Coleg Gwyddoniaeth a Pheirianneg

  1. Cyhoeddwyd

    Dual-peak long-period fibre gratings with enhanced refractive index sensitivity by finely tailored mode dispersion that uses the light cladding etching technique

    Chen, X., Zhou, K., Zhang, L. & Bennion, I., Chwef 2007, Yn: Applied Optics. 46, 4, t. 451-455

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  2. Cyhoeddwyd

    Real-time detection of DNA interactions with long-period fiber-grating-based biosensor

    Chen, X., Zhang, L., Zhou, K., Davies, E., Sugden, K., Bennion, I., Hughes, M. & Hine, A. V., Medi 2007, Yn: Optics Letters. 32, 17, t. 2541-2543

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  3. Cyhoeddwyd

    A Point-to-multipoint Flexible Transceiver for Inherently Hub-and-Spoke IMDD Optical Access Networks

    Chen, L., Jin, W., He, J., Giddings, R., Huang, Y. & Tang, J., 15 Gorff 2023, Yn: Journal of Lightwave Technology. 41, 14, t. 4743-4754 12 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  4. Cyhoeddwyd

    Ecological intensification measures to improve productivity and decrease nitrogen surplus in wheat-maize/watermelon intercropping system

    Chen, Y., Yang, X., Zhang, Y., Xu, Z., Cross, P. & Zhang, C., 17 Chwef 2023, Yn: International Journal of Sustainable Development and World Ecology. 30, 2, t. 140-151 12 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  5. Cyhoeddwyd

    Chemical sensor using Mach-Zehnder interferometer based on a pair of largely tilted fibre gratings

    Chen, X., Zhou, K., Adebayo, A. & Zhang, L., 2012.

    Allbwn ymchwil: Cyfraniad at gynhadleddPapuradolygiad gan gymheiriaid

  6. Cyhoeddwyd

    Optical chemsensors utilizing long-period fibre gratings UV-inscribed in D-fibre with enhanced sensitivity through cladding etching

    Chen, X., Zhou, K., Zhang, L. & Bennion, I., 4 Mai 2004, Yn: IEEE Photonics Technology Letters. 16, 5, t. 1352-1354

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  7. Cyhoeddwyd

    Bragg grating in a polymer optical fibre for strain, bend and temperature sensing

    Chen, X., Zhang, C., Webb, D. J., Peng, G. D. & Kalli, K., 28 Gorff 2010, Yn: Measurement Science and Technology. 21, 9, t. 1-5

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  8. Cyhoeddwyd

    RAMPVIS: Answering the Challenges of Building Visualization Capabilities for Large-scale Emergency Responses

    Chen, M., Abdul-Rahman, A., Archambault, D., Dykes, J., Ritsos, P. D., Slingsby, A., Torsney-Weir, T., Turkay, C., Bach, B., Borgo, R., Brett, A., Fang, H., Jianu, R., Khan, S., Laramee, R. S., Matthews, L., Nguyen, P. H., Reeve, R., Roberts, J. C., Vidal, F., Wang, Q., Wood, J. & Xu, K., Hyd 2023, IEEE VIS Workshop on Visualization for Pandemic and Emergency Responses 2023 (Vis4PandEmRes). IEEE

    Allbwn ymchwil: Pennod mewn Llyfr/Adroddiad/Trafodion CynhadleddCyfraniad i Gynhadleddadolygiad gan gymheiriaid

  9. E-gyhoeddi cyn argraffu

    Superlens-Assisted laser nanostructuring of Long Period optical fiber Gratings (LPGs) for enhanced refractive index sensing

    Chen, Y., Yue, L., Yang, X., Baxter, C. & Wang, Z., 1 Medi 2024, Yn: Optics and Laser Technology. 176, 111001.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  10. Cyhoeddwyd

    Analyzing Peak-to-Average Power Ratio Characteristics in Multi-Channel Intensity Modulation and Direct Detection Flexible Transceivers Deploying Inverse Fast Fourier Transform/Fast Fourier Transform-Based Processing

    Chen, L., Huang, X., Jin, W., Wang, X., Yang, G., Jiang, M., Huang, Y. & Tang, J., 13 Rhag 2023, Yn: Sensors. 23, 24, 9804.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  11. Cyhoeddwyd

