magnetometry measurements indicate very similar TC, magnetic anisotropy, and saturation magnetization for both freestanding and epitaxial films, and cannot discern any difference in dead layer. Therefore, polarized neutron reflectometry (PNR) was performed at multiple temperatures to understand the depth profile of both the nuclear and magnetic structure of these freestanding films.As shown in Fig. 1, the film transferred onto the LSAT substrate is very smooth and has an expected structure. Between the substrate and freestanding film is a region with slightly lower nuclear scattering length density (SLD); from STEM and EELS, we find that this region is caused by intermixing with or residue from the water-soluble layer. In contrast, the film transferred onto silicon has a much more complicated structure, as shown in Fig. 2. The PNR data can only be explained with a long-range corrugation or rumpling of the film when placed on the Si substrate, with water incorporated into the interfacial region. After chemical removal of the PMMA support, this corrugation remains.Regardless of the substrate material, the magnetic profile within the freestanding manganite film is uniform, and there is no evidence of the magnetically dead layers at the substrate/film interface seen in epitaxial films. Thus, although choice of substrate can still affect freestanding films through chemical effects – which may have implications for the integration of these freestanding films with silicon-based devices – removing the structural constraints imposed by epitaxial growth is a promising avenue to reduce the dead layer in manganites. **

van der Waals heterostructures (vdWHs) leverage the characteristics of two-dimensional (2D) material building blocks to create a myriad of structures with unique and desirable properties. Several commonly employed fabrication strategies rely on polymeric stamps to assemble layers of 2D materials into vertical stacks. However, the properties of such heterostructures frequently are degraded by contaminants, typically of unknown composition, trapped between the constituent layers. Such contaminants, therefore, impede studies of the intrinsic properties of heterostructures and hinder their application. Here, we use the photothermal induced resonance (PTIR) technique to obtain infrared spectra and maps of the contaminants down to a few attomoles and with nanoscale resolution. Heterostructures comprised of WSe2, WS2, and hexagonal boron nitride layers were found to contain significant amounts of poly(dimethylsiloxane) (PDMS) and polycarbonate, corresponding to the stamp materials used in their construction. Additionally, we verify that an atomic force microscope-based "nanosqueegee" technique is an effective method for locally removing contaminants by comparing spectra within as-fabricated and cleaned regions. Having identified the source of the contaminants, we demonstrate that cleaning PDMS stamps with isopropyl alcohol or toluene prior to vdWH fabrication reduces PDMS contamination within the structures. The general applicability of the PTIR technique for identifying the sources corrupting vdWHs provides valuable guidance for devising mitigation strategies (e.g., stamp cleaning or pre-/post-treatments) and enhances capabilities for producing materials with precisely engineered properties.

Nanoscale materials with multifunctional properties are necessary for the quick development of high-performance devices for a wide range of applications, hence theoretical research into new two-dimensional (2D) materials is encouraged. 2D materials have a distinct crystalline structure that leads to intriguing occurrences. Stacking diverse two-dimensional (2D) materials has shown to be an efficient way for producing high-performance semiconductor materials. We explored a 2D nanomaterial family, an MXO/MoX2 heterostructure (M = Hf, Ti and X = S, Se), for their various applications using first-principles calculations. We discovered that all of the heterostructure materials utilized are direct band gap semiconductors with band gaps ranging from 1.0 to 2.0 eV, with the exception of hexagonal HfSeO/MoSe2, which has a band gap of 0.525 eV. The influence of strain on the band gap of this HfSeO/MoSe2 material was investigated. In the visible range, we obtained promising optical responses with a high-power conversion efficiency. With fill factors of 0.5, MXO/MoX2 photovoltaic cells showed great PCE of up to 17.8%. The tunable electronic characteristics of these two-dimensional materials would aid in the development of energy conversion devices. According to our findings, the 2D Janus heterostructure of MXO/MoX2 (M = Hf, Ti and X = S, Se) material is an excellent choice for photovoltaic solar cells.

Recent advances have made it viable to fabricate optical surfaces that are not rotationally symmetric using a new generation of diamond-turning machines. This new fabrication capability allows for surfaces whose departure from a sphere varies both radially and azimuthally in the aperture to be machined into an optical surface. This new degree of freedom allows for the design of unobscured optical systems that are truly non-symmetric by tilting the optical surfaces themselves. With this new design degree of freedom, the aperture and field of view can be pushed to yield an order of magnitude increase in aerial coverage over current production while maintaining a compact solution. In one particular case, to be presented, a diffraction limited (< λ/10), long wave infrared (LWIR), F/1.9, 10° full field of view sensor telescope is designed by introducing these non-symmetric surfaces into the optical surface prescription.

