Brief Reviews of Nature: Volume 616, Issue 7956 (April 13, 2023)

This review is written by non-expert and used for personal study only.

Giant magnetoresistance of Dirac plasma in high-mobility graphene

• Electronic Properties and Materials

Introduction to Magnetoresistance and Carrier Mobility in Graphene

Magnetoresistance (MR) is a property of metallic systems that describes how their electrical resistance changes when exposed to a magnetic field. MR can be attributed to both intrinsic and extrinsic mechanisms, but understanding these mechanisms in new materials remains a challenge. High carrier mobility is essential for observing significant MR. A few materials, like doped graphene and indium antimonide, have highly mobile carriers at room temperature. Monolayer graphene, particularly at the neutrality point, exhibits exceptionally high mobility, making its magnetoresistivity stand out from other quantum-critical systems.

Investigating Graphene’s Electron-Hole Plasma: Observations and Features

Researchers conducted experiments on multiterminal Hall bars created from monolayer graphene (MLG) encapsulated in hexagonal boron nitride (hBN), a layered material that protects and preserves the unique electronic properties of graphene. These structures showed high mobility at low temperatures. In response to small magnetic fields, the Dirac plasma (electron-hole plasma in graphene) demonstrated a surprisingly large magnetoresistivity, reaching around 110% at 300 K near the neutrality point. This value is much larger than that seen in typical metals or even high-quality encapsulated graphene. Graphene’s electron-hole plasma maintains high mobility at various temperatures because electron scattering is less efficient due to the material’s small effective mass and low density of states, which are features of the Dirac spectrum.

Understanding the Factors Behind Graphene’s Large Magnetoresistivity

The high mobility of Dirac fermions (charge carriers in graphene) in low magnetic fields results in large magnetoresistivity. This is because electron-hole scattering in graphene is ineffective at suppressing Hall currents, which arise from the flow of charged particles in a magnetic field. The relative motions of electrons and holes in graphene differ when exposed to zero and finite magnetic fields. In high magnetic fields, graphene’s magnetotransport properties change from a parabolic to a linear relationship. This linear MR is almost independent of temperature and can be attributed to Landau quantization, a phenomenon that describes the behavior of electrons in a magnetic field. The linear MR is not due to complex current flows or edge effects but is instead a result of Coulomb interactions (electrostatic forces between charged particles) and the confinement of charge carriers within electron and hole puddles.

Revisiting Magnetotransport Theories and Looking Forward

Previous reports of linear MR in high-magnetic-field behavior were attributed to complex current flows that become non-uniform as the Hall resistivity increases proportionally to the magnetic field. Abrikosov’s linear MR, which was predicted to occur in 3D semimetals with Dirac-like spectra, cannot be justified for 2D transport in a smooth background potential. The Drude model, a classical model for describing electrical conduction, is used to explain the observed linear MR. The linearity in magnetic fields arises from the linear increase in carrier density in the zeroth Landau level. More research is needed to develop a detailed theory of magnetotransport in the 2D Boltzmann plasma at the zeroth Landau level.

Graphene’s Dirac plasma exhibits high magnetoresistivity due to its unique properties in quantizing fields, making it an ideal model system for studying relevant high-field physics. The ability to modify magnetotransport by adjusting electron-electron interactions using proximity screening offers exciting possibilities for future research and applications.

Required Additional Study Materials

• Castro Neto, A. H., Guinea, F., Peres, N. M. R., Novoselov, K. S. & Geim, A. K. The electronic properties of graphene. Rev. Mod. Phys. 81, 109–162 (2009).
• Yankowitz, M., Ma, Q., Jarillo-Herrero, P. & LeRoy, B. J. van der Waals heterostructures combining graphene and hexagonal boron nitride. Nat. Rev. Phys. 1, 112–125 (2019).
• Phillips, P. W., Hussey, N. E. & Abbamonte, P. Stranger than metals. Science 377, eabh4273 (2022).
• Pippard, A. B. Magnetoresistance in Metals(Cambridge Univ. Press, 1989).

