Tungsten Nitride (W5N6): An Ultraresilient 2D Semimetal.

Two-dimensional transition metal nitrides offer intriguing possibilities for achieving novel electronic and mechanical functionality owing to their distinctive and tunable bonding characteristics compared to other 2D materials. We demonstrate here the enabling effects of strong bonding on the morphology and functionality of 2D tungsten nitrides. The employed bottom-up synthesis experienced a unique substrate stabilization effect beyond van-der-Waals epitaxy that favored W5N6 over lower metal nitrides. Comprehensive structural and electronic characterization reveals that monolayer W5N6 can be synthesized at large scale and shows semimetallic behavior with an intriguing indirect band structure. Moreover, the material exhibits exceptional resilience against mechanical damage and chemical reactions. Leveraging these electronic properties and robustness, we demonstrate the application of W5N6 as atomic-scale dry etch stops that allow the integration of high-performance 2D materials contacts. These findings highlight the potential of 2D transition metal nitrides for realizing advanced electronic devices and functional interfaces.

Three-dimensional ultrafast charge-density-wave dynamics in CuTe.

Charge density waves (CDWs) involved with electronic and phononic subsystems simultaneously are a common quantum state in solid-state physics, especially in low-dimensional materials. However, CDW phase dynamics in various dimensions are yet to be studied, and their phase transition mechanism is currently moot. Here we show that using the distinct temperature evolution of orientation-dependent ultrafast electron and phonon dynamics, different dimensional CDW phases are verified in CuTe. When the temperature decreases, the shrinking of c-axis length accompanied with the appearance of interchain and interlayer interactions causes the quantum fluctuations (QF) of the CDW phase until 220 K. At T < 220 K, the CDWs on the different ab-planes are finally locked with each other in anti-phase to form a CDW phase along the c-axis. This study shows the dimension evolution of CDW phases in one CDW system and their stabilized mechanisms in different temperature regimes.

Nature of the bad metallic behavior of Fe1.06Te inferred from its evolution in the magnetic state.

We investigate with angle-resolved photoelectron spectroscopy the changes of the Fermi surface and the main bands from the paramagnetic state to the antiferromagnetic (AFM) state occurring below 72 K in Fe1.06Te. The evolution is completely different from that observed in Fe pnictides, as nesting is absent. The AFM state is a rather good metal, in agreement with our magnetic band structure calculation. On the other hand, the paramagnetic state is very anomalous with a large pseudogap of ~65 meV on the electron pocket that closes in the AFM state. We discuss this behavior in connection with spin fluctuations existing above the magnetic transition and the correlations predicted in the spin-freezing regime of the incoherent metallic state.

Large temperature dependence of the number of carriers in co-doped BaFe(2)As(2).

Using angle-resolved photoemission spectroscopy, we study the evolution of the number of carriers in Ba(Fe(1-x)Co(x))(2)As(2) as a function of Co content and temperature. We show that there is a k-dependent energy shift compared to density functional calculations, which is large below 100 K at low Co contents and reduces the volume of hole and electron pockets by a factor 2. This k shift becomes negligible at high Co content and could be due to interband charge or spin fluctuations. We further reveal that the bands shift with temperature, changing significantly the number of carriers they contain (up to 50%). We explain this evolution by thermal excitations of carriers among the narrow bands, possibly combined with a temperature evolution of the k-dependent fluctuations.

Enhance high harmonic generation (HHG) efficiency via compact multi-plate continuum post-compression for time-resolved angle-resolved photoemission spectroscopy.

Time-resolved angle-resolved photoemission spectroscopy (Tr-APRES) gives direct insight into electron dynamics by providing temporal-, energy-, and momentum-resolved information in one experiment. A major obstacle to using high harmonic generation (HHG) probe pulses for photoemission spectroscopy is the low conversion efficiency, that is, the low flux of probe photons. We use a Yb-KGW based duo-laser source with an oscillator to pump two separate amplifiers and generate two synchronized pulsed laser sources with average energies of 7.5 and 6 W. By using the multiplate continuum method and chirped mirrors, the resulting flux of HHG photons at 33-70 eV can be increased 50-fold (up to 1011 photons/s) by using post-compressed 30 fs pulses compared with the photon flux generated by the fundamental 190 fs pulses. Moreover, pulses from the 6 W amplifier are used to pump an optical parametric amplifier that can vary the wavelengths for photoexcitation. The system performance is demonstrated by applying Tr-ARPES to single-crystal graphite. The front tilt broadening is significantly suppressed by the off-plane mounted conical grating, leading to a 184 fs temporal resolution that is mainly limited by the pump pulse. The energy resolution is 176 meV.