We investigate the interfacial transport of water and hydrophobic solutes on van der Waals bilayers and heterostructures formed by stacking graphene, hBN, and MoS2 using extensive ab initio molecular dynamics simulations. We compute water slippage and the diffusio-osmotic transport coefficient of hydrophobic particles at the interface by combining hydrodynamics and the theory of the hydrophobic effect. We find that slippage is dominated by the layer that is in direct contact with water and only marginally altered by the second layer, leading to a so-called "slip opacity". The screening of the lateral forces, where the liquid does not feel the forces coming from the second nearest layer, is one of the factors leading to the "slip opacity" in our systems. The diffusio-osmotic transport of small hydrophobes (with a radius below 2.5 Å) is also affected by the slip opacity, being dramatically enhanced by slippage. Furthermore, the direction of diffusio-osmotic flow is controlled by the solute size, with the flow in the opposite direction of the concentration gradient for smaller hydrophobes, and vice versa for larger ones. We connect our findings to the wetting properties of two-dimensional materials, and we propose that slippage and wetting can be controlled separately: whereas the slippage is mostly determined by the layer in closer proximity to water, wetting can be finely tuned by stacking different two-dimensional materials. Our study advances the computational design of two-dimensional materials and van der Waals heterostructures, enabling precise control over wetting and slippage properties for applications in coatings and water purification membranes.

The recent observational data in cosmology seem to indicate that the universe is currently expanding in an accelerated way. This unexpected conclusion can be explained assuming the presence of a non-vanishing yet extremely fine tuned cosmological constant, or invoking the existence of an exotic source of energy, dark energy, which is not observed in laboratory experiments yet seems to dominate the energy budget of the Universe. On the other hand, it may be that these observations are just signalling the fact that Einstein's General Relativity is not the correct description of gravity when we consider distances of the order of the present horizon of the universe. In order to study if the latter explanation is correct, we have to formulate new theories of the gravitational interaction, and see if they admit cosmological solutions which fit the observational data in a satisfactory way. Quite generally, modifying General Relativity introduces new degrees of freedom, which are responsible for the different large distance behaviour. On one hand, often these new degrees of freedom have negative kinetic energy, which implies that the theory is plagued by ghost instabilities. On the other hand, for a modified gravity theory to be phenomenologically viable it is necessary that the extra degrees of freedom are efficiently screened on terrestrial and astrophysical scales. One of the known mechanisms which can screen the extra degrees of freedom is the Vainshtein mechanism, which involves derivative self-interaction terms for these degrees of freedom. In this thesis, we consider two different models, the Cascading DGP and the dRGT massive gravity, which are candidates for viable models to modify gravity at very large distances. Regarding the Cascading DGP model, we consider the minimal (6D) set-up and we perform a perturbative analysis at first order of the behaviour of the gravitational field and of the branes position around background solutions where pure tension is localized on the 4D brane. We consider a specific realization of this set-up where the 5D brane can be considered thin with respect to the 4D one. We show that the thin limit of the 4D brane inside the (already thin) 5D brane is well defined, at least for the configurations that we consider, and confirm that the gravitational field on the 4D brane is finite for a general choice of the energymomentum tensor. We also confirm that there exists a critical tension which separates background configurations which possess a ghost among the perturbation modes, and background configurations which are ghost-free. We find a value for the critical tension which is different from the value which has been obtained in the literature; we comment on the difference between these two results, and perform a numeric calculation in a particular case where the exact solution is known to support the validity of our analysis. Regarding the dRGT massive gravity, we consider the static and spherically symmetric solutions of these theories, and we investigate the effectiveness of the Vainshtein screening mechanism. We focus on the branch of solutions in which the Vainshtein mechanism can occur, and we truncate the analysis to scales below the gravitational Compton wavelength, and consider the weak field limit for the gravitational potentials, while keeping all non-linearities of the mode which is involved in the screening. We determine analytically the number and properties of local solutions which exist asymptotically on large scales, and of local (inner) solutions which exist on small scales. Moreover, we analyze in detail in which cases the solutions match in an intermediate region. We show that asymptotically flat solutions connect only to inner configurations displaying the Vainshtein mechanism, while non asymptotically flat solutions can connect both with inner solutions which display the Vainshtein mechanism, or with solutions which display a self-shielding behaviour of the gravitational field. We show furthermore that there are some regions in the parameter space of the theory where global solutions do not exist, and characterize precisely in which regions the Vainshtein mechanism takes place.