    Point-to-point intensity modulation and direct detection flexible transceivers incorporating cascaded inverse fast fourier transform/fast fourier transform-based multi-channel aggregation/de-aggregation techniques

    Chen, L., Wang, X., Jin, W., Huang, X., Yang, G., Jiang, M. & Tang, J., 27 Chwef 2024, Yn: IET Optoelectronics. 18, 1-2, t. 41-47 7 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  12. Cyhoeddwyd

    Subcarrier Index-Power Modulated Optical OFDM With Superposition Multiplexing for IMDD Transmission Systems

    Chen, L., Al Halabi, F., Giddings, R. & Tang, J., 12 Tach 2016, Yn: Journal of Lightwave Technology. 34, 22, t. 5284 - 5292

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  13. Cyhoeddwyd

    Multimodal Learning for Classification of Solar Radio Spectrum

    Chen, Z., Ma, L., Xu, L., Weng, Y. & Yan, Y. H., 9 Hyd 2015, t. 1035-1040.

    Allbwn ymchwil: Cyfraniad at gynhadleddPapur

  14. Cyhoeddwyd

    RAMPVIS: Answering the Challenges of Building Visualisation Capabilities for Large-scale Emergency Responses

    Chen, M., Abdul-Rahman, A., Archambault, D., Dykes, J., Slingsby, A., Ritsos, P. D., Torsney-Weir, T., Turkay, C., Bach, B., Borgo, R., Brett, A., Fang, H., Jianu, R., Khan, S., Laramee, R. S., Nguyen, P. H., Reeve, R., Roberts, J. C., Vidal, F., Wang, Q., Wood, J. & Xu, K., Meh 2022, Yn: Epidemics. 39, 15 t., 100569.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  15. Cyhoeddwyd

    Simultaneous measurement of temperature and external refractive index by use of a hybrid grating in D fibre with enhanced sensitivity by HF etching

    Chen, X., Zhou, K., Zhang, L. & Bennion, I., 2005, Yn: Applied Optics. 44, 2, t. 178-182

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  16. Cyhoeddwyd

    EDC-Mediated Oligonucleotide Immobilization on a Long Period Grating Optical Biosensor

    Chen, X., Liu, C., Hughes, M., Nage, D. A., Hine, A. V. & Zhang, L., 28 Meh 2015, Yn: Journal of Biosensors and Bioelectronics. 6, 2

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  17. Cyhoeddwyd

    Subcarrier Index-Power Modulated-Optical OFDM with Dual Superposition Multiplexing for Directly Modulated DFB-based IMDD PON Systems

    Chen, L., Halabi, F., Giddings, R. P., Zhang, J. & Tang, J., Rhag 2018, Yn: IEEE Photonics Journal. 10, 6, 7908113.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  18. Cyhoeddwyd

    Parental size and environmental conditions affect egg capsule production by Nassarius reticulatus (Linnaeus 1758) (Gastropoda: Nassariidae).

    Chatzinikolaou, E. & Richardson, C. A., 15 Gorff 2010, Yn: Journal of Experimental Marine Biology and Ecology. 390, 1, t. 14-21

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  19. Cyhoeddwyd

    Population dynamics and growth of Nassarius reticulatus (Gastropoda : Nassariidae) in Rhosneigr (Anglesey, UK).

    Chatzinikolaou, E. & Richardson, C. A., 1 Chwef 2008, Yn: Marine Biology. 153, 4, t. 605-619

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  20. Cyhoeddwyd

    Evaluating the growth and age of the netted whelk Nassarius reticulatus (Gastropoda: Nassariidae) using statolith growth rings.