Introductory material

• “Condensed Matter Field Theory” by Alexander Altland and Ben Simons
• “A Student’s Guide to the Ising Model” by James S. Walker
• “Quantum Mechanics in Nanoscience and Engineering” by Uri Peskin
• “Modern Condensed Matter Physics” by Steven M. Girvin and Kun Yang

Reference

Xin, N., Lourembam, J., Kumaravadivel, P. et al. Giant magnetoresistance of Dirac plasma in high-mobility graphene. Nature 616, 270–274 (2023).

Separating single- from multi-particle dynamics in nonlinear spectroscopy

• Optical Spectroscopy

Challenges in Time-Resolved Spectroscopy and Existing Approaches

Time-resolved spectroscopy is a powerful tool for studying single and multiple excitations, but it faces challenges in organic materials due to exciton-exciton annihilation (EEA). Multi-exciton signals can be used to monitor transport, but a model for exciton diffusion and interaction is needed to separate and analyze single and multi-exciton signals. Intensity-dependent experiments have been used to study multi-particle dynamics in various samples, but existing methods do not allow for the separation of single and multi-particle responses without prior knowledge of the system or fitting procedures.

A Novel Method for Separating Single and Multi-Particle Dynamics

Researchers have developed a new method to separate single and multi-particle dynamics in transient absorption, enabling artifact-free single-particle measurement at high excitation powers and providing systematic information on multi-particle correlations. The method involves using a pump pulse with varying intensity and a weak probe pulse. By measuring the pump-probe signal at different intensities, researchers can isolate all contributions of higher-order responses.

Measuring Higher-Order Nonlinearities and Isolating Orders

Inspired by phase cycling, researchers have devised a strategy to choose appropriate values of pump intensity for multi-pulse spectroscopy. These values allow for the addition of n-quantum signals and the evaluation of nonlinear order contributions. The matrix formed by the coefficients can be easily inverted, and the orders are isolated using a specific relation. This procedure is general and can be applied to any measured sample.

Applications and Implications of the New Method

The new method has been tested on various samples, including photosynthetic complexes and quantum dots, and has produced accurate results. It also provides a model-independent way to measure the rate of excitation transfer in photosynthetic complexes, which was previously inferred through models. By simultaneously acquiring signals for single-exciton, bi-exciton, tri-exciton, and higher excitations, researchers can better understand the competition between transport and annihilation. This knowledge can be applied to improve exciton mobility in organic photovoltaics and probe particle interactions in diverse research areas.

Insights into Energy Transfer Rates in Biological Samples

Researchers performed two experiments to study energy transfer rates in biological samples using ‘blue’ and ‘red’ excitation. They discovered that higher-order signals were dominated by EEA, even in molecules with only one possible electronic excitation. TA signals saturate as a function of pump intensity for two-level systems, and higher-order signals capture this non-linear dependence.

Understanding Quantum Dot Responses

Third- and fifth-order responses in CdSe/ZnS core-shell quantum dots exhibit opposite signs but similar spectral shapes and time dynamics. By using the developed equation, researchers can reconstruct the intensity dependence of the signal, which agrees with the fitted saturation curve. Higher-order responses in a two-level system represent the negation of lower-order responses rather than multi-particle dynamics. The combined third- and fifth-order signals allow better determination of the excitations of the system than is possible in third-order alone.

Exploring Exciton States in Quantum Dots

Third- and fifth-order signals in quantum dots reveal exciton states that were not visible in the third-order signal alone. Both fifth-order features decay exponentially on a timescale of about 130 ps, providing a measure of the bi-exciton lifetime. Highly nonlinear TA successfully extracts the single-excitation dynamics and provides insight into the multi-particle properties of Si nanocrystals in an SiO$_2$ matrix.

