Quantum critical points separating weak ferromagnetic and paramagnetic phases trigger many novel phenomena. Dynamical spin fluctuations not only suppress the long-range order, but can also lead to unusual transport and even superconductivity. Combining quantum criticality with topological electronic properties presents a rare and unique opportunity. Here, by means of ab initio calculations and magnetic, thermal, and transport measurements, it is shown that the orthorhombic CoTe2 is close to ferromagnetism, which appears suppressed by spin fluctuations. Calculations and transport measurements reveal nodal Dirac lines, making it a rare combination of proximity to quantum criticality and Dirac topology.

Charge transfer (CT) cocrystals, molecular crystals composed of electron donating and accepting species, are being developed for applications in optoelectronics. Here we present optical and electronic characterization of the CT cocrystal phenothiazine-tetracyanoquinodimethane (PTZ-TCNQ). This material has a broad NIR absorption peak with an optical band edge less than 0.6 eV. We used density functional theory calculations to identify the origin of the low energy CT states and changes in the Raman spectra. We also demonstrate the fabrication of long, ribbon-like oriented cocrystals using an evaporative alignment method. Cocrystals grown on Si substrates were fabricated into organic field effect transistors. Despite theoretical predictions of ambipolarity, only electron conduction was observed, with mobilities on the order of 10-4 cm2 V-1 s-1. Measurements of the temperature dependence of the mobility indicated a superexchange mediated hopping mechanism for charge transport, with a characteristic scale of 0.19 eV.

Layered materials enable the assembly of a new class of heterostructures where lattice-matching is no longer a requirement. Interfaces in these heterostructures therefore become a fertile ground for unexplored physics as dissimilar phenomena can be coupled via proximity effects. In this article, we identify an unexpected photoluminescence (PL) peak when MoSe2 interacts with TiSe2. A series of temperature-dependent and spatially resolved PL measurements reveal that this peak is unique to the TiSe2–MoSe2 interface, is higher in energy compared to the neutral exciton, and exhibits exciton-like characteristics. The feature disappears at the TiSe2 charge density wave transition, suggesting that the density wave plays an important role in the formation of this new exciton. We present several plausible scenarios regarding the origin of this peak that individually capture some aspects of our observations but cannot fully explain this feature. These results therefore represent a fresh challenge for the theoretical community and provide a fascinating way to engineer excitons through interactions with charge density waves.

We explored the electronic and magnetic properties of the lanthanide double perovskite Dy2FeCoO6 by combining magnetization, Raman and Mössbauer spectroscopy and neutron diffraction along with density functional theory (DFT) calculations. Our magnetization measurements revealed two magnetic phase transitions in Dy2FeCoO6. First, a paramagnetic to antiferromagnetic transition at T N = 248 K and subsequently, a spin reorientation transition at T SR = 86 K. In addition, a field-induced magnetic phase transition with a critical field of H c ≈ 20 kOe is seen at 2 K. Neutron diffraction data suggested cation-disordered orthorhombic structure for Dy2FeCoO6 in Pnma space group which is supported by Raman scattering results. The magnetic structures ascertained through representational analysis indicate that at T N, a paramagnetic state is transformed to Γ5(Cx, Fy, Az) antiferromagnetic structure while, at T SR, Fe/Co moments undergo a spin reorientation to Γ3(Gx, Ay, Fz). The refined magnetic moment of (Fe/Co) is 1.47(4) μ B at 7 K. The antiferromagnetic structure found experimentally is supported through the DFT calculations which predict an insulating electronic state in Dy2FeCoO6.