    Chatzinikolaou, E. & Richardson, C. A., 1 Gorff 2007, Yn: Marine Ecology Progress Series. 342, t. 163-176

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  21. Cyhoeddwyd

    Climatic similarity and genomic background shape the extent of parallel adaptation in Timema stick insects

    Chaturvedi, S., Gompert, Z., Feder, J., Osborne, O., Muschick, M., Riesch, R., Soria-Carrasco, V. & Nosil, P., Rhag 2022, Yn: Nature Ecology and Evolution. 6, 12, t. 1952-1964 13 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  22. Cyhoeddwyd

    Future mangrove carbon storage under climate change and deforestation

    Chatting, M., Al-Maslamani, I., Walton, M., Skov, M., Kennedy, H., Husrevoglu, S. & Le Vay, L., 10 Chwef 2022, Yn: Frontiers in Marine Science. 9, 14 t., 781876.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  23. Cyhoeddwyd

    Mangrove carbon stocks and biomass partitioning in an extreme environment

    Chatting, M., Le Vay, L., Walton, M., Skov, M., Kennedy, H., Wilson, S. & Al Maslamani, I., 5 Hyd 2020, Yn: Estuarine, Coastal and Shelf Science. 244, 106940.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  24. Cyhoeddwyd

    Preparation of 2,3-disubstituted-4,5-dihydrothiophenes and thiophenes using the intramolecular non-classical wittig reaction of thiolcarboxylates

    Chatterjee, P., Murphy, P. J., Pepe, R. & Shaw, M., 1994, Yn: Journal of the Chemical Society, Perkin Transactions 1. 17, t. 2403-2405 3 t.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  25. Cyhoeddwyd

    Network of hydrogen bonds in Pro-Ala-Pro and Pro-Phe-Pro diamides: A first principles study of Ala -> Phe point mutation in proline environment.

    Chasse, G. A., Wang, H., Csizmadia, I. G., Marsi, I., Chass, G. A., Fang, D. C. & Viskolcz, B., 21 Gorff 2009, Yn: Journal of Chemical Physics. 131, 3, t. 035105

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  26. Cyhoeddwyd

    Conversion of combustible municipal solid waste to methyl alcohol: an environmentally friendly technology.

    Chasse, G. A., Chass, G. A. & Csizmadia, I. G., 1 Hyd 2008, Yn: International Journal of Environmental Studies. 65, 5, t. 655-665

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  27. Cyhoeddwyd

    Ab initio conformational analysis of an N- an C-terminally-protected valyl-alanine dipeptide model

    Chasse, G. A., Chun, C. P., Connor, A. A. & Chass, G. A., 30 Medi 2005, Yn: Journal of Molecular Structure: THEOCHEM. 729, 3, t. 177-184

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  28. Cyhoeddwyd

    A quantitative scale for the extent of conjugation of the amide bond. Amidity percentage as a chemical driving force

    Chasse, G. A., Mucsi, Z., Tsai, A., Szori, M., Chass, G. A., Viskolcz, B. & Csizmadia, I. G., 20 Rhag 2007, Yn: Journal of Physical Chemistry A. 111, 50, t. 13245-13254

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  29. Cyhoeddwyd

    Pd-NHC (PEPPSI) complexes: Synthetic utility and computational studies into their reactivity.

    Chasse, G. A., Organ, M. G., Chass, G. A., Fang, D. C., Hopkinson, A. C. & Valente, C., 2 Medi 2008, Yn: Synthesis - Stuttgart. 2008, 17, t. 2776-2797

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  30. Cyhoeddwyd

    Density Functional Theory (DFT) Investigation of the Alkyl-Alkyl Negishi Cross-Coupling Reaction Catalyzed by N-heterocyclic Carbene (NHC)-Pd Complexes.

    Chasse, G. A., Chass, G. A., O'Brien, C. J., Hadei, N., Kantchev, E. A., Mu, W. H., Fang, D. C., Hopkinsn, A. C., Csizmadia, I. G. & Organ, M. G., 20 Ebr 2009, Yn: Chemistry - A European Journal. 15, 17, t. 4281-4288

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  31. Cyhoeddwyd

    Characterisation of the conformational probability of N-Acetyl-Phenylalanine-NH2 by RHF, DFT, MP2 computation and AIM Analyses, confirmed by jet-cooled infrared data

    Chasse, G. A., Chass, G. A., Mirasol, R. S., Setiadi, D. H., Tang, T. H., Chin, W., Mons, M., Dimicoli, I., Dognon, J. P., Viskolcz, B., Lovas, S., Penke, B. & Csizmadia, I. G., 1 Mai 2005, Yn: Journal of Physical Chemistry A. 109, 24, t. 5289-5302