Broader Applications and Future Research

The novel method for isolating nonlinear orders in the TA signal can be applied in any TA experiment on any sample. This approach allows for the systematic measurement of multi-particle dynamics and enables researchers to probe particle interactions in a wide range of research areas. Future studies can further refine this method and explore its potential applications in other fields, such as materials science, chemistry, and biology.

Required Additional Study Materials

• Mukamel, S. Principles of Nonlinear Optical Spectroscopy (Oxford Univ. Press, 1995).
• Smith, M. B. & Michl, J. Singlet fission. Chem. Rev. 110, 6891–6936 (2010).
• Polman, A., Knight, M., Garnett, E. C., Ehrler, B. & Sinke, W. C. Photovoltaic materials: present efficiencies and future challenges. Science 352, aad4424 (2016).
• Diels, J.-C. & Rudolph, W. Ultrashort Laser Pulse Phenomena: Fundamentals, Techniques, and Applications on a Femtosecond Time Scale (Academic Press, 1996).

Introductory material

• “An Introduction to Transient Absorption Spectroscopy and Nonlinear Photochemical Behavior of Polymer Systems” by Hiroshi Masuhara

Reference

Malý, P., Lüttig, J., Rose, P.A. et al. Separating single- from multi-particle dynamics in nonlinear spectroscopy. Nature 616, 280–287 (2023).

Mechanistic formulation of inorganic membranes at the air–liquid interface

• Surfaces and Thin Films

Introduction to Solid-Liquid Interface Shielding Strategy

The article explores a method called interfacial polymerization, which is used to create ultrathin polymeric and freestanding inorganic membranes. These membranes are thin layers of material with selective permeability. The solid-liquid interface shielding (SLIS) strategy is introduced, which involves using a hydrogel coating to promote nucleation, or the formation of new solid particles, at the air-liquid interface rather than on the walls of the container. This strategy was tested using the silver mirror reaction, which resulted in a homogenous silver membrane. Further testing led to the creation of a library of membranes composed of 42 elements and representatives from eight classic material categories.

Understanding Membrane Growth and Formation Process

While the SLIS technique effectively positions building blocks at the air-liquid interface, it doesn’t provide guidance for maintaining in-plane continuity during membrane growth. Two important factors influencing membrane formation are Brownian motion, the random movement of particles in a fluid, and capillary attraction, a force that draws particles together. Researchers developed a fast spectroscopic technique to study critical structural changes in the membrane formation process, which involves observing how silver particles absorb and reflect light. This information helped researchers understand how nanoparticles transform into two-dimensional clusters and eventually form a freestanding membrane.

The Role of Collision Dynamics in Membrane Formation

The Cheerios effect, a phenomenon where floating objects tend to cluster together, plays a significant role in the formation of connections in polycrystalline silver membranes. The energy needed for mechanical welding, or the joining of particles, increases with the size difference between floating objects. In situ observations, or observations made in the actual environment where the process occurs, confirmed the dynamic growth of membranes using various building blocks. This smooth network evolution allows for continuous adjustment of the through-hole ratio, or the proportion of holes in the membrane, by controlling the reaction time.

Exploring New Membranes and Expanding Applications

The study investigates the synthesis of previously unexplored membranes within complex dynamic systems and constructs a phase diagram, a graphical representation of the different states of a system, based on graph theory. This diagram helps identify distinct regimes and reveals that successful membrane formation with adjustable thickness can be achieved in the closed network phase. This qualitative phase diagram provides guidance for inorganic membrane preparation using the SLIS system. The approach broadens the traditional understanding of membranes in terms of composition, structure, and functionality, unlocking new dimensions in inorganic membrane design and acting as a catalyst for rejuvenating mature membrane technology.

Required Additional Study Materials

• Genet, C. & Ebbesen, T. W. Light in tiny holes. Nature 445, 39–46 (2007).
• Lu, X. & Elimelech, M. Fabrication of desalination membranes by interfacial polymerization: history, current efforts, and future directions. Chem. Soc. Rev. 50, 6290–6307 (2021).
• Yang, C. & Suo, Z. Hydrogel ionotronics. Nat. Rev. Mater. 3, 125–142 (2018).
• Wolfram, S. Cellular automata as models of complexity. Nature 311, 419–424 (1984).