Neutral and charged excitons (trions) in atomically thin materials offer important capabilities for photonics, from ultrafast photodetectors to highly efficient light-emitting diodes and lasers. Recent studies of van der Waals (vdW) heterostructures comprised of dissimilar monolayer materials have uncovered a wealth of optical phenomena that are predominantly governed by interlayer interactions. Here, we examine the optical properties in NbSe2-MoSe2 vdW heterostructures, which provide an important model system to study metal-semiconductor interfaces, a common element in optoelectronics. Through low-temperature photoluminescence (PL) microscopy, we discover a sharp emission feature, L1, that is localized at the NbSe2-capped regions of MoSe2. L1 is observed at energies below the commonly studied MoSe2 excitons and trions and exhibits temperature- and power-dependent PL consistent with exciton localization in a confining potential. This PL feature is robust, observed in a variety of samples fabricated with different stacking geometries and cleaning procedures. Using first-principles calculations, we reveal that the confinement potential required for exciton localization naturally arises from the in-plane band bending due to the changes in the electron affinity between pristine MoSe2 and NbSe2-MoSe2 heterostructure. We discuss the implications of our studies for atomically thin optoelectronics devices with atomically sharp interfaces and tunable electronic structures.

Synthetic two-dimensional (2D) materials provide an opportunity to realize large-scale applications in next generation electronic and optoelectronic devices. One of the biggest challenges of synthetic 2D materials is the lateral heterogeneity such as non-uniform strain, composition and defect density. The electronic and optical properties are found to be not uniform in many cases, even within a single crystalline domain, potentially limiting synthetic 2D materials in advanced devices. In this work, we probe the origin of the widely observed lateral heterogeneities in synthetic monolayer MoS2. Epitaxial single crystalline domains (similar to 10 mu m) are optically homogeneous and uniform with 0.3%-0.4% tensile strain, while misoriented domains (> 20 mu m) exhibit distinct photoluminescence (PL) emissions from the center to the edge, along with released strain at the center. Temperature-dependent Raman and PL mapping reveals that the center of non-epitaxial domains exhibits an enhanced PL due to increased defect density. Density function theory (DFT) calculations suggest that oxygen defects can readily lead the loss of epitaxy, consistent with our observation of a MoOx core-shell structure that only exists in misoriented domains. Combining experiment and DFT, we hypothesize that two growth mechanisms, solid-solid and vapor-solid growth, may be responsible for the lateral heterogeneities.

Jaydeep Joshi,1,2 Heather M. Hill,3 Sugata Chowdhury,4 Christos D. Malliakas,5 Francesca Tavazza,4 Utpal Chatterjee,6 Angela R. Hight Walker,3 and Patrick M. Vora1,2,* 1Department of Physics and Astronomy, George Mason University, Fairfax, Virginia 22030, USA 2Quantum Materials Center, George Mason University, Fairfax, Virginia 22030, USA 3Physical Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 4Materials Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 5Department of Chemistry, Northwestern University, Evanston, Illinois 60208, USA 6Department of Physics, University of Virginia, Charlottesville, Virginia 22904, USA

2H-TaS2 undergoes a charge density wave (CDW) transition at ~ 75 K, however key questions regarding the onset of CDW order remain under debate. In this study, we explore the CDW transition through a combination of temperature and excitation-dependent Raman spectroscopy, angle resolved photoemission spectroscopy (ARPES), and density functional theory (DFT). Below we identify two CDW amplitude modes that redshift and broaden with increasing temperature and one zone-folded mode that disappears above . Above , we observe a strong two-phonon mode that softens substantially upon cooling, which suggests the presence of substantial lattice distortions at temperatures as high as 250 K. This correlates with the ARPES observation of the persistence of CDW energy gap above and finite-temperature DFT calculations of the phonon band structure that indicate an instability occurring well above the CDW transition temperature. DFT also provides the atomic displacements of the CDW amplitude modes and reproduces their temperature-dependence. From these observations we suggest that short range CDW order exists well-above which poses new questions regarding the interplay between electronic structure and vibrational modes in layered CDW materials.

In article number 1706950, Joshua A. Robinson and co-workers demonstrate in-situ Re doping of synthetic monolayer MoS2 with ≈1 at% Re on r-plane sapphire. Electronic measurements and density functional theory calculations show that 1 at% Re doping achieves nearly degenerate n-type doping. Low-temperature photoluminescence reveals a suppression of defect emission, resulting from the passivation of defects due to the presence of Re. Picture Credit: Donna Deng.