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  32. Cyhoeddwyd

    The gas-phase dipeptide analogue acetyl-phenylalanyl-amide: A model for the study of side chain/backbone interactions in proteins

    Chasse, G. A., Chin, W., Mons, M., Dognon, J. P., Mirasol, R., Chass, G., Dimicoli, I., Piuzzi, F., Butz, P., Tardivel, B., Compagnon, I., von Helden, G. & Meijer, G., 23 Meh 2005, Yn: Journal of Physical Chemistry A. 109, 24, t. 5281-5288

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  33. Cyhoeddwyd

    Systemic Energy Management by Strategically Located Functional Components within Molecular Frameworks, Determined by Systems Chemistry.

    Chasse, G. A., Mucsi, Z., Chass, G. A. & Csizmadia, I. G., 30 Gorff 2009, Yn: Journal of Physical Chemistry B. 113, 30, t. 10308-10314

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  34. Cyhoeddwyd

    A Synergy between Experiment and Theory for the Formation of Pyridine and Pyrrole Derivatives from Selected Butadienes and Organolithium Reagents: Mechanism, Solvent, and Substituent Effect.

    Chasse, G. A., Mu, W. H., Chass, G. A. & Fang, D. C., 26 Hyd 2009, Yn: Organometalics. 28, 20, t. 5848-5856

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  35. Cyhoeddwyd

    High level a initio exploration on the conversion of carbon dioxide into oxazolidinones: The mechanism and regioselectivity.

    Chasse, G. A., Mu, W. H., Chass, G. A. & Fang, D. C., 24 Gorff 2008, Yn: Journal of Physical Chemistry A. 112, 29, t. 6708-6714

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  36. Cyhoeddwyd

    Quantitative scale for the extent of conjugation of carbonyl groups: "Carbonylicity" percentage as a chemical driving force.

    Chasse, G. A., Mucsi, Z., Chass, G. A., Viskolcz, B. & Csizmadia, I. G., 25 Medi 2008, Yn: Journal of Physical Chemistry A. 112, 38, t. 9153-9165

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  37. Cyhoeddwyd

    Geometric, electronic and elastic properties of dental silver amalgam γ-(Ag3Sn), γ1-(Ag2Hg3), γ2-(Sn8Hg) phases, comparison of experiment and theory.

    Chasse, G. A., Davies, R. A., Ardalan, S., Mu, W. H., Tian, K., Farsaikiya, F., Darvell, B. W. & Chass, G. A., 1 Mai 2010, Yn: Intermetallics. 18, 5, t. 756-760

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  38. Cyhoeddwyd

    Towards the rational design of palladium-N-heterocyclic carbene catalysts by a combined experimental and computational approach

    Chasse, G. A., O'Brien, C. J., Kantchev, E. A., Chass, G. A., Hadei, N., Hopkinson, A. C., Organ, M. G., Setiadi, D. H., Tang, T. H. & Fang, D. C., 10 Hyd 2005, Yn: Tetrahedron. 61, 41, t. 9723-9735

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  39. Cyhoeddwyd

    Toward a computed peptide structure database: The role of a universal atomic numbering system of amino acids in peptides and internal hierarchy of database

    Chasse, G. A., Chass, G. A., Sahai, M. A., Law, J. M., Lovas, S., Farkas, O., Perczel, A., Rivali, J. L. & Csizmadia, I. G., 1 Gorff 2002, Yn: International Journal of Quantum Chemistry. 90, 2, t. 933-968

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  40. Cyhoeddwyd

    The fine balance between one cross-coupling and two β-hydride elimination pathways: a DFT mechanistic study of Ni(π-allyl)2-catalyzed cross coupling of alkyl halides and alkyl Grignard reagents.

    Chasse, G. A., Chass, G. A., Kantchev, E. A. & Fang, D. C., 28 Ebr 2010, Yn: Chemical Communications. 46, 16, t. 2727-2729

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  41. Cyhoeddwyd

    Recycling the Waste: The Development of a Catalytic Wittig Reaction.

