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We investigate spin and optical properties of individual nitrogen-vacancy centers located within 1-10 nm from the diamond surface. We observe stable defects with a characteristic optically detected magnetic resonance spectrum down to lowest depth. We also find a small, but systematic spectral broadening for defects shallower than about 2 nm. This broadening is consistent with the presence of a surface paramagnetic impurity layer [Tisler et al., ACS Nano 3, 1959] largely decoupled by motional averaging. The observation of stable and well-behaved defects very close to the surface is critical for single-spin sensors and devices requiring nanometer proximity to the target.

We report a demonstration of two-dimensional (2D) terahertz (THz) magnetic resonance spectroscopy using the magnetic fields of two time-delayed THz pulses. We apply the methodology to directly reveal the nonlinear responses of collective spin waves (magnons) in a canted antiferromagnetic crystal. The 2D THz spectra show all of the third-order nonlinear magnon signals including magnon spin echoes, and 2-quantum signals that reveal pairwise correlations between magnons at the Brillouin zone center. We also observe second-order nonlinear magnon signals showing resonance-enhanced second-harmonic and difference-frequency generation. Numerical simulations of the spin dynamics reproduce all of the spectral features in excellent agreement with the experimental 2D THz spectra. DOI: 10.1103/PhysRevLett.118.207204 Nonlinear manipulation of spins is the basis for all advanced methods in magnetic resonance including multidimensional nuclear magnetic resonance and electron spin resonance (ESR) spectroscopies [1,2], magnetic resonance imaging, and, in recent years, quantum control over individual spins [3]. The methodology is facilitated by the ease with which the regime of strong coupling can be reached between radio frequency or microwave magnetic fields and nuclear or electron spins, respectively, typified by sequences of magnetic pulses that control the magnetic moment directions [1][2][3]. The capabilities meet a bottleneck, however, for far-infrared magnetic resonances characteristic of correlated electron materials, molecular magnets, and metalloproteins.ESR in the terahertz (THz) frequency region can reveal rich information content in chemistry, biology, and materials science [1,2,[4][5][6][7]. In molecular complexes and metalloproteins, THz-frequency zero-field splittings (ZFS) of high-spin transition-metal and rare-earth ions show exquisite sensitivity to ligand geometries, providing mechanistic insight into molecular magnetic properties [4] and protein catalytic function [5]. With strong applied magnetic fields (∼10 T), resonances of unpaired electron spins in molecular complexes can be shifted from the usual microwave regime into the THz range, drastically improving the resolution due to enhanced spectral splittings [2,6,7]. In many ferromagnetic (FM) and antiferromagnetic (AFM) materials, intrinsic magnetic fields in the same range put collective spin waves (magnons) in the THz range. Current ESR spectroscopy remains limited at THz frequencies because the weak sources used only permit measurements of free-induction decay (FID) signals that are linearly proportional to the excitation magnetic field strength. In some cases, including most proteins, even linear THz-frequency ESR signals may not be measurable because the THz spectrum includes much stronger absorption features due to low-frequency motions of polar segments [8]. However, the fast dephasing of such motions ensures that they would not compete with nonlinear spin echo signals [9]. Two-dimensional (2D) THz ESR spectroscopy, like 2D ESR at lower frequenci...

Materials research with a focus on enhancing the minority-carrier lifetime of the light-absorbing semiconductor is key to advancing solar energy technology for both early-stage and mature material platforms alike. Tin sulfide (SnS) is an absorber material with several clear advantages for manufacturing and deployment, but the record power conversion efficiency remains below 5%. We report measurements of bulk and interface minority-carrier recombination rates in SnS thin films using optical-pump, terahertz (THz)-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H2S gas increases the minority-carrier lifetime, and oxidation of the surface reduces the surface recombination velocity. However, the minoritycarrier lifetime remains below 100 ps for all tested combinations of growth technique and postgrowth processing. Significant improvement in SnS solar cell performance will hinge on finding and mitigating as-yet-unknown recombination-active defects. We describe in detail our methodology for TPC experiments, and we share our data analysis routines in the form freelyavailable software.For a more sophisticated solar cell figure of merit, we consider the dimensionless ratio (FPV) of minority-carrier diffusion length to optical absorption length:D is the minority carrier diffusivity,  is the bulk minority-carrier lifetime, and  is the optical absorption coefficient. In Figure 1b we present compiled data for FPV and solar cell efficiency, including results for wafer-based silicon technologies. With the exception of silicon, all of the materials represented strongly absorb light at energies above their respective band gaps. For each material we calculate FPV using  as measured at the knee the curve of log10((E)). For the thin film materials we require that lifetime, diffusivity, absorption coefficient, and device measurements were reported for samples that were synthesized in the same laboratory and using as close to the same procedure as is reasonably possible. This requirement greatly reduces the number of data points compared to Figure 1a. For crystalline silicon we assume the values D = 30 cm 2 s -1 and  = 300 cm -1 .SnS is an absorber with several inherent advantages compared to materials that are widely used in solar cells, but its demonstrated efficiency is too low for commercial relevance. It is composed of non-toxic, Earth-abundant and inexpensive elements. SnS is an inert and waterinsoluble semiconducting mineral (Herzenbergite) with an indirect bandgap of 1.1 eV, strong light absorption for photons with energy above 1.4 eV ( > 10 4 cm -1 ), and intrinsic p-type conductivity with carrier concentration in the range 10 15 -10 17 cm -3 . 5-7 SnS evaporates congruently and is phase-stable up to 600 °C. 8,9 This means that SnS thin films can be deposited by thermal evaporation and its high-speed cousin, closed space sublimation (CSS), as is employed in the manufacture of CdTe solar cells. It also means that SnS phase control is simpler than for most thin film PV materials, nota...