Doping is a fundamental requirement for tuning and improving the properties of conventional semiconductors. Recent doping studies including niobium (Nb) doping of molybdenum disulfide (MoS2) and tungsten (W) doping of molybdenum diselenide (MoSe2) have suggested that substitutional doping may provide an efficient route to tune the doping type and suppress deep trap levels of 2D materials. To date, the impact of the doping on the structural, electronic, and photonic properties of in situ-doped monolayers remains unanswered due to challenges including strong film substrate charge transfer, and difficulty achieving doping concentrations greater than 0.3 at%. Here, in situ rhenium (Re) doping of synthetic monolayer MoS2 with approximate to 1 at% Re is demonstrated. To limit substrate film charge transfer, r-plane sapphire is used. Electronic measurements demonstrate that 1 at% Re doping achieves nearly degenerate n-type doping, which agrees with density functional theory calculations. Moreover, low-temperature photoluminescence indicates a significant quench of the defect-bound emission when Re is introduced, which is attributed to the Mo-O bond and sulfur vacancies passivation and reduction in gap states due to the presence of Re. The work presented here demonstrates that Re doping of MoS2 is a promising route toward electronic and photonic engineering of 2D materials.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T′ semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the Td Weyl semimetal phase. The Mo1−xWxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study Mo1−xWxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T′, and Td structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2-WTe2 system, including single-phase 2H, 1T ′, and Td regions, as well as a two-phase 1T ′ + Td region. Disorder arising from compositional fluctuations in Mo1−xWxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T-MoTe2 mode and the enhancement of a double-resonance Raman process in 2H-Mo1−xWxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo1−xWxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T$^\prime$ semimetallic phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy barrier between these two phases, while also allowing access to the T$_d$ Weyl semimetal phase. The MoWTe$_2$ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe$_2$-WTe$_2$ system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe$_2$ alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T$^\prime$, and T$_d$ structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe$_2$-WTe$_2$ system, including single-phase 2H, 1T$^\prime$, and T$_d$ regions, as well as a two-phase 1T$^\prime$ + T$_d$ region. Disorder arising from compositional fluctuations in MoWTe$_2$ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T$^\prime$-MoTe$_2$ mode and the enhancement of a double-resonance Raman process in 2H-MoWTe$_2$ alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe$_2$ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T' semimetallic phase at high temperatures. Alloying MoTe2 with WTe2 reduces the energy barrier between these two phases, while also allowing access to the T-d Weyl semimetal phase. The Mo1-xWxTe2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe2-WTe2 system. We combine polarization-resolved Raman spectroscopy with x-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study bulk Mo1-xWxTe2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T', and T-d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe2-WTe2 system, including single-phase 2H, 1T', and T-d regions, as well as a two-phase 1T' + T-d region. Disorder arising from compositional fluctuations in Mo1-xWxTe2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T'-MoTe2 mode and the enhancement of a double-resonance Raman process in 2H-Mo1-xWxTe2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in Mo1-xWxTe2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