    Chasse, G. A., O'Brien, C. J., Tellez, J. L., Nixon, Z. S., Kang, L. J., Carter, A. L., Kunkel, S. R., Przeworski, K. C. & Chass, G. A., 1 Medi 2009, Yn: Angewandte Chemie - International Edition. 48, 37, t. 6836-6839

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  42. Cyhoeddwyd

    Amidicity change as a significant driving force and thermodynamic selection rule of transamidation reactions. A synergy between experiment and theory.

    Chasse, G. A., Mucsi, Z., Chass, G. A. & Csizmadia, I. G., 3 Gorff 2008, Yn: Journal of Physical Chemistry B. 112, 26, t. 7885-7893

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  43. Cyhoeddwyd

    A Quantitative Scale for the Extent of Conjugation of Substituted Olefines.

    Chasse, G. A., Mucsi, Z., Chass, G. A., Viskolcz, B. & Csizmadia, I. G., 9 Gorff 2009, Yn: Journal of Physical Chemistry A. 113, 27, t. 7953-7962

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  44. Cyhoeddwyd

    Easily prepared air- and moisture-stable Pd-NHC (NHC = N-heterocyclic carbene) complexes: a reliable, user-friendly, highly active palladium precatalyst for the Suzuki-Miyaura reaction

    Chasse, G. A., O'Brien, C. J., Kantchev, E. A., Valente, C., Hadei, N., Chass, G. A., Lough, A., Hopkinson, A. C. & Organ, M. G., 14 Meh 2006, Yn: Chemistry - A European Journal. 12, 18, t. 4743-4748

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  45. Cyhoeddwyd

    Optimising storage conditions and processing of sheep urine for nitrogen cycle and gaseous emission measurements from urine patches

    Charteris, A. F., Marsden, K. A., Evans, J. R., Barrat, H. A., Loick, N., Jones, D. L., Chadwick, D. R. & Cárdenas, L. M., 9 Meh 2021, Yn: Scientific Reports. 11, 1, 12116.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  46. Cyhoeddwyd

    Within-field spatial variability of greenhouse gas fluxes from an extensive and intensive sheep-grazed pasture

    Charteris, A. F., Harris, P., Marsden, K. A., Harris, I. M., Guo, Z., Beaumont, D. A., Taylor, H., Sanfratello, G., Jones, D. L., Johnson, S. C. M., Whelan, M. J., Howden, N., Sint, H., Chadwick, D. R. & Cardenas, L. M., 1 Meh 2021, Yn: Agriculture, Ecosystems and Environment. 312, 107355.

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  47. Cyhoeddwyd

    Ontogeny of osmoregulation, physiological plasticity and larval export strategy in the grapsid crab Chasmagnathus granulata (Crustacea, Decapoda).

    Charmantier, G., Gimenez, L., Charmantier-Duares, M. & Anger, K., 1 Maw 2002, Yn: Marine Ecology Progress Series. 229, t. 185-194

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  48. Cyhoeddwyd

    Editorial: Quaternary revolutions

    Charman, D. J., Duller, G. A., Long, A. J., Schreve, D. C. & Scourse, J. D., 30 Maw 2015, Yn: Journal of Quaternary Science. 30, 2, t. 101-103

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid

  49. Cyhoeddwyd

    Plants to Products BBSRC NIBB - Proof of Concept award (HD-RD0300E): Evaluation of the thermo-mechanical pulping of biomass to facilitate release of fermentable sugars for advanced biofuel production

    Charlton, A., Skinner, C., Baker, P., Chesworth, M., Wilkinson, C., Hanslip, Z., Richards, D., Kelly, S. & Zein, S., 10 Mai 2017, 73 t.

    Allbwn ymchwil: Cyfraniad arallCyfraniad Arall

  50. Cyhoeddwyd

    An improved procedure for the synthesis of N-substituted 3,4-dichloromaleimides

    Charlton, A., Edge, S., Kruse, T., Varma, K. S., Underhill, A. E., Kathirgamanathan, P. & Becher, J., 1991, Yn: Chemistry and Industry. 4, t. 130-131

    Allbwn ymchwil: Cyfraniad at gyfnodolynErthygladolygiad gan gymheiriaid