Introductory material

• “Inorganic Chemistry” by Gary L. Miessler, Paul J. Fischer, and Donald A. Tarr
• “Materials Science and Engineering: An Introduction” by William D. Callister Jr. and David G. Rethwisch
• “Membrane Technology and Engineering for Water Purification” by Rajindar Singh

Reference

Zhang, C., Lu, W., Xu, Y. et al. Mechanistic formulation of inorganic membranes at the air–liquid interface. Nature 616, 293–299 (2023).

Orbital-resolved observation of singlet fission

• Light Harvesting

Harnessing Singlet Fission for Solar Cell Efficiency Improvement

Singlet fission (SF) presents a promising approach to enhance the performance of third-generation solar cells by tapping into the exciton doubling effect. In the primary step of SF, a singlet exciton transforms into a bitriplet exciton, which is believed to involve delocalized charge-transfer states in the process.

Utilizing Momentum Maps for Distinguishing Transient

States in SF To tackle the challenges associated with matching experimental observations to proposed mechanisms, it is crucial to obtain orbital- and localization-resolved information. This allows for a clearer understanding of SF in crystalline pentacene. By employing time- and angle-resolved PE spectroscopy, researchers can generate momentum maps that differentiate transient and energetically overlapping states in SF, providing an orbital-resolved visualization of the SF process.

Examining Momentum-Integrated Dynamics and Disentangling Excited States

In this study, the momentum-integrated dynamics of excited states were investigated, revealing the simultaneous population of the singlet exciton and a lower energy signal. The lower energy signal was found to be a combination of two different, energetically overlapping transitions, which could be separated based on their orbital character. By utilizing a decomposition procedure, the researchers successfully disentangled the excited states, paving the way for a kinetic analysis of SF.

Ruling Out the Coherent Mechanism and Exploring Implications for Other Materials

The findings of this research suggest that the formation of the bitriplet exciton occurs after the optical excitation, following the decay of the singlet exciton. This observation contradicts the coherent mechanism, which would involve instantaneous population. The significant charge-transfer (CT) character of the singlet exciton aligns with the purely electronic CT-mediated mechanism. This orbital-resolved approach offers insights into SF in other materials like crystalline tetracene and hexacene and could potentially impact a variety of molecular processes and ultrafast dynamics of topological matter.

Required Additional Study Materials

• Smith, M. B. & Michl, J. Singlet fission. Chem. Rev. 110, 6891–6936 (2010).
• Miyata, K., Conrad-Burton, F. S., Geyer, F. L. & Zhu, X.-Y. Triplet pair states in singlet fission. Chem. Rev. 119, 4261–4292 (2019).
• Damascelli, A., Hussain, Z. & Shen, Z.-X. Angle-resolved photoemission studies of the cuprate superconductors. Rev. Mod. Phys. 75, 473–541 (2003).
• Fratini, S., Nikolka, M., Salleo, A., Schweicher, G. & Sirringhaus, H. Charge transport in high-mobility conjugated polymers and molecular semiconductors. Nat. Mater. 19, 491–502 (2020).

Introductory material

• “Atoms, Molecules and Photons: An Introduction to Atomic-, Molecular- and Quantum Physics” by Wolfgang Demtröder

Reference

Neef, A., Beaulieu, S., Hammer, S. et al. Orbital-resolved observation of singlet fission. Nature 616, 275–279 (2023).

Dehydration of a crystal hydrate at subglacial temperatures

• Materials Chemistry

The Role of Crystal Hydrates in Material Sciences

Crystal hydrates are essential in various applications, such as pharmaceuticals and desiccation materials. They can exchange water with the atmosphere under certain conditions. Understanding how these materials hydrate and dehydrate is important for the development of new materials. A unique vapochromic channel hydrate has been discovered that can release water vapor at temperatures as low as -70°C. This expands our knowledge about the possible temperature ranges for dehydration and may impact future material design.