The structural polymorphism intrinsic to select transition metal dichalcogenides provides exciting opportunities for engineering novel devices. Of special interest are memory technologies that rely upon controlled changes in crystal phase, collectively known as phase change memories (PCMs). MoTe$_2$ is ideal for PCMs as the ground state energy difference between the hexagonal (2H, semiconducting) and monoclinic (1T’, metallic) phases is minimal. This energy difference can be made arbitrarily small by substituting W for Mo on the metal sublattice, thus improving PCM performance. Therefore, understanding the properties of Mo$_{1-x}$W$_x$Te$_2$ alloys across the entire compositional range is vital for the technological application of these versatile materials. We combine Raman spectroscopy with aberration-corrected scanning transmission electron microscopy and x-ray diffraction to explore the MoTe$_2$-WTe$_2$ alloy system. The results of these studies enable the construction of the complete alloy phase diagram, while polarization-resolved Raman measurements provide phonon mode and symmetry assignments for all compositions. Temperature-dependent Raman measurements indicate a transition from 1T’-MoTe$_2$ to a distorted orthorhombic phase (T$_d$) below 250 K and facilitate identification of the anharmonic contributions to the optical phonon modes in bulk MoTe$_2$ and Mo$_{1-x}W$_x$Te$_2$ alloys. We also identify a Raman-forbidden MoTe$_2$ mode that is activated by compositional disorder and find that the main WTe$_2$ Raman peak is asymmetric for x<1. This asymmetry is well-fit by the phonon confinement model and allows the determination of the phonon correlation length. Our work is foundational for future studies of Mo$_x$W${1-x}$Te$_2$ alloys and provides new insights into the impact of disorder in transition metal dichalcogenides.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe_2 crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T^' semimetallic phase at high temperatures. Alloying MoTe_2 with WTe_2 reduces the energy barrier between these two phases, while also allowing access to the T_d Weyl semimetal phase. The MoWTe_2 alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe_2-WTe_2 system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe_2 alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T^', and T_d structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe_2-WTe_2 system, including single-phase 2H, 1T^', and T_d regions, as well as a two-phase 1T^' + T_d region. Disorder arising from compositional fluctuations in MoWTe_2 alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T^'-MoTe_2 mode and the enhancement of a double-resonance Raman process in 2H-MoWTe_2 alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe_2 alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

The structural polymorphism in transition metal dichalcogenides (TMDs) provides exciting opportunities for developing advanced electronics. For example, MoTe$_2$ crystallizes in the 2H semiconducting phase at ambient temperature and pressure, but transitions into the 1T$^\prime$ semimetallic phase at high temperatures. Alloying MoTe$_2$ with WTe$_2$ reduces the energy barrier between these two phases, while also allowing access to the T$_d$ Weyl semimetal phase. The MoWTe$_2$ alloy system is therefore promising for developing phase change memory technology. However, achieving this goal necessitates a detailed understanding of the phase composition in the MoTe$_2$-WTe$_2$ system. We combine polarization-resolved Raman spectroscopy with X-ray diffraction (XRD) and scanning transmission electron microscopy (STEM) to study MoWTe$_2$ alloys over the full compositional range x from 0 to 1. We identify Raman and XRD signatures characteristic of the 2H, 1T$^\prime$, and T$_d$ structural phases that agree with density-functional theory (DFT) calculations, and use them to identify phase fields in the MoTe$_2$-WTe$_2$ system, including single-phase 2H, 1T$^\prime$, and T$_d$ regions, as well as a two-phase 1T$^\prime$ + T$_d$ region. Disorder arising from compositional fluctuations in MoWTe$_2$ alloys breaks inversion and translational symmetry, leading to the activation of an infrared 1T$^\prime$-MoTe$_2$ mode and the enhancement of a double-resonance Raman process in 2H-MoWTe$_2$ alloys. Compositional fluctuations limit the phonon correlation length, which we estimate by fitting the observed asymmetric Raman lineshapes with a phonon confinement model. These observations reveal the important role of disorder in MoWTe$_2$ alloys, clarify the structural phase boundaries, and provide a foundation for future explorations of phase transitions and electronic phenomena in this system.

We examine anharmonic contributions to the optical phonon modes in bulk T_d-MoTe_2 through temperature-dependent Raman spectroscopy. At temperatures ranging from 100 K to 200 K, we find that all modes redshift linearly with temperature in agreement with the Grüneisen model. However, below 100 K we observe nonlinear temperature-dependent frequency shifts in some modes. We demonstrate that this anharmonic behavior is consistent with the decay of an optical phonon into multiple acoustic phonons. Furthermore, the highest frequency Raman modes show large changes in intensity and linewidth near T≈ 250 K that correlate well with the T_d → 1T^' structural phase transition. These results suggest that phonon-phonon interactions can dominate anharmonic contributions at low temperatures in bulk T_d-MoTe_2, an experimental regime that is currently receiving attention in efforts to understand Weyl semimetals.