Two-dimensional (2D) heterostructures (HSs) offer unlimited possibilities for playing with layer number, order, and twist angle. The realization of high-performance optoelectronic devices, however, requires the achievement of specific band alignments, k-space matching between conduction and valence band extrema, and efficient charge transfer between the constituent layers. Fine-tuning mechanisms to design ideal HSs are lacking. Here, we show that layer-selective strain engineering can be exploited as an extra degree of freedom to tailor the band alignment and optical properties of 2D HSs. To that end, strain is selectively applied to MS2 (M = Mo or W) monolayers in InSe/MS2 HSs, triggering a giant photoluminescence enhancement of the highly tunable but weakly emitting InSe of up to >2 orders of magnitude. Resonant excitation measurements, supported by first-principles calculations, provide evidence of a strain-activated charge transfer from the MS2 monolayers toward InSe. The huge emission enhancement of InSe widens its range of applications for optoelectronics.

Manipulation of valley-pseudospin in 2D WTe2/CrI3 van der Waals heterostructure by magnetic proximity effect

Abstract: Next-Gen Semiconductors: Quantum Dots, 2D Materials, and Spintronics for AI, 5G and Beyond offers a unified framework linking quantum‐scale phenomena to manufacturable devices and system performance. The book integrates three pillars—colloidal quantum dots for photonics and imaging, van der Waals (2D) materials for RF/logic/sensing, and spintronics/magnonics for non-volatile and wave-based compute—into heterogeneous packages that meet the latency, energy, and reliability targets of AI accelerators and 5G/6G front ends. We translate materials descriptors (bandgap, spin Hall angle, dielectric screening) into device metrics (PLQY, TMR, contact resistivity) and system KPIs (pJ/MAC, EVM/ACLR, J/bit, gCO₂e/GB). Beyond performance, the text embeds security (measured boot, PUFs, PQC agility), safety (reliability and failure containment), and sustainability (circular materials, repairability) as first-class requirements. Methods chapters specify metrology, ML-assisted TCAD, benchmark suites, and accreditation gates from pilot line to qualification. Case studies across Asia, Europe, the Americas, and Africa demonstrate repeatable paths from discovery to deployment. The result is a rigorous, end-to-end playbook for researchers, practitioners, and policymakers seeking to convert nanoscale innovation into trusted, low-carbon infrastructure for intelligent sensing, connectivity, and computation. Keywords: quantum dots, perovskite QDs, 2D materials, graphene, TMDCs, spintronics, SOT-MRAM, magnonics, van der Waals heterostructures, Si photonics, co-packaged optics, antenna-in-package, AI accelerators, 5G/6G RF, energy efficiency, reliability, hardware security, post-quantum cryptography, sustainability, circular electronics, ML-assisted TCAD

Spintronics addresses the spin degree of freedom of electrons as an analog of the traditional information technology approach toward the charge degree of freedom. Two-dimensional (2D) materials have provided a new platform for spintronics since the successful isolation of graphene in 2004. In this review, we discuss the recent progress of chromium tri-iodide (CrI3), phosphorene, silicene, germanene, stanene, animonene, and borophene in spintronics. Among these 2D materials, CrI3 has intrinsic magnetism, while the others are single-element 2D materials (Xenes) without intrinsic magnetism. A review is presented of works based on different research methods, including experimental, theoretical, and simulation studies. Various morphologies of 2D materials are included, such as monolayers, bilayers, nanoribbons, heterojunctions, and nanoflakes of different shapes. The effect of atomic doping, vacancies, line defects, strain, and external fields is also summarized. Among the properties and applications of these materials, the spin polarization, spin/valley-dependent resistance, various Hall effects, RKKY interaction, Rashba effect, and topological phase are the most widely studied topics. Over the past two years, various novel spintronics devices have been designed based on 2D materials, and numerous fundamental functions in spintronics have been realized. Thus, we also summarize the emerging novel devices in this field. Finally, we present a summary of the remaining challenges and possible future directions.