Developing Nanoporous Molecular Solids and Trianglimine T1

Designing nanoporous molecular solids is challenging because they require the precise arrangement of molecules to create empty spaces or channels for guest molecules. Two methods involve using irregularly shaped host molecules or molecules with built-in cavities, such as rigid macrocycles. Trianglimine T1 is a Schiff-base macrocycle designed with salicylimine components that provide structural rigidity and create channels that can potentially host other molecules.

T1 Crystal Color Change Due to Humidity

T1 crystals change color from yellow to red as humidity increases because of water molecules entering the channels. The color change is due to a shift in the balance between two different molecular structures (enolimine and ketoenamine) as water molecules enter the channel and form hydrogen bonds with exposed hydroxyl groups on the host molecule. This color-changing property can help track water absorption in these materials.

Water Behavior and Dehydration Properties of T1

Researchers studied how water molecules behave within T1-R channels under different temperatures and humidity levels. At higher temperatures, water molecules are loosely bound, while at lower temperatures, they form more stable clusters that don’t match the host molecules’ arrangement. Experiments revealed that hydrous T1 crystals change color even at temperatures below 0°C, suggesting that water molecules can form a glass-like state when confined within these channels.

Dehydration kinetics of T1 were examined across a wide temperature range. The rate at which water molecules exit the channels depends on temperature, with an activation energy significantly lower than that required for the sublimation of ice and dehydration of other crystal hydrates. The study found that water molecules move more freely within the channels of T1 compared to other materials.

Understanding dehydration kinetics and the total dehydration time ($t_{tot}$) is crucial for optimizing material properties and energy efficiency. The new findings reveal that the temperature at which water release begins ($T_{on}$) can be as low as -70°C. This discovery has implications for designing channel hydrates, including metal-organic frameworks and covalent organic frameworks, which can be customized to meet specific needs.

Required Additional Study Materials

• Ball, P. H2O: A Biography of Water (Weidenfeld & Nicolson, 1999).
• Morris, K. R. & Rodriguez-Hornedo, N. in Encyclopedia of Pharmaceutical Technology (eds Swarbrick, J. & Boylan, J. C.) 393–440 (Marcel Dekker, 1993).
• Morris, K. R. Structural Aspects of Hydrates and Solvates in Polymorphism in Pharmaceutical Solids (Marcel Dekker, 1999).
• Feng, A. et al. Recent development of atmospheric water harvesting materials: a review. ACS Materials Au 2, 576–595 (2022).
• Shi, W., Guan, W., Lei, C. & Yu, G. Sorbents for atmospheric water harvesting: from design principles to applications. Angew. Chem. Int. Edn 61, e202211267 (2022).
• Kitiagorodski, A. I. Molecular crystals and molecules (Physical Chemistry A series of monographs, Vol. 29) (Academic, 1973).
• Steed, K. M. & Steed, J. W. Packing problems: high Z′ crystal structures and their relationship to cocrystals, inclusion compounds, and polymorphism. Chem. Rev. 115, 2895–2933 (2015).
• Little, M. A. & Cooper, A. I. The chemistry of porous organic molecular materials. Adv. Funct. Mater. 30, 1909842 (2020).

Introductory material

• “Structure of Materials: An Introduction to Crystallography, Diffraction and Symmetry” by Marc De Graef and Michael E. McHenry
• “Crystalology: basic concepts about crystals, crystalline matter, and methods of their study” by E.N. Zav’yalov
• “Foundations of Materials Science and Engineering” by William Smith and Javad Hashemi

Reference

Eaby, A.C., Myburgh, D.C., Kosimov, A. et al. Dehydration of a crystal hydrate at subglacial temperatures. Nature 616, 288–292 (2023).